Butterworth–Heinemann An imprint of Elsevier Limited © 2004, Elsevier Limited. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without either the prior permission of the publishers or a licence permitting restricted copying in the United Kingdom issued by the Copyright Licensing Agency, 90 Tottenham Court Road, London W1T 4LP. Permissions may be sought directly from Elsevier’s Health Sciences Rights Department in Philadelphia, USA: (+!) 215 238 7869, fax: (+1) 215 238 2239, e-mail:
[email protected]. You may also complete your request on-line via the Elsevier Science homepage (http://www.elsevier.com), by selecting ‘Customer Support’ and then ‘Obtaining Permissions’. First published 2004 ISBN 0 7506 5560 7 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress. Note Medical knowledge is constantly changing. As new information becomes available, changes in treatment, procedures, equipment and the use of drugs become necessary. The author/contributors and the publishers have taken great care to ensure that the information given in this text is accurate and up to date. However, readers are strongly advised to confirm that the information, especially with regard to drug usage, complies with the latest legislation and standards of practice.
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Preface Recent years have seen an exponential growth in the field of refractive surgery. In 1996, the US Federal Drug Administration’s approval of certain excimer lasers to correct myopia lead to a rapid increase in the uptake of excimer laser procedures. Further approvals have been granted for other laser manufacturers, and for the correction of astigmatism and hyperopia. Popularity increases as patients hear about the successful outcomes for friends and relatives. Famous people who undergo this surgery are a further boost, and laser clinics are quick to state the names of famous patients who have undergone treatment. Techniques have improved, better lasers and other equipment are available and there seems to be an unlimited supply of patients willing to undertake surgical alternatives to wearing spectacles or contact lenses. Practitioners, both optometrists and ophthalmologists, seem to be split over refractive surgery. Some advocate its usefulness as a viable alternative, whereas others feel it is nothing more than cosmetic surgery for refractive error. It may be oversimplifying the issue to call refractive surgery a cosmetic procedure, as patients often state that their disability requires the use of optical aids, almost like using an aid to assist hearing. However, it would be correct to say that refractive sur-
gery is an elective procedure, as the patient chooses to undergo surgical intervention on an otherwise healthy eye, and the surgeon agrees to operate on an eye that is without pathology. In the UK refractive surgery is offered on a private basis only. There have been attempts to treat higher refractive errors on the NHS, but these schemes tend to be regional and not the norm. Patients who decide to undergo refractive surgery either book in to a refractive surgery clinic or go to see a consultant ophthalmologist who offers treatment privately. Surgeons who offer refractive surgery do not need to be consultant ophthalmologists accredited with the Royal College of Ophthalmologists, although many are. However, they must have suitable qualifications, such as Member or Fellow or the Royal College of Ophthalmologists (MRCOphth or FRCOphth), Fellow of the Royal College of Surgeons (FRCS) or be listed on the European Specialist Register. Most laser manufacturers ensure that doctors who use their lasers have attended the relevant training courses to use that particular apparatus. In the UK the most common types of refractive surgery currently employed are photorefractive keratectomy (PRK) and laser in-situ keratomileusis
(LASIK), although PRK is being replaced by laser epithelial keratectomy (LASEK). Both of these use the same currently widespread excimer laser technology. Other techniques are available, but to a lesser degree. This book examines various aspects that may be relevant to those interested in learning more about the current status of refractive surgery, with particular attention paid to patient selection, available surgical techniques and the evaluation of patients pre-operatively and post-operatively (details of some specialist instrumentation are also outlined). Clinicians with a degree of knowledge in refractive surgery may be interested in the chapters that discuss wound healing after refractive surgery and case reports from surgeons. Of general interest, the book also discusses legal issues and future trends in this fastchanging area. Notes for the CD-ROM The large size of the videos means that the loading of video clips 4 and 5, in particular, may take 1–2 minutes on some computers, and users of Mac OS9 may see a white screen while the videos are loading. When the videos finish loading, the screen will change and the Play, Pause, and other buttons will appear Shehzad A Naroo
Contributors Alejandro Cerviño DOO (EC) W Neil Charman DSc, PhD Paul MH Cherry MBBS, LRCP, FRCS(Ed), FRCS, FRCSC, FRCOphth Catharine Chisholm PhD, MCOptom Sandip Doshi PhD, MCOptom Stephen J Doyle BSc(Hons), MRCOphth Balasubraminiam Ilango FRCS(Ed), MRCOphth Mohammad Laiquzzaman MBBS, PhD Shehzad A Naroo MSc, PhD, MCOptom, FIACLE Sunil Shah FRCS(Ed), FRCOphth Baldev K Ubhi BSc(Hons)
1 Patient selection and pre-operative assessment Shehzad A Naroo
For many patients who want to find out more about refractive surgery the first port of call is often the local ophthalmic practice, while others call dedicated clinic phone lines or browse the Internet. Prospective patient interest can be classed into two categories: those who make casual enquiries to see if they are suitable and those who have decided that this is definitely something they will opt for. The first group may progress to become part of the second group when they feel they are more informed. The latter group can sometimes be difficult to dissuade from surgery if they are found to be unsuitable. Those patients who make casual enquiries often seek general advice only and can usually be referred to websites or professional bodies that produce this type of information. Whereas for patients who have decided to opt for refractive surgery, it is usually advisable to make a specific appointment to discuss the surgical options and perform the relevant tests (or else refer the patient to a colleague who is able to do this). Some patients may suspect that optometrists have their own agenda and advise against refractive surgery because they want to protect their own livelihood. Furthermore, many optometrists may feel that their knowledge about the current state of affairs is not adequate and thus choose not to become involved at all and advise patients not to proceed with this option. A proactive approach towards refractive surgery by optometrists is advised by some refractive surgery clinics, and more recently the number of optometrists who have become involved in co-management schemes with refractive surgery providers has increased (discussed in Chapter 7). However, a careful balance needs to be struck by optometrists in rou-
tine practice to ensure they are best able to serve their patient’s needs.1 Optometrists are in a unique primary care position in eye health from which they can offer an unbiased opinion. Various studies have shown the average age of prospective patients to be the mid-to-late thirties with an almost equal ratio of male to female patients.2,3 The author recently completed a study (not yet published) that shows the average patient age to be creeping up to around 40 years, and there seems to a shift towards more female patients. Since refractive surgery usually involves an initial financial outlay comparable to that for contact lenses, which in the UK are often paid by monthly bank debits, most studies seem to show a prevalence of patients from higher socio-economic groups. This may partly explain the age groups of refractive surgery patients too. Figure 1.1 shows the breakdown of the occupational groups of new patients who presenting for refractive surgery.
Retired 6%
Student 3%
Most studies highlight that many patients who present for refractive surgery are former contact lens users.4,5 Often the reasons why people want contact lenses are similar to the motivation for patients to have refractive surgery, so it is not surprising that there are some similarities in the types of patients who present for both types of refractive correction. Both groups of patients often say that they want the freedom from spectacles or they want to achieve a cosmetic look that spectacles do not allow, or perhaps the reasons are related to certain activities (work or sports, etc.). Patients who cease contact lens use in favour of refractive surgery often complain of the inconvenience of contact lenses and/or complications with contact lenses, which is a primary motivation for their decision. Often, many of the less serious complications with contact lenses that patients complain of could be minimized with improved contact lens management, which requires the appropriate input from their contact lens practitioners.
Unemployed 5%
Unskilled 12%
Professional 19% Management 15%
Semi-skilled 9% Clerical 31%
Figure 1.1 Breakdown of the occupational groups of new patients who present for refractive surgery3
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Refractive surgery: a guide to assessment and management
Also, some patients choose refractive surgery as a primary alternative to spectacles and present for surgery even though they have not tried contact lenses. This may result, in part, from the way that laser refractive surgery is marketed. In many cases it would be useful for the patient to try contact lenses first. Laser refractive surgery clinics advertise on the radio, newspapers and television. There seems to be an interesting shift in the way that advertisers have portrayed refractive surgery over the years. In the early days the convenience of refractive surgery was used to herald it as being a ‘quick’, ‘painless’ and ‘safe’ treatment that only took a few seconds or minutes to complete and the patient would return to work shortly afterwards. The next wave of advertising seemed to use people that patients could relate to, either celebrities who would advocate a certain clinic or ‘real’ people that were respected in the community, such as firemen, nurses and even priests. The most recent advertising trend seems to focus on the technology that a particular centre uses, although in the UK this approach has come under the scrutiny of the Advertising Standards Association. Patients who opted for refractive surgery gave the main factors shown in Figure 1.2 as those that influenced their decision to cease contact lens use; the values relate to the percentage of patients who offered the particular reason as an influential factor.3 Patients who are former contact lens wearers are advised to remove their contact lenses prior to their pre-operative refractive surgery consultation. The time period for lens removal depends on the type and modality of lens worn. Typically, soft lenses are removed for 7–14 days prior to the appointment and gas permeable lenses for
Costs 21%
10–21 days. Users who wear hard polymethylmethacrylate (PMMA) lenses may find that they have to leave their lenses out for a few months, especially if they are longterm wearers, to ensure that any corneal distortion is eliminated. Patients are often asked to produce past refraction details, for up to the previous 3 years, to show that they have some level of stability. A patient with a large recent change in refraction would probably be advised to wait until two or three consecutive prescriptions were similar. If patients undergo refractive surgery and then find that a year later their prescriptions naturally became worse, they will often be dissatisfied with the outcome. It has been suggested that refractive surgery may aid visual development in children with squints that are purely accommodative. This type of service would not typically be offered by most commercial refractive surgery clinics and currently it is not widely available in hospital refractive clinics either. Patients under the age of 21 years who present for refractive surgery are often advised to wait until they reach 21, or until their prescription has stabilized.6 Although there is no upper age limit for refractive surgery, it may be unwise to perform a corneal procedure on late presbyopic patients with lens sclerosis, as they may be better suited to clear lens extraction with an accurately calculated intraocular lens implant. Patients with only one ‘seeing’ eye are considered a contraindication to refractive surgery, as infection in the good eye would seriously compromise the patient’s visual function, although the risk of sight-threatening infection is extremely rare after refractive surgery.7,8 Also, in photorefractive keratectomy (PRK) surgery the two
Red eye 14%
Dry eye 18%
Overwear 17%
Intolerance to solutions 7%
Intolerance to lenses 17%
Professional advice 5% Advice from friends 1%
Figure 1.2 Main factors that influenced the decision to cease contact lens use and opt for refractive surgery (values relate to the percentage of patients who offered the particular reason as an influential factor)3
eyes are operated on over an interval of around 3 months, and the operated eye does not achieve its final prescription for a few weeks and often is quite blurred during the first week after surgery. So patients who are amblyopic in the non-treated eye may experience some degree of visual disability while the first treated eye reached its final prescription.9,10 Conical corneas, such as keratoconus or keratoglobus, are considered as contraindications to refractive surgery. Both of these conditions have associated thinning at the apex of the conical cornea, which may lead to ectasia after corneal refractive surgery. However, a corneal topography pattern that appears to indicate keratoconus without any other clinical sign of the disease may not necessarily be a contraindication.11,12 An irregular corneal surface, possibly caused by other types of disease or dystrophy such as Fuchs’ endothelial dystrophy, is also considered a reason not to proceed with refractive surgery. However, many corneal surgeons use an excimer laser to perform a phototherapeutic keratectomy (PTK) on patients with conditions such as recurrent corneal erosions or band keratopathy. In this an even layer of stromal tissue is removed to smooth off the irregularities at the anterior stroma, with a wide ablation diameter, without altering the overall corneal curvature and refraction greatly. Patients with known, current viral infections are not suitable for treatment while they have an active disease process. Patients undergoing drug therapy or treatment that may affect their corneal healing should consider refractive surgery only when they have completed their other therapy. Glaucomatous patients may be thought unsuitable for PRK, as they might require the use of corticosteroid drops post-operatively.13–15 Patients with a family history of glaucoma should be made aware that after corneal surgery the measurement of intraocular pressure (IOP) can be affected.16–19 Similarly, pregnancy is considered a contraindication to refractive surgery as there may be subtle changes in refraction during gestation, and also many patients may be wary if drug treatment is indicated after refractive surgery. Inappropriately motivated patients should not be encouraged to have refractive surgery as they may have unrealistic expectations that cannot be met. Motivation for treatment should be assessed carefully preoperatively, and patients should not feel coerced into proceeding. This can be especially difficult, as most refractive surgery takes place in a very commercial environment in which competition, pricing and advertising is often fierce. Nonetheless, it
Patient selection and pre-operative assessment
should be remembered that an unhappy patient is more likely to tell his or her friends about the experience than a happy patient. It would almost be a false economy to treat patients who were unsure about going ahead. Many refractive surgery clinics allow a cooling off period for potential patients between the time of their initial consultation and the actual surgery so that they do not feel pressurized. This tends to be the norm for laser in situ keratomileusis (LASIK) surgery, but opinions vary on this for surface-based laser treatments like PRK and laser epithelial keratomileusis (LASEK; see Chapters 3 and 4 for details of the types of surgery). Patients who are unable to comprehend the rationale of treatment should not be treated, unless for therapeutic reasons. This includes anyone who is unable to give informed consent, such as minors or mentally disadvantaged individuals. When assessed subjectively, it appears as though the majority of patients are satisfied with the outcome of refractive surgery.20, 21 The complications of refractive surgery are mentioned in patient literature and detailed in ophthalmic literature. Patients with realistic expectations are more likely to be successful candidates.22 Often it is asked why patients are willing to undergo refractive surgery knowing the potential risks associated with it and not knowing if there will be any long-term effects that are yet to be uncovered.23 Studies to carry out recognized psychometric personality tests on a group of refractive surgery patients and compare them to a control group, or maybe compare them to patients who present for other types of elective or cosmetic surgery are currently underway.24,25 Is there an underlying trait in some refractive surgery patients that leads them on a compulsive drive for perfection?26 Practitioner’s who evaluate prospective patients for refractive surgery should first assess that the patient is suitably motivated towards undergoing surgery, as highlighted above. It is usually advisable that the patient be armed with some information before attending for consultation. The actual pre-operative assessment routine may differ slightly from clinic to clinic, but the essence of the examination is the same. The individual tests that are usually performed are mentioned below, although this list is not conclusive and some tests can be omitted depending on the type of refractive surgery that the patient is to undergo. Most of these tests, unless indicated, do not require equipment additional to that currently available in the routine ophthalmic practice.
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Visual acuity
Full refraction
It is important to know the patient’s visual acuity (VA) before refractive surgery, as it can be used as a guide to post-operative success, and also to detect amblyopia. Loss of best-corrected visual acuity (BCVA) can occur after excimer laser refractive surgery and can result from one or more of the complications of the procedure mentioned here. Independent loss of BCVA may be attributed to the alteration that occurs in the magnification from the patient’s spectacle lenses. In the case of a moderate hyperope, the patient does not receive the extra magnification, after refractive surgery, in their VA that they previously had as a result of their hyperopic spectacles. Conversely, in refractive surgery for moderate myopia the patient does not have the reducing effect of their spectacle lenses after surgery. This means that the patient shows an improvement in the BCVA or, in the presence of other postoperative problems, the patient does not show a reduction in BCVA.27,28 Most clinicians use Snellen acuity, although better analyzes could be made if Bailey–Lovie charts were used. Often the figures quoted suggest that patients lose or gain lines of BCVA based on Snellen acuity. This may hold true for a Snellen chart, but it is not as accurate as quoting Bailey–Lovie charts (Figure 1.3) in which the lines of letters have equal numbers of letters and an equal rate of change exists between each line of letters.29,30
It is vitally important that an accurate prescription is measured for all prospective patients. A patient whose prescription is too minus will end up with a result that is overcorrected and thus will become hyperopic. A patient with an undetected latent hyperopia will also end up with a result that is hyperopic. This is especially important in presbyopes and pre-presbyopes, as a small hyperopic result will be more detrimental to them than a small myopic result. Cycloplegic refraction is often useful to eliminate any concerns of latent hyperopia or an over-minus of the refraction. It is not unreasonable to assume that some of the hypocorrections and hypercorrections that occur after refractive surgery result from an incorrect pre-operative refraction. The author routinely performs cycloplegia on all potential patients to avoid any refractive surprises.
Figure 1.3 High-contrast (90 per cent) distance Bailey–Lovie chart
Pupil diameter Early excimer laser refractive surgery used smaller diameter ablations of up to 3–4mm, so that the depth of the ablation keratectomy was kept to a minimum. The downside of this was noted in some patients with larger pupils, who found, at night especially, that their pupil would dilate to beyond the treatment zone.31 The result of this was a ghosting around bright objects and lights.32,33 This is very similar to the ghosting that a patient may experience from a decentred corneal contact lens, where the optic zone diameter crosses over the pupil margin. Nowadays, this is less of a problem as most excimer laser refractive surgery uses larger diameter ablations,34 but it still may be an issue in cases for which a small diameter ablation is used (possibly because the patient has a relatively thin cornea; see Corneal pachymetry below). Usually, a central stromal area is ablated with the full refractive correction and a blended zone is ablated around it, similar to the optic zone and carrier portion of a contact lens. This allows the depth of the ablation keratectomy to be kept to a minimum.35,36 However, there remains the problem that this creates substantial spherical aberration in the outer zones of the dilated pupil, so that some degradation of the retinal images occurs.37–39 Pupil diameters are measured either with a ruler under normal lighting conditions or, preferably, using a pupillometer such as the Colvard unit (Oasis Technologies, California, USA) or Keeler pupillometer (Keeler Ltd, Windsor,
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Refractive surgery: a guide to assessment and management
Berkshire, UK) or similar. One clinic in the UK exclusively uses a computerized pupillometer device to measure pupil diameter under different lighting levels.
Corneal topography Most routine optometric practices use a keratometer to assess central corneal curvature for contact-lens fitting. In the preassessment of patients for refractive surgery a keratometer is inadequate since it takes measurements from the central 3–4mm of the anterior cornea only.40 Excimer laser refractive surgery involves removal of corneal tissue by ablation over a wide area. In myopic refractive surgery this tissue is removed from the central corneal area (up to about 7mm), and in hyperopic surgery the tissue is removed from the mid-peripheral cornea (up to about 9mm). The net result of the surgery means there is a change in the anterior corneal profile. It is important to measure the full anterior corneal shape before refractive surgery, to check for any contraindications, such as corneal conditions or dystrophies, and corneal irregularities. All refractive surgery clinics use a corneal topography unit to measure the whole corneal shape to obtain baseline data for the cornea, but also a very flat cornea may prove to be more difficult in flap creation with a microkeratome.31,41,42 Contact lens users who present for refractive surgery are advised to remove their lenses for a period of time before surgery to eliminate warpage
induced by contact lenses. For a patient in whom warpage is observed, the topography measurements are repeated on subsequent visits until no further changes are seen in the topography maps; only then is the patient considered suitable for surgery. Most corneal topography units use Placido disc technology (Figure 1.4), which allows measurements of the anterior surface only. The change in the anterior curvature is dependent upon the amount of initial refractive error.43–47 Recent developments in corneal biometry include slit scan topography machines, which use light slits across the cornea to take a threedimensional image.48 Until very recently, the Orbscan corneal topography system (Figure 1.5), developed by Orbtek, Salt Lake City, Utah (Bausch and Lomb, Rochester, New York, USA), was the only commercially available machine able to assess the posterior corneal shape, but a recent unit by Oculus (Giessen, Germany) uses a rotating Scheimplug camera to take similar measurements. A map is produced by these newer devices that may be more representative of the true corneal shape, with attention given to the posterior surface topography and corneal thickness. This allows a better evaluation of anterior corneal and posterior surface astigmatism, and of residual lenticular astigmatism. More information on corneal topography is presented in Chapter 2. Recent literature shows that there can be an associated change in the posterior corneal curvature, too, which is also related to the amount of treatment.49–51 In the LASIK procedure the microkeratome cuts
Figure 1.4 Eyesys 2000 topography unit, which uses a large Placido disk and is able to give information about the radius of curvature on the anterior corneal surface
approximately one-quarter to one-third into the depth of the cornea to create a flap. Although there are no reported incidences of corneal ectasia after LASIK, there is concern over what happens to the posterior corneal curvature after this procedure, especially in high refractive corrections. It is not unreasonable to assume that an alteration in posterior corneal curvature occurs in LASIK also.52,53
Slit-lamp examination Detailed slit-lamp biomicroscopy examination is important prior to refractive surgery. Contraindications to refractive surgery should be identified, and include anterior corneal scars and opacities, clinical signs of conditions such as keratoconus (e.g., Vogt’s striae and Fleischer’s ring) and lenticular changes.54 A patient with nuclear sclerosis may be deemed unsuitable for excimer laser surgery, but may benefit from clear lens extraction with an appropriately calculated intraocular lens.55 Previous contact-lens complications, such as neovascularization, do not usually contraindicate refractive surgery.
Corneal pachymetry As mentioned above, PRK and LASIK involve the removal of small areas of corneal tissue by ablation with an excimer laser, which results in an alteration of the overall corneal curvature. If a patient has a very thin cornea, then
Figure 1.5 Original Orbscan corneal analysis unit, which uses scanning slit technology. The Orbscan allows the posterior corneal surface curvature and corneal pachymetry to be viewed. Note the acquisition head does not use Placido technology, but contains two scanning slit lights. (Courtesy of Bausch and Lomb)
Patient selection and pre-operative assessment
Ablation depth depends on correction and diameter
175 150
4.0mm
125
4.5mm
100
5.0mm
75
5.5mm
50
6.0mm
25
7.0mm
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
Spherical refractive error (D)
Figure 1.6 Amount of ametropia on the horizontal axis and the estimated laser ablation depth on the vertical axis for different ablation diameters. (Courtesy of Stefan Pieger)
Intraocular pressure measurement Fundus examination To identify abnormal ocular conditions of the fundus, patients should undergo full ophthalmoscopic examination. Some clinicians warrant dilated fundus examination with an indirect ophthalmoscope, such as a Volk lens, in addition to direct ophthalmoscopy. Many patients who elect to undergo refractive surgery are high myopes. In the case of high myopia the likelihood of spontaneous retinal detachment is about 1 per cent. 57,58 After laser refractive surgery the retina is unchanged and the retina is as likely to detach spontaneously as before surgery. However, very often the patient’s lifestyle may change, especially if this was one of the primary motivations for having refractive surgery, and the patient may partake in activities and sports that before were hindered by the use of spectacles. Retinal detachment after laser refractive surgery has been reported and patients should be warned about the risks, in the same way as high myopes would be warned routinely. Authors have quoted incidences of retinal problems after LASIK of between 0.06 and 0.25 per cent of eyes, and of about 0.08 per cent after PRK.59–62 The low incidence of retinal problems after refractive surgery may reflect careful pre-operative assessment of patients to assess potential risks. Some clinics apply prophylactic treatment to patients deemed at risk of later retinal detachment problems.63,64
5
200 Theoretical ablation depth (m)
cutting a flap with a microkeratome may not leave sufficient cornea under the flap to sustain corneal strength. Most surgeons like to leave a bed of at least 250–300μm under the ablated stroma left untouched. Pachymetry is also important for cases in which repeated PRK is warranted for similar reasons. Conditions that lead to areas of corneal thinning, such as keratoconus or pellucid margin degeneration, may also be detected by carefully positioned pachymetry measurements. Corneal thickness is usually measured with an ultrasonic pachymeter using an appropriate anaesthetic, since it is a contact device. The amount of tissue removed during laser refractive procedures depends on the level of ametropia to be corrected and the diameter of the laser ablation. The relationship between diameter and depth of ablation was investigated by Munnerlyn et al., 56 and many clinicians still use their formula to estimate the amount of treatment (Figure 1.6).
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Active glaucoma is a contraindication to refractive surgery, although most refractive surgery clinics do not assess visual fields on all potential patients, unless warranted. IOP is of interest to refractive surgeons as there have been suggestions in the literature of instances of altered IOP readings after PRK. It is thought that the thinner cornea still has the same mechanical forces acting on it and that regular tonometers do not make an allowance for thinner corneas.65–71 Hence, a lower tonometer force may be required to applanate the cornea by the required amount, and so the IOP reading is falsely low. Attempts have been made by some workers to quantify the change in IOP readings with the amount of ablation received by the cornea.16,17,19,72–79 The altered IOP reading is of particular importance if a patient who has undergone refractive surgery develops glaucoma in future years. For this reason other contributing factors towards glaucoma should be noted, such as positive family history, refractive error, age, race and anterior chamber depth.
breakdown of binocularity. A patient who is a moderate-to-high myope has a base-in prismatic effect when performing near tasks with spectacles on. After refractive surgery the patient loses this additional base-in prism and may develop a fixation disparity. This is likely to be more problematic in pre-presbyopic patients, who may find the need for a reading add if their base-in prism for near is removed from their habitual state. Furthermore, in early and pre-presbyopes a change in accommodative demand when moving from spectacles to refractive surgery (or contact lenses) can occur and may be problematic for the myopic patient. As hyperopia increases, the demand on ocular accommodation increases. Hence, as the spectacle refraction is moved towards the ocular
Muscle balance Although not essential, it can be useful to check the muscle-balance status of patients. A post-surgical problem, which may only be theoretical, since it has not been described in the literature, is the
Figure 1.7 Pelli–Robson CSF chart
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Refractive surgery: a guide to assessment and management
plane the hyperope benefits from the lower demand on accommodative effort, whereas the myopic patient places a higher demand on the accommodative effort.80
Contrast sensitivity function Reduced contrast sensitivity function (CSF) has been described after refractive surgery, and hence its measurement with a suitable test, such as a Pelli–Robson chart (Figure 1.7), is useful.81 In PRK patients this may be the result of corneal haze. Haze is thought to be an immune response of the stroma and forms precisely at the level of the site of laser ablation (i.e., the epithelial–stromal interface). To combat haze, some surgeons use corticosteroids prophylactically with all patients, some use them only if haze is beginning to appear and others prefer to use them with patients who are deemed to be more likely to develop haze, such as patients with higher refractive errors.14,15 It has been suggested that newly synthesized cells cause haze, and an aggregation of keratocytes may play a part in the aetiology of
References 1 Conway R (1994). PRK counselling in the optometric practice. Optician 208, 32–34. 2 Orr D, Sidiki SS and McGhee CNJ (1998). Factors that influence patient choice of an excimer laser treatment center. J Cataract Refract Surg. 24, 335–340. 3 Naroo SA, Shah S and Kapoor R (1999). Factors that influence patient choice of contact lens or photorefractive keratectomy. J Refract Surg. 15, 132–136. 4 Tan DT and Tan JT (1993). Will patients with contact lens problems accept excimer laser photorefractive keratectomy? CLAO J. 19, 174–177. 5 Whittaker G (1996). Are contact lensassociated problems a primary motivation factor for PRK patients? J Br Contact Lens Assoc. 19, 21–23. 6 Simensen B and Thorud LO (1994). Adultonset myopia and occupation. Acta Ophthalmol. 72, 469–471. 7 Holland SP, Srivannaboon S and Reinstein DZ (2000). Avoiding serious corneal complications of laser assisted in situ keratomileusis and photorefractive keratectomy. Ophthalmology 107, 640–652. 8 Hill VE, Brownstein S, Jackson WB and Mintsioulis G (1998). Infectious keratopathy complicating photorefractive keratectomy. Arch Ophthalmol. 116, 1382–1384. 9 Price FW Jr, Belin MW, Nordan LT, McDonnell PJ and Pop M (1999). Epithelial haze, punctate keratopathy, and induced hyperopia after photorefractive keratectomy for myopia. J Refract Surg. 15, 384–387.
Supplementary tests
haze. The new stromal tissue deposited is not laid in a regular pattern, which leads to a reticular pattern of fibres. Studies have shown that severe haze is more likely with patients who have high refractive corrections, since the ablation depth is deeper. Lasers that use scanning micro-beam technology appear to produce less haze than older broad-beam lasers,82–85 but this may be partly because these newer lasers make a central optic zone and a peripheral blended zone.35,36,86 Thus, the actual change in contour profile of the corneal shape is less severe. Another factor may be the laser beam itself. If the beam is able to produce a smoother ablation, the newly synthesized cells may be able to form a more regular pattern of fibres. Haze does not appear to form on eyes that have undergone LASIK, which suggests that when the flap is replaced some smoothing of the underlying tissue occurs, although altered CSF has been described after LASIK.29,87 Reduced CSF may occur in some older patients with early lens-ageing changes, in which case laser refractive surgery may be contraindicated and lens exchange may be warranted.
Altered tear secretion has been reported after LASIK,88,89 and it is useful to assess tear-film quality and measure tear breakup time. Appropriate instruments, such as the Keeler Tearscope (Keeler Ltd, Windsor, Berkshire, UK) could be useful in identifying patients with potentially low tear volumes or break-up times. This may be important for patients who undergo PRK, for whom an incidence of recurrent erosions of about 3 per cent is quoted.90–92 Many patients find ocular lubricants useful for a period of time after corneal laser refractive surgery (corneal wound healing after these types of surgery is discussed in Chapter 3). There have been reports in the literature of changes in corneal sensation after PRK and LASIK, although most authors suggest the corneal sensation is usually at its pre-operative level within a year, or sooner.93–95 However, it is not common to take aesthesiometry measurements before refractive surgery using devices such as the Cochet–Bonnet aesthesiometer mounted on a slit lamp.
10 Whittaker G (1994). Post-treatment follow-up for the PRK patient. Optician 208, 20–26. 11 Bilgihan K, Ozdek SC, Konuk O, Akata F and Hasanreisoglu B (2000). Results of photorefractive keratectomy in keratoconus suspects at 4 years. J Refract Surg. 16, 438–443. 12 Doyle SJ, Hynes E, Naroo SA and Shah S (1996). PRK in patients with a keratoconic topography picture: The concept of a physiological displaced apex syndrome. Br J Ophthalmol. 80, 25–28. 13 O’Brart DP, Lohmann CP, Klonos G, et al. (1994). The effects of topical corticosteroids and plasmin inhibitors on refractive outcome, haze, and visual performance after photorefractive keratectomy. A prospective, randomized, observer-masked study. Ophthalmology 101, 1565–1574. 14 Corbett MC, O’Brart DP and Marshall J (1995). Do topical corticosteroids have a role following excimer laser photorefractive keratectomy? J Refract Surg. 11, 380–387. 15 Gartry DS, Kerr-Muir M, Lohmann CP and Marshall J (1992). The effect of corticosteroids on refractive outcome and corneal haze after photorefractive keratectomy: A prospective, randomized, double-blind trial. Arch Ophthalmol. 110, 944–952. 16 Rao SK, Ratra V and Padmanabhan P (1999). How and where should intraocular pressure be measured after photorefractive keratectomy? J Cataract Refract Surg. 25, 1558–1560. 17 Chatterjee A, Shah S, Bessant DAR, Naroo SA and Doyle SJ (1997). Reduction in
intraocular pressure after excimer laser photorefractive keratectomy: Correlation with pre-treatment myopia. Ophthalmology 104, 355–359. 18 Patel S and Aslanides IM (1996). Main causes of reduced intraocular pressure after excimer laser photorefractive keratectomy. J Refract Surg. 12, 673–674. 19 Mardelli PG, Piebenga LW, Whitacre MM and Siegmund KD (1998). The effect of excimer laser photorefractive keratectomy on intraocular pressure measurements using the Goldmann applanation tonometer. Ophthalmology 104, 945–949. 20 McGhee CN, Craig JP, Sachdev N, Weed KH and Brown AD (2000). Functional, psychological, and satisfaction outcomes of laser in situ keratomileusis for high myopia. J Refract Surg. 26, 497–509. 21 McGhee CN, Orr D, Kidd B, Stark C, Bryce IG and Anastar CN (1996). Psychological aspects of excimer laser surgery for myopia. Reasons for seeking treatment and patient satisfaction. Br J Ophthalmol. 80, 874–879. 22 McGhee C, Sachdev N and Craig J (1999). Photorefractive surgery – assessing patient satisfaction. Optician 218, 27–30. 23 Kahle G, Seiler T and Wollensak J (1992). Report on psychosocial findings and satisfaction amongst patients 1 year after excimer laser photorefractive keratectomy. Refract Corneal Surg. 8, 286–289. 24 West SG and Finch JF (1997). Personality measurement: Reliability and validity issues. In: Handbook of Personality Psychology, p. 143–164, Eds Hogan R,
Patient selection and pre-operative assessment Johnson J and Briggs S (San Diego: Academic Press). 25 Young FA, Singer RM and Foster D (1975). The psychological differentiation of male myopes and non-myopes. Am J Optom Physiol Opt. 52, 679–686. 26 Serano N (2000). Operation overkill. Elle 16, 250–254. 27 Applegate RA and Chundru U (1995). Experimental verification of computational methods to calculate magnification in refractive surgery. Arch Ophthalmol. 113, 571–577. 28 Applegate RA and Howland HC (1993). Magnification and visual acuity in refractive surgery. Arch Ophthalmol. 111, 1335–1342. 29 Moniz N, Fernandes T, Narayanan KK and Sreedhar A (2000). Visual outcome in high myopia after laser in situ keratomileusis. J Refract Surg. 16, S247–S250. 30 Black H (1997). Low contrast tests may more accurately determine visual acuity post-PRK. Ocul Surg News 8, 53. 31 Maloney RK (1990). Corneal topography and optical zone location in photorefractive keratectomy. Refract Corneal Surg. 6, 363–371. 32 Lohmann CP, Fitzke F, O’Brart D, KerrMuir M, Timberlake G and Marshall J (1993). Corneal light scattering and visual performance in myopic individuals with spectacles, contact lenses, or excimer laser photorefractive keratectomy. Am J Ophthalmol. 115, 444–453. 33 Lohmann CP, Fitzke FW, O’Brart D, Kerr Muir MG and Marshall J (1993). Halos – a problem for all myopes? A comparison between spectacles, contact lenses and photorefractive keratectomy. Refract Corneal Surg. 9, S72–S75. 34 Martinez CE, Applegate RA, Klyce SD, McDonald MB, Medina JP and Howland HC (1998). Effect of pupillary dilation on corneal optical aberrations after photorefractive keratectomy. Arch Ophthalmol. 116, 1053–1062. 35 Pop M and Aras M (1995). Multizone/multipass photorefractive keratectomy: Six month results. J Refract Surg. 21, 633–643. 36 Pop M and Payette Y (1999). Multipass versus single pass photorefractive keratectomy for high myopia using a scanning laser. J Refract Surg. 15, 444–450. 37 Applegate RA and Gansel KA (1990). The importance of pupil size in optical quality measurements following radial keratotomy. Refract Corneal Surg. 6, 47–54. 38 Fay AM, Trokel SL and Myers J (1992). Pupil diameter and the principal ray. J Cataract Refract Surg. 18, 348–351. 39 Maeda N, Klyce SD, Smolek MK and McDonald MB (1997). Disparity between keratometry-style readings and corneal power within the pupil after refractive surgery for myopia. Cornea 16, 517–524. 40 Bennett AG and Rabbetts RB (1989). Measurements of ocular dimensions. In: Clinical Visual Optics, Second Edition, p. 457–483, Eds Bennett AG and Rabbetts RB (London: Butterworths). 41 Hersh PS, Scher KS and Irani R (1998). Corneal topography of photorefractive keratectomy versus laser in situ keratomileusis. Ophthalmology 106, 612–619.
42 Schallhorn SC, Reid JL, Kaupp SE, et al. (1998). Topographic detection of photorefractive keratectomy. Ophthalmology 105, 507–516. 43 Dutt S, Steinert RF, Raizman MB and Puliafito CA (1994). One-year results of excimer laser photorefractive keratectomy for low to moderate myopia. Arch Ophthalmol. 112, 1427–1436. 44 Maldonado A and Onnis R (1998). Results of laser in situ keratomileusis in different degrees of myopia. Ophthalmology 105, 606–611. 45 Maguen E, Salz JJ, Nesburn AB, et al. (1994). Results of excimer laser photorefractive keratectomy for the correction of myopia. Ophthalmology 101, 1548–1555. 46 Lindstrom RL, Lineberger EJ, Hardten DR, Houtman DM and Samuelson TW (2000). Early results of hyperopic and astigmatic laser in situ keratomileusis in eyes with secondary hyperopia. Ophthalmology 107, 1858–1863. 47 Salz JJ, Maguen E, Nesburn AB, et al. (1993). A two-year experience with excimer laser photorefractive keratectomy for myopia. Ophthalmology 100, 873–882. 48 Dave T (1998). Current developments in measurements of corneal topography. Contact Lens Anterior Eye 21, S13–S30. 49 Shimmura S, Yang HY, Miyajima HB, Shimazaki J and Tsubota K (1997). Posterior corneal protrusion after PRK. Cornea 16, 686–688. 50 Kamiya K, Oshika T, Amano S, Takahashi T, Tokunaga T and Miyata K (2000). Influence of excimer laser photorefractive keratectomy on posterior corneal surface. J Cataract Refract Surg. 26, 867–871. 51 Naroo SA and Charman WN (2000). Changes in posterior corneal curvature after photorefractive keratectomy. J Cataract Refract Surg. 26, 872–878. 52 Wang Z, Chen J and Yang B (1999). Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness. Ophthalmology 106, 406–409. 53 Maloney RK (1999). Discussion of article by Wang Z, Chen J, Yang B. Ophthalmology 106, 409–410. 54 Lawless M, Coster DJ, Phillips AJ and Loane M. Keratoconus: Diagnosis and management. ANZ J Ophthalmol. 17, 33–45. 55 Colin J and Robinet A (1997). Clear lensectomy and implantation of a lowpower posterior chamber intraocular lens for correction of high myopia: A four-year follow-up. Ophthalmology 104, 73–78. 56 Munnerlyn CR, Koons SJ and Marshall J (1988). Photorefractive keratectomy: A technique for laser refractive surgery. J Cataract Refract Surg. 14, 46–52. 57 Ho P and Tolentino F (1984). Pseudophakic retinal detachment: Surgical success rate with various types of IOLs. Ophthalmology 91, 847–852. 58 Colin J, Robinet A and Cochener B (1999). Retinal detachment after clear lens extraction of high myopia. Ophthalmology 106, 2281–2285. 59 Arevelo JF, Ramirez E, Suarez E, et al. (2000). Incidence of vitreoretinal pathologic conditions within 24 months
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after laser in situ keratomileusis. Ophthalmology 107, 258–262. 60 Mansour AM and Ojeimi GK (2000). Premacular subhyaloid hemorrhage following laser in situ keratomileusis. J Refract Surg. 16, 371–372. 61 Ruiz-Moreno JM, Artola A and Alio JL (2000). Retinal detachment in myopic eyes after photorefractive keratectomy. J Cataract Refract Surg. 26, 340–344. 62 Ruiz-Moreno JM, Perez-Santonja JJ and Alio JL (1999). Retinal detachment in myopic eyes after laser in situ keratomileusis. Am J Ophthalmol. 128, 588–594. 63 Charteris D, Cooling R, Lavin M and McLeod D (1997). Retinal detachment following excimer laser. Br J Ophthalmol. 81, 759–761. 64 Farah ME, Hofling-Lima AL and Nascimento E (2000). Early rhegmatogenous retinal detachment following laser in situ keratomileusis for high myopia. J Refract Surg. 16, 739–743. 65 Dohadwala AA, Munger R and Damji KF (1998). Positive correlation between TonoPen intraocular pressure and central corneal thickness. Ophthalmology 105, 1849–1854. 66 Doughty MJ and Zaman ML (2000). Human intraocular thickness and its impact on intraocular pressure measurement: A review and metaanalysis approach. Surv Ophthalmol. 44, 367–408. 67 Ehlers N, Bramsen T and Sperling S (1975). Applanation tonometry and central corneal thickness. Acta Ophthalmol. 53, 652–659. 68 Foster PJ, Baasanhu J, Alsbirk PH, et al. (1998). Central corneal thickness and intraocular pressure in a Mongolian population. Ophthalmology 105, 969–973. 69 Mark HH (1973). Corneal curvature in applanation tonometry. Am J Ophthalmol. 76, 223–224. 70 Mills RP (2000). If intraocular pressure measurement is only an estimate – then what? Ophthalmology 107, 1807–1808. 71 Shah S (2000). Accurate intraocular pressure measurement – the myth of modern ophthalmology. Ophthalmology 107, 1805–1807. 72 Cennamo G, Rosa N, La Rana A, Bianco S and Adolfi S (1997). Non-contact tonometry in patients that underwent photorefractive keratectomy. Ophthalmologica 211, 341–343. 73 Cho P and Liu T (1998). Comparison of the performance of the Nidek NT-2000 non-contact tonometer with the Keeler Pulsair 2000 and the Goldman applanation tonometer. Optom Today 38, 28–36. 74 Damji KF and Munger R (1997). Reduction of IOP after PRK: Letter to the Editor. Ophthalmology 104, 1525–1526. 75 Garcia J and Sherry R (1997). Reduction in IOP after PRK. Ophthalmology 104, 1526–1527. 76 Montes-Mico R and Charman WN (2001). Intraocular pressure after excimer laser myopic refractive surgery. Ophthalmic Physiol Opt. 21, 228–235. 77 Patel S and McLaughlin JM (1999). Effects of central corneal thickness on
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measurements of intraocular pressure in keratoconus and post-keratoplasty. Ophthalmic Physiol Opt. 19, 236–241. 78 Rosa N, Cennamo G, Breve A and La Rana A (1998). Goldmann applanation tonometry after myopic photorefractive keratectomy. Acta Ophthalmol. 76, 550–554. 79 Tuunanen TH, Hamalainen P, Mali M, Oksala O and Tervo T (1996). Effect of photorefractive keratectomy on the accuracy of pneumatonometer readings in rabbits. Invest Ophthalmol Vis Sci. 37, 1810–1814. 80 Whittaker G (1994). Pre-assessment of prospective PRK patients by optometrists. Optician 208, 28–31. 81 Verdon W, Bullimore M and Maloney RK (1996). Visual performance after photorefractive keratectomy. Arch Ophthalmol. 114, 1465–1472. 82 Caubet E (1993). Cause of subepithelial corneal haze over 18 months after photorefractive keratectomy for myopia. Refract Corneal Surg. 9, S65–S70. 83 Corbett MC, Prydall JI, Verma S, Oliver KM, Pande M and Marshall J (1996). An in vivo investigation of the structures responsible for corneal haze after
photorefractive keratectomy and their effect on visual function. Ophthalmology 103, 1366–1380. 84 Lohmann CP, Gartry D, Kerr-Muir M, Timberlake G, Fitzke F and Marshall J (1991). ‘Haze’ in photorefractive keratectomy: Its origins and consequences. Laser Light Ophthalmol. 4, 15–34. 85 Lohmann CP, Gartry D, Kerr-Muir M, Timberlake G, Fitzke F and Marshall J (1991). Corneal haze after excimer laser refractive surgery: Objective measurements and functional implications. Eur J Ophthalmol. 1, 173–180. 86 Piovella M, Camesasca FI and Fattori C (1997). Excimer laser photorefractive keratectomy for high myopia: Four year experience with a multiple zone technique. Ophthalmology 104, 1554–1565. 87 Mutyala S, McDonald, Scheinblum KA, Ostrik MD, Brint SF and Thompson H (2000). Contrast sensitivity evaluation after laser in situ keratomileusis. Ophthalmology 107, 1864–1867. 88 Aras C, Ozdamar A, Bahcecioglu H, Karacorlu M, Sener B and Ozkan S (2000). Decreased tear secretion after laser in situ keratomileusis for high myopia. J Refract Surg. 16, 362–364.
89 Patel S, Perez-Santoja JJ, Alio JL and Murphy PJ (2001). Corneal sensitivity and some properties of the tear film after laser in situ keratomileusis. J Refract Surg. 17, 17–24. 90 Loewenstain A, Lipshitz I, Varssano D and Lazar M (1997). Complications of excimer laser photorefractive keratectomy for myopia. J Cataract Refract Surg. 23, 1174–1176. 91 Seiler T, Holschbach A, Derse M, Jean B and Genth U (1994). Complications of myopic photorefractive keratectomy with the excimer laser. Ophthalmology 101, 153–160. 92 Stevens JD and Steele ADM (1993). Indications, results and complications of refractive corneal surgery with lasers. Curr Opin Ophthalmol. 4, 91–98. 93 Murphy PJ, Corbett MC, O’Brart DPS, Verma S, Patel S and Marshall J (1999). Loss and recovery of corneal sensitivity following photorefractive keratectomy for myopia. J Refract Surg. 15, 38–45. 94 Chuck RS, Quiros PA, Perez AC and McDonnell PJ (2000). Corneal sensation after laser in situ keratomileusis. J Cataract Refract Surg. 26, 337–339. 95 Sun R and Gimbel HV (1997). Effects of topical ketorolac and diclofenac on normal corneal sensation. J Refract Surg. 13, 158–161.
2 Corneal topography and its role in refractive surgery Shehzad A Naroo and Alejandro Cervino
The cornea plays a fundamental role in both the structural integrity and the refractive state of the eye. Thus, both the determination and representation of its shape are important for refractive and surgical purposes, as well as in the diagnosis and evolution of several pathologies that express corneal shape alterations, such as keratoconus, marginal degeneration and other ectasias. The adult cornea is characterized by its specific distributions of curvature and thickness along the different meridians, distributions that are essential for the correct function of the cornea as the most important and powerful refractive element of the human eye.
but it served as a base for the development of the keratometers. In 1880 Antonio Placido introduced a flat disc with a series of concentric black and white rings, with the corneal reflections of the rings examined through a central aperture. It is illuminated from a light source above or beside the patient’s head. The Placido disc, as it became known, must be held normal to the line of sight or it will give a false impression of the toricity of the cornea. Gullstrand (1896) was the first to photograph the corneal image formed with the Placido disc.1,2
Classification of corneal topography History Early interest in corneal topography dates back to Father Christopher Scheiner, who in 1619 compared corneal images to marbles. Using daylight he viewed the image formed when daylight shone through the cross-shaped glazing frame bars of his windows onto corneas and compared the images formed to those formed on marbles of a known size. Senff introduced the first concepts about human corneal topography in 1846, reporting that the anterior corneal surface flattens towards the limbus and compared the anterior surface of the cornea with a revolution ellipsoid. Henry Goode (1847) described the first keratoscope, which comprised a small luminous square held near to the eye. Helmholtz (1853) invented the ophthalmometer, and introduced the first doubling image system to avoid the problems caused by the continuous micro-movements of the eye that existed until then. This ophthalmometer was difficult to use,
Since the early investigations of Javal and Helmholtz, a basic model of corneal topography has been established that uses the ellipse as a first-order approximation to the normal corneal profile. This classic model of the corneal contour corresponds to a surface with two zones, a central spherical zone of 4–5mm diameter and a peripheral zone that flattens towards the limbus. The central zone is responsible for the foveal image formation, and within this area of the cornea the changes in curvature are small, so often uniformity is assumed. However, it has been demonstrated by Bennett that this is not actually correct,3 but rather each point on the cornea is conical (as mentioned below). The anatomic centre of the apical zone rarely corresponds to the visual centre or the geometric centre, although most instruments assume this to be true. The position of the apex is independent of the geometric centre and is usually located 0.5mm on the temporal side with respect
to the geometric centre.4 Other classifications have also been developed, such as that of Rowsey and co-workers who considered the quantity and symmetry of peripheral flattening,5 and classified the corneas into essentially four types: • Type A: paracentral zone is symmetrical (nasal–temporal difference less than 0.2mm), peripheral zone is symmetrical and the difference in flattening between the paracentral and peripheral zones is less than 0.2mm. • Type B: paracentral zone is symmetrical, as is the peripheral zone, but the difference in flattening between both zones is more than 0.2mm. • Type C: paracentral zone has a trace asymmetry (about 0.2mm), the peripheral zone is symmetrical and the difference in flattening between them is less than 0.2mm. • Type D: paracentral zone has a nasal–temporal asymmetry and the peripheral zone is symmetrical, but the difference in flattening between the paracentral and peripheral zones is greater than 0.2mm. The classification of the cornea into anatomic zones is considered inappropriate by several authors, because the cornea is a smooth surface, the curvature of which is submitted constantly to subtle changes,3,6,7 which suggests that at any individual point the cornea is conical and represented by Equation (2.1) y2 = 2ro – px2 (2.1) where p is the shape factor of the cornea (see below) and ro is the central radius of curvature of the cornea. However, in the central 3–4mm the changes are small, as mentioned above, and hence some level of uniformity is often assumed. The anterior peripheral cornea
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flattens with respect to the central curvature, a pattern mimicked by the posterior corneal curvature. The rate of flattening may be different along different meridians. The corneal asphericity is described in mathematical terms as being a prolate shape or a flattening ellipse. This shape of the cornea partially compensates the spherical aberration of the eye and improves the quality of the retinal image. The technical requirements for a correct and reliable measurement of corneal topography were established by Bibby:8 • The units used to describe the corneal topography must not depend on the method of obtaining the values. • The instrument must measure the total area of interest. • All the information must be acquired simultaneously. • The technique employed must be precise and reproducible. Following these requirements, his work suggested mean values for the corneal shape of 0.85 ± 0.18 in 2100 eyes and, later, of 0.79 ± 0.15 in 32,000 eyes. In the 20th century, the growth of the field of contact lenses, and later of refractive surgery, led to an increased interest in corneal topography. This, along with the parallel development of the computer technologies, resulted in great advances in corneal topographical analysis. Various workers have helped to develop better designs of photokeratoscopic systems and better graphic presentation and analysis of the data. Colour-coded topography maps were introduced by Klyce and later developed further by Maguire.9,10
Corneal shape Evaluation of the corneal shape is of great importance in the monitoring and followup of corneal pathologies, contact lens fitting and refractive surgery, and in the evaluation of sequential temporal changes induced by contact lens wear, refractive surgery or orthokeratology. However, the description of the cornea may not be the same for a contact lens fit as for refractive surgery purposes, for example. Mandell described the cornea in three ways, according to the viewpoint required:11 • From a qualitative point of view, several corneal zones are considered: the central, paracentral, peripheral and limbal. Also, a division into optic and peripheral zones can be made for practical purposes, in which the central optic zone, with an almost constant curvature, is surrounded by a peripheral zone with a radius that progres-
Table 2.1 Corneal descriptors and their mathematical relations Mathematical description
Shape factor (p)
Hyperbola shape
p<0
Parabola shape
p=0
Prolate shape (flattening ellipse)
0
Q<0
Circle
p=1
Q=0
Oblate shape (steepening ellipse)
p>1
Q>0
sively increases. Near the apex the degree of change in the radius of curvature is very low, but it increases quickly towards the periphery. To establish the size of the central zone a 1D change criterion is usually accepted or, in other words, the area of the corneal surface at which the dioptric powers differ by less than 1D. In most cases this is a 4mm diameter central portion of the cornea. • From a mathematical point of view, a simple mathematical expression is used to define the cornea as an ellipse or polynomial expression. In most normal corneas, the central zone is more curved than the peripheral zone, which means it has the form of a section of a prolate ellipse (with a positive shape factor and an asphericity in the range 0 to 1). There are studies that draw the conclusion that the different refractive groups have similar corneal eccentricity values, but different values for the apical radius.12 • The third way to describe the corneal surface, as Mandell reports,11 is point by point, which consists simply of a collection of values for the corneal radius of curvature or power found at different positions on the cornea. If all the adjacent numbers with the same value are connected in a contour map, the result obtained is transformed into an easily comprehensible pattern that gives a global image of the particular corneal shape. A series of descriptors of the corneal shape has been defined to unify the different criteria of the range of normal corneal shapes. Also, some investigators have defined a number of corneal indexes to give a better understanding of corneal topography and its variations. The commonly used descriptors of corneal shape
Asphericity (Q) Corneal example
Normal corneal shape
Post-myopic laser surgery or postradial keratotomy
are eccentricity (e), shape factor (p) and asphericity (Q). These indices are related by simple mathematical equations: p = 1 – e2 (2.2) Q = –e2 (2.3) and Q=p–1 (2.4) Table 2.1 shows corneal descriptors and their mathematical relations. Some of the information collected by corneal topography is used to describe the corneal shape in easy-to-understand terms, which can both aid interpretation of the data and decipher the colour maps. A few examples of these are given below, although many corneal topography units have their own individual indices. Corneal uniformity index or surface regularity index The corneal uniformity index (CUI) or surface regularity index (SRI) represents the smoothness of the surface, a relation of the change in local corneal radius of curvature or corneal power over a determined area. It is evaluated from the frequency distribution of powers along the different meridians. This index is sometimes used to give a value to the visual acuity (VA), based on the assumption that the cornea is the only limiting factor in the patient’s VA; this is called the predicted corneal acuity (PCA). Simulated keratometry readings Simulated keratometry (SimK) readings are calculated using the steepest meridian from the central area along every meridian (SimK1), and the power and axis of the meridian orthogonal to the steepest (SimK2). These readings are useful substitutes of traditional keratometric measurements and have been reported as useful in the calculation of intraocular lens power.13 They are often taken as the steepest and flattest profiles, although if they
Corneal topography and its role in refractive surgery
are calculated perpendicular to each other this may not be exactly correct. Elevation is a relative measurement of corneal topography, and is described as the elevation difference with respect to a plane or to a flatter or steeper surface. Elevation may also be taken in relation to a reference sphere, which may be a floating sphere (i.e., related only to that cornea) or a fixed sphere (used to calculate the elevation of all corneas with that machine). Usually a floating sphere is used, and its radius of curvature is the mean radii for that cornea; all other data points are related to this reference sphere, which is termed the best-fit sphere (BFS). Figure 2.1 shows a Holliday diagnostic summary (HDS) from the EyeSys 2000 machine.
Corneal topography measurement methods Currently, numerous evaluation methods for corneal topography exist, some more precise than the others, but basically all are developments of the same fundamental theme. The idea is to make a three-dimensional reconstruction of the corneal surface, but difficulties arise when trying to represent a three-dimensional shape using a two-dimensional image. To do this some assumptions and simplifications are made:
•
The working distance from the object point to the image is constant. • The instrument axis is perpendicular to the corneal surface. • The light from the object is reflected in the same meridian onto the plane on which the image is created (i.e., it is assumed that no circular inclination of the corneal surface occurs). • The position of the image on the plane is unique for a determined surface. • The image point is on a non-curved plane. • The refractive index of the cornea is the same for all individuals and remains constant in a particular patient. Image capture in corneal topography can be divided into two basic types: • Reflection techniques: the cornea works as a curved mirror and the reflected image is viewed directly or captured and analyzed. Examples of this technique are keratometry, keratoscopy and videokeratoscopy. • Projection techniques: in this group of techniques, the cornea acts as a projection screen. An example of this is rasterstereography, which is used successfully in other areas of medicine such as spinal curvature measurement. Another technique is that of interferometry.
Figure 2.1 Holliday diagnostic summary (HDS) from the EyeSys 2000 machine. The box underneath the four maps shows some corneal parameters for this cornea measured over 3mm, except for the asphericity value (Q), which is measured over an area of 4.5mm. Note that the steepest and flattest refractive profiles (column 1) are not the same axes as the SimK axes (column 2)
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Using the cornea as a reflector system Nearly all optometry practices have a keratometer. The main function of the keratometer is to measure the radius of curvature of the central portion of the front surface of the cornea.14 This result is usually obtained indirectly by measuring the angular size of the reflected image, formed by the cornea, of an object of known angular size; this is the first Purkinje image.6 In most instruments, this is an object with a linear size that is fixed or measurable at a predetermined distance from the image plane. Since it would be difficult to read off the reflected image height from the cornea, because of involuntary eye movements, the principle of doubling is used in keratometers. The image size is measured by lateral displacement of a doubled image (doubling may be achieved using a series of lenses, mirrors or, more commonly, prisms). In most keratometers, doubling takes place in one meridian only, along the line that joins the mires. Such an instrument must be rotated about its axis to align it with each of the principal meridians of the cornea in turn, and is therefore known as a two-position keratometer, such as a Javal–Schiötz type. A one-position keratometer (such as the Bausch and Lomb type, see Figures 2.2 and 2.3) is an instrument in which variable doubling of mutually perpendicular pairs of mires is produced by two doubling devices in the corresponding meridians. The instrument is rotated about its axis to align the mires with both principal meridians of the cornea, and the images in each can then be brought into contact without further rotation.6 The primary use of the keratometer in contact lens practice is to measure the central radius of the cornea to determine the back optic zone radius of a contact lens
Figure 2.2 Bausch and Lomb style keratometer
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a
b
Figure 2.3 Reflection of the mires from a Bausch and Lomb style keratometer. (a) The mires incorrectly aligned. (b) The mires correctly aligned when the keratometry readings are taken
that will produce the best fit. It is also used to check the radii of a corneal lens and to assess the fit of soft contact lenses. Changes in central corneal shape can also be detected with the keratometer, both quantitatively and qualitatively (by assessing the regularity of the mires), and in this capacity it is often used by clinicians to assist in the diagnosis of keratoconus.14 Keratometry has a number of inherent problems. The one-position keratometer described above assumes that the two principal meridians of the cornea are perpendicular. All keratometers measure corneal radius with pencils of light reflected by small areas, each situated not less than 1mm and up to about 1.7mm from the centre. The keratometer does not allow for decentration of the corneal apex or for corneal asphericity. The main source of error is focusing. If the mire images formed by the object are not focused accurately in the intended primary image plane, the radius measurement will be incorrect, since the object–image separation is then incorrect, and the unfocused mire images have a different separation from the sharply focused ones.15 These blurred images may not appear to be so if compensated for by the observer’s accommodation and his or her own uncorrected ametropia (especially astigmatism). Also, local distortion of the cornea in the region of the reflection area causes a corresponding distortion of the mire and renders focusing of the instrument uncertain.6,16 Adaptations of keratometers have been used to assess the peripheral corneal shape. New keratometers have been designed and modifications made to older designs. A modified Bausch and Lomb keratometer with the mire separation reduced from 64mm to 26mm and a series of offaxis fixation points was able to take measurements of the corneal periphery.11 Bennett describes a keratometer based on the Drysdale effect and used to measure the central and peripheral cornea.3
Videophotokeratoscopy Modern corneal topography devices are an accumulation of techniques learned from the historical methods of keratoscopy and photokeratoscopy, described above. Highresolution video cameras record the reflection of the Placido disc mires from the patient’s cornea. Once the patient is aligned in front of the videophotokeratoscope, with the chin on the rest, the images are captured. The system is aligned when the tracking lights of the two superimposed laser beams reflected by the cornea are placed in the centre of the cross-hair target located in a box displayed on the monitor screen. The reflected image of the mires is recorded on a close-circuit video camera and analyzed by computer software to yield a representation of the corneal contour. Examples of different machines The machines mentioned here are just a few of the many types of topography units currently available. This is not intended to be an exhaustive list, but merely a representation of the variety of designs around. The two most widely used computer-assisted videophotokeratography systems are the EyeSys Corneal Analysis System (by EyeSys Technologies) and the Topographic Modelling System (TMS, by Tomey instruments), and these represent the two extremes of design. The EyeSys machines have a Placido disc and a longer working distance than their Tomey rivals, which have a Placido cone. A larger working distance makes the instrument less sensitive to small displacements of the eye, but has the disadvantage that the instrument is less compact. The cone systems use a shortened working distance, and the size of the cone means they are able to move closer to the eye, which allows a larger corneal coverage. The TMS-1 uses a 25-ring Placido cone, with a total of 6400 data points, and utilizes a short working distance of approximately 70mm. The EyeSys-1 uses 16 rings, each
giving rise to 360 data points, a total of 5760 data points. The latest EyeSys topography unit, EyeSys-2000, uses 18 rings, but maintains a longer working distance. It still collects data from 360 points along each ring, to give a total of 6480 data points. The TMS-2, like its predecessor, uses a Placido cone, but increases the number of rings to 34, while maintaining 256 data points per ring over a maximum corneal diameter of 11.5mm. The latest offering from Tomey, the TMS-3, boasts an impressive automated image-capture system. It has 31 rings with 256 data points per ring, to give a total of 7936 data points. The automated image capture of the TMS-3 leads to a small sacrifice in corneal coverage, and offers up to a maximum diameter of 9.5mm.17,18 As corneal topography has become less of a research tool and more clinically widespread, the number of models available has increased. All use either the Placido cone system, as in the TMS units, or the Placido disc system, as in the EyeSys units. Most use a working distance that is between the two extremes of these two units. Haag-Streit and Oculus both offer the same unit, but packaged slightly differently. This machine has 22 rings on a Placido disc and offers 10,000 data points, which is currently the most of any topography unit; this device also has a very accurate collimating measurement for more accurate SimK readings. While a greater number of sampling points, in principle, allows the topography to be assessed in more detail, the validity of the device’s data depends on the algorithms used and, as yet, few comparative studies have been made on the performance of different units. Topcon offer a novel system, the KR7000P, which is a combined autorefractor–autokeratometer–topographer. As well as providing automated refraction and central corneal curvature readings along the two principal meridians, it gives the topography over a corneal diameter of 7mm, but it only offers 2600 data points in total. This unit can be used as a standalone machine with a built-in printer or can be linked to a desktop computer. Once a patient is aligned on the topography machine and the cornea is in focus the actual image capture (automated or manual) is very quick, typically 33 milliseconds, as with the TMS-3 unit. Each data collection point measures the curvature at that reflected point on the cornea and all the data points are represented on a colour map display. Presenting topography data Two scales are commonly used to display topographic features; the absolute (also called standard scale) and the normalized
Corneal topography and its role in refractive surgery
(also called the relative scale or autoscale). The absolute scale generates a colour-coded map with 1.0D increments on a pre-set scale, usually between 37D and 51D, and thus allows comparison of different corneas and different machines. The normalized scale uses smaller increments to span the range of dioptric powers of an individual cornea, and thus the same colour may not represent the same numerical value on different corneal pictures. The normalized scale is created by assigning the red range of colours to the steepest curvature of the cornea being examined, and the blue range of colours to the flattest curvature. The remaining colours are divided into equal step sizes and assigned their particular ranges (Figures 2.4 and 2.5). The normalized scale is intended to render similar contours similar in appearance, irrespective of their absolute radius of curvature. Hence, the normalized scale, being more specific to an individual, is more sensitive in detecting subtle topographic changes in the anterior corneal surface. With both scales, steep areas are depicted by so-called ‘hot colours’ (i.e., reds and browns) and flatter areas are represented by ‘cold colours’ (i.e., blues and greens). The keratometric data display gives details of the steepest and flattest corneal curvature in the central 3mm, 5mm and 7mm (the central data may be represented on the colour map). The profile map shows the corneal curvature in dioptric powers over the corneal surface by calculating the profile along the steepest and flattest meridians from the central 3mm zone.19 Most software algorithms assume that the corneal contour changes smoothly and hence ‘average’ the curvature over an area of a few square millimetres. Unfortunately, little information is available on this effect, which may be of importance in relation to ablation geometry in laser refractive surgery. Each local area of the cornea is generally a toric surface, rather than a sphericalone, and hence possesses both spherical and cylindrical power. Sagittal and tangential data In corneal topography, the light from the topographer mires is directed onto the cornea. Off-axis light, when reflected from a curved surface, gives rise to two focal points. One represents the radius of curvature normal to the reflected mires, known as the sagittal (or axial) reflection. The other focal point represents the radius of curvature that contains the reflected mires, the tangential image. Sagittal and tangential data can be represented as different types of topography maps, and for the same cornea
Figure 2.4 Absolute (or standardized) scale of a corneal topography map. The scale is preset by the machine manufacturer. (Note the large range of the dioptric scale)
a
b
they may look different (Figure 2.6), although the main features of the map remain the same. Using the cornea as projector system The cornea was first used as a projector system to determine the corneal topography by Bonnett and Cochet (1962).20 It consists of the projection of a diffraction grid onto the corneal surface and the pattern produced by the grid is a function of the corneal topography. However, the cornea must first be made opaque to allow the grid pattern to be detected. Initially, talcum powder (in conjunction with topical anaesthesia) was used for this purpose, but more recently sodium fluorescein has been used. Accuracy of the measurements taken depends on the magnification used on the slit lamp and the way that the image is viewed or captured.7
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Figure 2.5 Normalized (or relative) scale of the same cornea as in Figure 2.4. In the map the dioptric scale has a much smaller range, which enables differences in the radius of curvature to be detected more easily Figure 2.6 Corneal topography maps of a patients’ eyes taken with the Orbscan topography unit. The map on the left in both (a) and (b) represents sagittal data and that on the right represents tangential data. Both (a) and (b) show clearly how sagittal and tangential data can look very different for the same eye, but the main emphasis remains the same
Similar technology is adopted in the Orbscan topography unit (Orbtek Inc., Salt Lake City, Utah). The Orbscan takes 40 slit sections of the cornea during two scans, each scan lasting 0.7 seconds. Each slit section is similar to an optical section viewed through a slit lamp. Similar to Placido-based topography, the patient rests on a chin rest and the instrument is aligned using an XYZ manipulator base (see Figures 1.5 and 10.1). The image capture takes a total of 1.4 seconds and any eye movements render the image void. The corneal curvature results are usually presented in the form of a contour map that shows height deviations from the best-fitting spheres, but a variety of other numerical descriptions can also be obtained. It has been shown that the measurement of anterior surface curvature, as assessed using calibrated standards, has a high accuracy and that the
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Figure 2.7 The quad map consists of maps of the anterior corneal height, the posterior corneal height, the keratometric data and the pachymetry, but these maps can be altered to suit the user’s preferences (see text)
thickness measurement on human corneas has a high reproducibility.21,22 The default topography map that the Orbscan produces, the quad map, consists of four pictures (Figure 2.7). The quad map has maps of the anterior corneal height, the
Figure 2.8 Surgical options, ‘birds-eye’ view of a normal cornea (note the central steepening of the cornea)
Figure 2.9 Surgical options, view of cornea that has undergone myopic PRK, with the associated central corneal flattening
Figure 2.10 Surgical options, view of an eye after radial keratotomy. (Courtesy of Orbtek)
posterior corneal height, the keratometric data and the pachymetry, but these maps can be altered to suit the user’s preferences. Height maps indicate the relative height above or below a mean of the radii of curvature of the surface (anterior or posterior). The mean radius of curvature of the corneal surface, the BFS, is subtracted from all other radii of curvature points of the surface. Thus, the height maps do not indicate the curvature of the cornea at a particular point, but rather the relative height with regard to the BFS (similar to an Ordnance survey map, in which heights are shown with respect to sea level). The height maps use ‘hot’ colours to indicate areas that are higher than the BFS, and ‘cold’ colours to indicate areas that are lower than the BFS. The keratometric map shows the radius of curvature data of the cornea at any point. In the quad map, it is viewed as an overall value for the anterior and posterior corneal surfaces, but these surfaces can be viewed individually. The final map in the quad selection is a pachymetry map that shows the thickness of the entire area of cornea assessed.
Another option that the Orbscan allows is called ‘surgical options’. This view produces a three-dimensional schematic image of the examined eye. It can be adjusted to produce a view of the anterior or posterior cornea, or both simultaneously. The anterior lens can also be imaged using this option, although lens curvature data are not available directly. This type of three-dimensional schematic view is available on other types of topography systems too, but is usually calculated from radius of curvature data, whereas the Orbscan uses elevation data. Figure 2.8 shows a ‘birds-eye’ view of a normal cornea (note the central steepening of the cornea). Figure 2.9 shows a cornea that has undergone myopic photorefractive keratectomy (PRK), with the associated central corneal flattening. Figure 2.10 shows an eye after refractive keratectomy. A new device recently available from the Birmingham Optical Group is the Oculus Pentacam system (Oculus, Giessen, Germany). This is discussed again in Chapter 10, as currently no published studies have used it. Essentially, it is a rotating Scheimplug camera and is able to image up to the fourth Purkinje image in a patient with a dilated pupil; otherwise, it is able to at least obtain data from three surfaces, like the Orbscan. The image creation and caption system is different to that of the Orbscan, so it remains to be seen how the two systems compare. The Pentacam is a table-mounted device and, similar to standard topography units, it uses an XYZ manipulator base with the patient lined up in front of the instrument with his or her chin upon a chin rest (Figure 2.11). The data are collected in around 2 seconds and approximately 25,000 data points are taken. The data can be represented as elevation data or radius of curvature data.
Figure 2.11 The Oculus Pentacam system is a tablemounted device that uses an XYZ manipulator base with the patient lined up in front of the instrument and his or her chin upon a chin rest
Corneal topography and its role in refractive surgery
Corneal topography in refractive surgery Irregular and regular astigmatism can be observed using topography after cataract surgery and post-penetrating keratoplasty, which allows the surgeon to assess stability. For cases in which there is a lot of post-surgical astigmatism, corneal topography maps can be used to indicate potential areas of suture removal (Figure 2.12). Corneal topography is a vital tool in refractive surgery. Pre-operative assessment checks for any contraindicated corneal conditions and dystrophies. Contact lens users who present for refractive surgery are advised to remove their lenses for a period of time before surgery to eliminate warpage induced by the contact lens. Warpage appears as an irregular topography picture with distortion that does not have a regu-
lar pattern. For a patient in whom warpage is observed, the topography is repeated on subsequent visits until no further changes are seen in the topography maps and only then is the patient considered suitable for surgery. Figure 2.13 shows a patient with corneal warpage in the right eye, but a relatively normal left eye. Post-operative assessment is essential to check astigmatic results, stability and irregular healing. Different techniques of refractive surgery show characteristic postoperative changes. For example, myopic excimer laser surgery shows central flattening, whereas after hyperopic surgery a mid-peripheral flattening is seen. Post-keratotomy, steepening of the areas of surgical incision and an accompanying flattening of other areas of the cornea are seen. Topography pictures taken at different visits allow the clinician to observe the
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healing changes that occur to an eye over a period of time post-surgery (Figure 2.14). During aftercare appointments topography is often conducted to pick up abnormalities such as central islands, which can be identified clearly. Decentred zone ablations can also be identified with corneal topography. Areas of surface scarring, such as complications of corneal flaps, erosions and sutures, are also detectable.
Limitations of corneal topography The quality of the anterior reflective surface of the cornea and inaccuracies in numerical assumptions can limit the usefulness of topography. Images are restricted nasally and superiorly because the recording mechanisms are eclipsed by the
Figure 2.12 A patient’s right eye after penetrating keratoplasty. The map labelled A was taken 1 week after surgery. Her corrected VA was 6/18, as there was some irregular astigmatism present. The surgeon removed one suture to try and make the astigmatism more regular. Picture B was taken a few minutes later, and gives corrected VA of 6/9 (Refraction: +6.00/–3.00 × 100). The main map shows the difference map obtained by subtracting map B from map A
Figure 2.13 Patient with corneal warpage in the right eye, but a relatively normal left eye
Figure 2.14 A cornea before and after photorefractive keratectomy. The initial refraction was –3.25/–0.25 × 175 and the 12 weeks post-surgical refraction was +1.00/–0.50 × 10. Picture A is the post-surgical map and picture B is the pre-surgical map. The larger picture is the difference between the two. The pre-surgical map has been subtracted from the postsurgical picture to demonstrate the area of cornea removed by ablation. It can be seen that a central area of approximately 5mm has been flattened (the actual laser setting for the ablation diameter was a 5.5mm optic zone and a total ablation zone of 6.5mm)
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nose, brow and upper eyelid. Superficial corneal scars and similar abnormalities confuse the topographies, especially if they are central. The patient’s ability to maintain fixation is vital.
Corneal topography and aberrometry Currently, practically aberrometry is becoming a very popular technique in refractive surgery. It is used to create better ablation profiles and also to assess postoperative patients, especially those with complications. In fact, pre-operative wavefront aberrometry examination should help the surgeon decide whether a traditional refractive surgery procedure would solve the visual problems of the patient, or if a customized ablation is indicated. Some aber-
References 1 Shah S and Naroo SA (1998). Corneal topography. Continued Medical Education. J Ophthalmol. 2, 16–19. 2 Corbett MC, Rosen ES and O’Brart D (1999). Corneal Topogrography.Principles and Applications, Ch 1, p. 3–11 (London: British Medical Journal Books). 3 Bennett AG (1964). A new keratometer and its application to corneal topography. Br J Physiol Opt. 21, 234–235. 4 Edmund C (1987). Determination of the corneal thickness profile by optical pachometry. Acta Ophthalmol. 65, 147–152. 5 Rowsey JJ (1984). Corneal topography. In Contact Lenses: The CLAO Guide to Basic Science and Clinical Practice, Ed. Dabezies OH Jr, Ch 4. (Orlando: Grune and Stratton). 6 Bennett AG and Rabbetts RB (1989). Measurements of ocular dimensions. In Clinical Visual Optics, Second Edition, p. 457–483, Eds Bennett AG and Rabbetts RB (London: Butterworths). 7 Dave T (1998). Current developments in measurements of corneal topography. Contact Lens Anterior Eye 21, S13–S30. 8 Bibby MM (1976). The Wesley–Jenssen System 2000 photokeratoscope. Contact Lens Forum 1, 37–45.
rometers are able to separate aberrations of the cornea from those of the whole eye, whereas other devices only give the wholeeye aberrations. The custom ablation techniques allow a method of correcting the whole eye’s aberrations on the corneal surface. In a similar way, traditional refractive surgery would correct a patient’s full ocular refraction on the cornea, even though some components of the prescription may be elsewhere, such as the crystalline lens. Most corneal topography devices now have software that enable radius of curvature data to be interpreted to express the corneal high-order aberrations, often in terms of Zernike polynomials or Fourier analysis. Some newer topography devices now take aberrometry measurements in addition to corneal curvature data. The Nidek OPD device (Figure 2.15) has a Placido disc and, in addition to that, uses a skiascopy
technique to measure ocular high-order aberrations. The combination of both techniques means that corneal aberrations can be separated from whole-eye aberrations.
9 Klyce SD (1984). High resolution graphic presentation and analysis of keratoscopy. Invest Ophthalmol Vis Sci. 25, 1426–1435. 10 Maguire LJ, Singer Dem and Klyce SD (1987). Graphic presentation of computer-analyzed keratoscope photographs. Arch Ophthalmol. 105, 223–230. 11 Mandell RB (1962). Methods to measure the peripheral corneal curvature. Part 3: Ophthalmometry. J Am Optom Assoc. 33, 889–892. 12 Sheridan M and Douthwaite WA (1989). Corneal asphericity and refractive error. Ophthalmic Physiol Opt. 9, 235–238. 13 Ilango B, Shah S and Clark IH (2001). A review of biometry techniques. Eye News 8, 24–28. 14 Sheridan M (1989). Keratometry and slit lamp biomicroscopy. In Contact Lenses, Third Edition, p. 243–259. Eds Phillips A and Stone J (London: Butterworths). 15 Applegate RA and Howland HC (1995). Non-invasive measurement of corneal topography. IEEE Eng Med Biol Mag. 14, 30–42. 16 Douthwaite WA and Evardson WT (2000). Corneal topography by keratometry. Br J Ophthalmol. 84, 842–847.
17 Douthwaite WA, Hough T, Edwards K and Notay H (1999). The EyeSys videokeratoscopic assessment of apical radius and p-value in the normal human cornea. Ophthalmic Physiol Opt. 19, 467–474. 18 Hilmantel G, Blunt RJ, Garrett BP, Howland HC and Applegate RA (1999). Accuracy of Tomey topographic modelling system in measuring surface elevations of asymmetric objects. Optom Vis Sci. 76, 108–114. 19 Roberts C (1998). A practical guide to the interpretation of corneal topography. Contact Lens Spectrum 13, 25–33. 20 Bonnet R and Cochet P (1962). New method of topographical ophthalmometry – its theoretical and clinical applications. Am J Optom Arch Am Acad Optom. 39, 227–251. 21 Lattimore MR Jr, Kaupp S, Schallhorn SS and Lewis IV RB (1999). Orbscan pachymetry: Implications of a repeated measures and diurnal variation analysis. Ophthalmology 106, 977–981. 22 Yaylali V, Kaufman SC and Thompson HW (1997). Corneal thickness measurements with the Orbscan topography system and ultrasonic pachymetry. J Cataract Refract Surg. 23, 1345–1350.
Figure 2.15 The Nidek OPD
3 Corneal anatomy, physiology and response to wounding Sandip Doshi
The mainstay of current laser refractive surgery centres on manipulation of the properties of the cornea to achieve the desired optical affect. As a result it is essential that the clinician has a strong understanding of the intricate architecture and physiological properties of the organ. Moreover, a firm understanding of the basic science of the cornea allows the clinician to plan treatments that result in a minimal disruption to its structure, and hence achieve a preferred visual outcome. The cornea occupies approximately 7 per cent of the outer coat of the eye. It is a highly organized five-layered structure (Figure 3.1) that consists of:
• The epithelium; • Bowman’s layer; • The stroma; • Descemet’s layer; and • The endothelium. With a standard slit-lamp biomicroscope and appropriate magnification and observation techniques only the epithelium, stroma and endothelium are visible. Each corneal layer is discussed separately in this chapter.
Corneal epithelium The corneal epithelium is the outermost layer (Figure 3.2) and is an anatomical continuation of the conjunctival epithelium. It is thinnest centrally, where it is typically around 50–60μm in thickness, and thickens to around 70–80μm in the periphery. Thus, the central corneal epithelium constitutes approximately 10 per cent of the total corneal thickness. The cornea is the major refractive component of the eye. The epithelium is arguably the most important layer for this property.
The reported refractive index of the epithelium varies for different researchers, but commonly is stated to range from 1.375 to 1.543. A uniform regularity and transparency of the epithelium is essential if the cornea is to be a perfect optical surface. Being the outermost layer of the cornea, the epithelium functions as a barrier to protect the deeper layers from various insults. It also provides a barrier against fluids from the tear film. As any refractive surgeon knows, the epithelium is remarkably tough and resilient to significant trauma, but when damage does occur the epithelium has an excellent recovery rate (see the section on corneal wound healing later). Epithelium microanatomy The normal human corneal epithelium can be described as a non-keratinized, stratified, squamous epithelium. Typically, the corneal epithelium is five to six cell layers thick. Three distinct cell types can be identified in the epithelium: basal, wing (or umbrella) and squamous cells.
Figure 3.2 Corneal epithelium and Bowman’s layer
Figure 3.1 Transverse section cornea
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Basal cells are the innermost cells of the epithelium and form a monolayer of large, columnar cells. They are relatively uniform in size and are typically 15–20μm in height. Basal cells produce the basement membrane of the corneal epithelium. The organization of the basement membrane allows the epithelium to attach to the remaining layers of the cornea. Superior (and so more external) to the basal cell layer is the wing cell layer. This consists of two or three layers of cells characterized by a flattened dome-shaped appearance. The wing cell layer tends to be 20–25μm in height. Wing cells are thought to be more mature than basal cells. At the outermost aspect of the epithelium is the squamous cell layer. This superficial layer consists of one or two cell layers. Typically, this layer tends to be about 10–15μm. These cells are the most mature of the epithelium and complete their lifespan by sloughing off into the tear film. Epithelium ultrastructure Resident cells of the corneal epithelium Basal cells are large, columnar cells with a slightly apically displaced nucleus that is spherical or slightly oval and contains dispersed chromatin. The cytoplasm contains many intermediate filaments (mostly grouped into bundles known as tonofilaments), free ribosomes, sparse mitochondria, little granular endoplasmic reticulum, glycogen granules and occasionally Golgi complexes. Basal cells are attached to the basement membrane via a series of hemidesmosomes. Normally, the latter are so numerous that they occupy at least one-third of the area of the membrane. Interestingly, these bonds are readily broken and reformed – a feature that proves to be vital as part of the wound-healing process.1 Hemidesmosomes are macula junctions that are continuous with basal cell tonofilaments and consist of local thickenings of the plasma membrane (lamina densa) opposite a thickened zone of basement membrane. A lighter interval between the two thickened membranes (lamina lucida) contains numerous fine, connecting filaments, which in turn are attached posteriorly to fine, branched collagen fibrils, called attachment (or anchoring) plaques, that anchor the basal lamina and epithelium to Bowman’s layer. Anchorage of the epithelium to the basal lamina occurs via a large number of unevenly distributed hemidesmosomes. This results in a strong adhesion of the epithelium to the underlying surface. The
attachment of the basement membrane is very stubborn and normally a blunt trauma does not remove it. However, in some varieties of refractive surgery, such as photorefractive keratectomy (PRK), the removal of the epithelium and the basement membrane (and Bowman’s layer) allows the anterior stroma to become involved in the healing process, which can lead to scarring. Moreover, the presence of a normal basal lamina is essential for reepithelialization. Basal epithelial cells continue to lay down the basement membrane throughout life and, therefore, there is an increase in its thickness with age. Bergmanson suggested that this might be a reason for a weakening with age of the attachment between the corneal stroma and epithelium.2 Away from the innermost aspect, basal cell borders are characterized by shallow interlocking ridges that cover most of their surfaces, with little or no space between cells. These ridges are least frequent in apposed membranes of basal cells. Adjacent cells are joined by numerous desmosomes. This results in the epithelium being able to withstand a considerable amount of abuse. A relatively small number of gap junctions are also seen throughout the basal layer. The wing cell layer is characterized by dome-shaped cells with central round nuclei. These cells flatten as they move anteriorly away from the basal layer. The number of cell layers varies from two to three in the central cornea, but may increase to four or five in the periphery. Cells of this layer are derived from the basal layer and represent more mature cells. Cell organelles are more sparse than in the basal layer, which suggests metabolic activity in this layer is slower than that in the basal layer. This is of some relevance when the corneal epithelium is examined in relation to stem cell theory (see later). Like all cells of the corneal epithelium, wing cells are tightly packed and there is very little intracellular space. Cells attach to their neighbours via numerous membrane interdigitations and desmosomes. As cells continue to mature through the wing cell layer they become flatter and ultimately move more superficially to the squamous cell layer. This layer is characterized by the presence of long, thin, flattened cells that display an intensely stained, elongated nuclei. Squamous cells contain the fewest organelles of the epithelium, which indicates that they possess the lowest metabolic activity. Cells are joined to their neighbours via membrane interdigitations and desmosomes and gap junc-
tions on the inner and lateral aspects. Gap junctions contribute to intercellular adhesions and are probably communicating junctions that permit ionic exchange. Additionally, the surface cells display tight junctions (zonula occludentes) on their most superficial surface. Unlike the other junctions, these girdle the cells and form the closest of contacts without achieving complete fusion. This limits permeability and permits access to the intracellular space from the tear film or, in reverse, through discrete pores only.3 Unlike surface cells of the skin, those of the normal corneal epithelium are non-keratinized and retain some organelles, which indicates that even at this late stage of maturation their metabolic processes are still functioning. This is particularly relevant as part of the normal wound-healing process. Squamous cells display numerous microvilli (and rarely microplicae). In humans, these can be quite substantial, reaching up to 0.75μm in height. Pedler suggested that as cells slough from the surface, desmosomes must split to achieve detachment, and cytoplasmic extrusions at these points may be responsible for microvilli development.4 Microplicae represent fusion of adjacent microvilli. The number of microvilli on a cell surface is a good indicator of its age – the greater the number of microvilli on a cell surface, the older it is. Microvilli provide an increased surface area for the attachment of a fine glycoprotein layer, the glycocalyx. This layer provides anchorage for the pre-ocular tear film. Damage to microvilli during the flapmaking process in laser in situ keratomileusis (LASIK) and the resultant lack of anchorage for the tear film components is thought to be one of the reasons for dry eye after the procedure. Non-native cells of the corneal epithelium A non-native cell in any epithelia can be described as one that does not form any junctional complexes with its neighbours. In humans, the normal corneal epithelium has a complement of non-native cells present at any time. Langerhans’ cells are found in the basal layer of the corneal epithelium. These dendritic, polymorphous cells are thought to play an important role in the ocular surface immune response.5 Although they perform a similar function to their counterparts in skin, ocular Langerhans’ cells are known to vary in that they lack the thymocyte antigen (T6).6 More recently, Doshi suggested that ocular Langerhans’ cells might also vary morphologically from those in the skin, in
Corneal anatomy, physiology and response to wounding
that they lack the Birbeck granules characteristic in skin.7 It is normal to see occasional lone lymphocytes and macrophages within a normal corneal epithelium, but it appears that their presence is of no clinical consequence. When present, these cells tend to be confined to the peripheral cornea. Corneal epithelium stem cell theory The concept of corneal epithelial stem cells is now firmly established. Among many other classifications, this theory describes cells in relation to their proliferative capacity and their state of maturation. The cornea is unique in that its progenitor cell, the stem cell, is located away from the organ itself and is found in the basal layer of the limbal conjunctival epithelium.8,9 Stem cells are immature and slow to multiply under normal conditions. Division of a stem cell produces two offspring, one that is a replica of itself and a second, transient amplifying cell (TAC). The TAC is responsible, by relatively rapid division, for increasing cell volumes. The sequestration of corneal epithelial stem cells to the limbus requires their offspring to migrate centripetally to reach the cornea. Evidence of this centripetal flow is seen in individuals with pigmented conjunctivae, who sometimes display pigment slide into the peripheral cornea.10 Further support for this observation was given by Thoft et al.,11 who suggested that corneal epithelial stem cells are located predominantly in the vertical meridian. Additionally, they suggested that the entire basal layer of the corneal epithelium represents TACs that had migrated centripetally from the limbus. Lauweryns et al. corroborated this observation and suggested that, consequently, centripetal movement of epithelial cells was limited to the vertical meridian.12 Stem cell theory and the subsequent centripetal migration that must exist for this theory to hold true are the focal points of corneal wound healing (see later).
Bowman’s layer Bowman’s layer lies beneath the epithelial basement membrane and separates the epithelium from the stroma (Figure 3.2). It is acellular and uniform in thickness, which typically measures around 12μm. Its thickness remains constant in a healthy cornea. Bowman’s layer is present throughout the cornea and terminates at the periphery, which marks the beginning of the limbus.
Normally, epithelial damage occurs readily without the involvement of Bowman’s layer. This provides evidence of its relative toughness. However, if damage does occur to Bowman’s layer, fibrous scar tissue is laid down and results in an opacity, which tends to reduce in density with time. Bowman’s layer does not regenerate, and significant damage or surgical removal of this layer, such as in PRK, results in a permanent loss. However, in such cases the anterior stroma becomes more compact and loses its cellularity to form a pseudo Bowman’s layer. Bowman’s ultrastructure Electron microscopy reveals a fine, randomly orientated mesh of collagen fibrils. These fibrils are finer than those of the stroma. Bowman’s layer modifies anteriorly, where the anchoring filaments of the basement membrane insert. Over the whole of its area, Bowman’s layer is penetrated by fine unmyelinated nerve fibres that pass from the stroma to the epithelium. To maintain transparency, the nerve fibres loose their Schwann cell sheaths as they leave the stroma.
Corneal stroma The stroma constitutes 90 per cent of the corneal thickness and gives the cornea its strength. Despite its apparent acellularity, the stroma is far from being passive. Stromal cells are important in the production and maintenance of corneal transparency. This property is further aided by the regularity of this layer and the absence of blood vessels within it. Stromal microanatomy The stroma consists of around 200 layers of lamellae of collagen fibrils. The fibres, which are buried in a matrix of proteoglycans, have a periodicity that is characteristic of collagen. The adult human cornea lacks elastic fibres.13 Collagen fibrils are of a regular size at any given depth of the cornea, and typically measure 34nm in humans.14 Cells occupy 2–10 per cent of the corneal volume. Keratocytes, nerve fibres and occasionally cells of a vascular origin lie between the lamellae. Keratocytes predominate and are responsible for secreting the proteoglycan matrix and procollagen. Blood-borne cells are relatively small in number. During inflammation, polymorphonuclear leukocytes (PMNs) produce localized opacities called infiltrates, which may contribute to the induction of stromal oedema.15 Interestingly, keratocytes
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also have phagocytic capabilities and congregate towards sites of inflammation. Stromal ultrastructure The majority of the stromal lamellae have a similar thickness (1.5–2.5μm) and lie parallel to each other, except in the anterior third of the stroma where some of the lamellae run obliquely. Fibrils within a lamina are parallel, but the fibrillar orientation in adjacent lamellae is angled, and tends to be arranged orthogonally.16 A bias of lamellar orientation has been claimed, but the direction is not agreed as the result differs according to the technique used. Lamellae widths are difficult to measure; most are up to 250μm, but some appear to be in excess of 1mm. Although discreteness of adjacent lamellae prevails, at least in the posterior two-thirds of the stroma, occasionally slightly oblique branches connect one lamella to another. This arrangement explains the ease with which the stroma may be split parallel to the surface, as in the preparation of flaps for LASIK or lamellar grafts. At the corneoscleral margin, the stromal lamellar undulate, branch and interweave. The fibrils of single lamellae remain parallel to each other, but their diameters increase significantly, up to tenfold. The matrix of the stroma is composed largely of glycosaminoglycans (GAGs) covalently bound to protein, which constitutes proteoglycans. The two major types are keratan sulphate and chondroitin sulphate with a filamentous structure demonstrated by Hirsch et al.17 It is the filaments that attach through their core proteins to collagen fibrils, bridging the spaces between them. Details of this bottle organization are unclear, but Scott proposed a model that may explain the manner in which the matrix influences the regular separation of collagen fibrils.18 During oedema that results from a compromised endothelium, GAGs are lost from the cornea.19 Keratocytes are positioned in the interface between adjacent lamellae. In a single interface, keratocyte cell bodies are spaced well apart across the cornea, but their thin, lengthy processes are so extensive that they may come into contact with processes from neighbouring cells, which gives the appearance of a fine, wide-mesh network. This is repeated at each lamellar interface. The nuclei of these cells are flat, oval and embedded in a sparse perikaryon. More than one nucleus may often be present. In a normal eye there is little or no proliferative activity among keratocytes.20
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Descemet’s layer
Figure 3.3 Corneal endothelium, Descemet’s membrane and posterior stroma
Descemet’s layer (or membrane) is the basement membrane of the corneal endothelium and can be found in embryos as early as 8 weeks gestation (Figure 3.3). There is a two-part formation and structure to this layer: the anterior striated or banded portion is formed in utero and the non-banded section is laid down after birth. Descemet’s layer is around 5μm thick at the first post-natal year and increases by approximately 1.3μm each decade.21 Stromal thickness, by comparison, remains unchanged. Descemet’s ultrastructure Under the light microscope, Descemet’s layer appears void of any cells and lacks internal structure. However, the anterior part of this membrane displays a fine, regular organization under the electron microscope. In tangential section it has a two-dimensional lace network appearance with a repeating hexagonal unit in which seven dense nodes mark the angles; fine filaments of equal length connect these. The networks are stacked in depth register, as revealed by transverse sections. Dark bands are discernible perpendicular to the plane of the cornea and consist of dark granules, which are the nodes of the tangential section network.16 In contrast, the posterior part of this layer has the same fine, granular appearance in whichever plane it is sectioned and shows no sign of patterned organization. The biochemical composition of Descemet’s layer is not understood well in humans. In other species it has been found to consist primarily of type IV collagen.22 There is evidence for the presence of types I, II and V collagens in nonhuman models.23–25
Corneal endothelium The corneal endothelium is a monolayer of hexagonal cells that lines the inside of the cornea. These cells assume a hexagonal array that varies with age, trauma and disease. This layer plays a pivotal role in the maintenance of corneal clarity because its function is to maintain stromal deturgescence (Figure 3.4). Endothelial microanatomy Seen in tangential section, cell borders are ill-defined because of the oblique cell interfaces and the interdigitation of the broad processes of adjacent cells. Cell nuclei are often oval or kidney-bean shaped and the cytoplasm appears grainy. The endothelial
mosaic is not always regular because of the variation in cell size or as a result of polymegathism. This increases with age and is exaggerated by some forms of contact lens wear. Endothelial ultrastructure Svedbergh and Bill reported that most primate endothelial cells averaged 20–25μm in diameter.26 Therefore, with a corneal surface of approximately 100mm2 to cover, there are about 400,000 endothelial cells in the typical cornea. Endothelial cells are well stocked with organelles, especially mitochondria. Cells also display a prominent endoplasmic reticulum. Both indicate an extensive metabolic activity. Near the posterior border of this layer, the intercellular space is reduced to form a tight junction of width about 10nm that restricts movement in and out of the cornea between adjacent endothelial cells. Endothelial replication, regeneration and healing after wounding The importance of the corneal endothelium in the maintenance of stromal deturgescence and clarity means a considerable
amount of literature has been published on the response of this organ to wounding. Spontaneous mitosis and migration occur in young rabbit endothelium after trauma. This is followed by mitotic activity to replenish the normal cellular density. Early specular microscopic studies suggested that there was no mitotic activity in human endothelium, and wound healing was accomplished by the spreading, enlargement and finally contact inhibition of adjacent endothelial cells. Over time an equilibration of endothelial cell size across a large area was seen to occur, even when the initial wound was limited to a small central area. Treffers challenged this view on finding that tritated thymidine was incorporated both in vitro and in vivo in humans,27 which indicated that the corneal endothelium does have a proliferative capacity. Some cells typical of those seen in the Mphase of mitosis were observed by specular microscopy28, which corroborated the Treffer’s study27. Human corneal endothelial cells grown in culture have responded favourably to the administration of epidermal and fibroblast growth factors.29 In fetal tissue, endothelial cells are seen to respond to
Figure 3.4 Tangential section of the corneal endothelium
Corneal anatomy, physiology and response to wounding
similar agents.30 Cytofluorometric techniques have indicated that most human endothelium may be stable in the postmitotic G1 phase.31 Whether human corneal endothelial cells actually undergo mitosis and assist in wound healing or in the repair of natural endothelial cell loss remains unclear, with strong arguments for and against. Specular microscopic studies strongly suggest that if mitosis does occur, it does not appear to result in the formation of endothelial cells of normal size. It is likely that the response to endothelial wounding (e.g., after surgery) is an initial lag phase followed by migration and enlargement of endothelial cells that surround the defect until these cells re-establish contact with one another. At this stage, intercellular junctions are reformed and the eventual establishment of endothelial function by thinning of the overlying corneal stroma is observed.
Corneal innervation The cornea is served by 70–80 small sensory nerves that issue from ciliary nerves which branch from the ophthalmic division of the trigeminal nerve. They enter the sclera from the uvea at the level of the ciliary body and pass anteriorly to enter the cornea radially and predominantly in the middle layers of the cornea. Other nerves from the same source enter the cornea more superficially. They enter the conjunctival epithelium from the subepithelial tissue at the limbus and pass directly into the corneal epithelium at basal level.32 A minority of the nerve fibres that enter the cornea possess a myelin sheath, but this is lost at the limbus or within 1mm of entering the cornea. Rarely, myelin persists a little further. The perineurium and the fibres and cells of the endoneurium also terminate at the limbus. Only the nerve fibre bundles advance into the cornea. Each bundle consists of several axons enclosed by a Schwann cell sheath. Initially, the fibre bundles of each nerve are grouped together. These separate and spread, overlapping and running together with branches of neighbouring nerves to produce the plexiform arrangements seen in full thickness preparations of the cornea. The plexus is particularly dense beneath Bowman’s layer. Axons separate and some divide at intervals and form fine terminal branches, some of which may lose their Schwann cell covering; these terminal axons follow a lengthy course between the stromal fibrils. They possess numerous small, bead-
like varicosities, with a final, often larger, one that marks the end of the axon. Fibres from a single nerve bundle at the limbus may be distributed to as much as two-thirds the area of the cornea. Consequently, there is considerable overlap of nerve fibres from different bundles. This arrangement explains why sensitivity persists in all areas of the cornea subsequent to large surgical incisions, and is also the reason why the cornea localizes stimuli poorly. The epithelium receives a prolific supply of terminal fibres that pass perpendicularly from the anterior stromal plexus and penetrate Bowman’s layer. The small nerve fibre bundles lose their Schwann cell covering before they enter the epithelium, where the fine, naked axons disperse and turn sharply to lie nearly parallel to Bowman’s layer. Varicosities similar to those in the stroma occur in the epithelium. Such axons may run a course up to 2mm long, and the fine beaded branches that issue from them are directed through successive layers of the epithelium to almost the surface of the cornea. Matsuda observed two types of epithelial nerve terminal beads in rabbits and humans.33 One contained mitochondria and the other contained mitochondria and vesicles. He suggested that beads without vesicles serve a sensory function, while those with vesicles were probably motor. There is no reliable evidence of parasympathetic fibres in the cornea. There is, however, a strong body of evidence for some sympathetic innervation to the cornea. Variety in their chemistry suggests that sensory fibres may consist of functionally distinct subgroups; some contain the neuropeptide substance P, and others of unknown chemical identity do not. Calcitonin gene-related protein (CGRP) also exists within nerves. It is thought to coexist with substance P in the same terminal.34
Corneal sensitivity It is generally undisputed that the sensitivity of the normal cornea is unsurpassed by any other organ of the body. Aesthesiometry has furnished the clinician with essential data relating to the sensitivity of the corneal surface. From such data it is now accepted that the sensitivity of the cornea varies from a maximum apically to a minimum at the periphery, with a further considerable drop in sensitivity at the limbal conjunctiva. Sensitivity varies with age. In a study of patients between 10 and 90 years of age Boberg-Ans found peak sensitivity to be up to three times greater in the younger indi-
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viduals than that in the eldest.35 Most sensitivity reduction occurs between the ages of 50 and 70 years.36,37 Sensitivity variations between the two eyes are normally minimal. Millodot and Lamont found significant reduction (almost half) in a sample of pre-menstrual and menstrual women,38 which indicates the possibility of a variation, albeit for a limited time, between the sexes. The cornea displays a diurnal variation in sensitivity, with about a third greater sensitivity as the day progresses from morning to evening.39 However, perhaps the most striking variation displayed is that between individuals with different iris colour. Blue-eyed individuals have a greater sensitivity than those with darkbrown irides. Non-white people with darkbrown irides have less sensitive corneas than Caucasians with a similar iris colour. Generally, non-white people have fourtimes less sensitive corneas than blue-eyed individuals and half as sensitive corneas as those of brown-eyed Caucasians.40 A normal nervous supply is essential to maintain regular corneal function. Without this several key features of the cornea are diminished or absent. Epithelial cell migration diminishes and epithelial cell turnover is hampered. It is widely accepted that both LASIK and PRK cause corneal hypoaesthesia, but there seems to be disagreement in the literature as to which procedure has a more profound and longer effect. The variety in results may reflect differences within surgical techniques. Moreover, variation in clinical techniques and corneal location in aesthesiometric results cannot be excluded. Yang et al. reported that corneal sensitivity was more reduced in PRK than in LASIK in the early stages post-procedure.41 In eyes that had undergone PRK, they found it took 6 months to recover baseline (pre-operative) levels. By contrast, eyes that had undergone LASIK recovered to baseline levels within 1 month. The majority of the literature, however, seems to corroborate the findings of Perez-Santonja et al.42 They found that corneal sensitivity was reduced for the first 3 months after LASIK and only recovered to pre-operative levels after 6 months. In PRK, corneal sensitivity recovered its preoperative values within 1 month, except at the central cornea, which took 3 months. In comparing both groups, they found that corneal sensitivity was more depressed after LASIK than after PRK during the first 3 months. No differences were found between the groups at 6 months. Matsui et al. reported recovery in corneal sensitivity as early as 1 week after PRK,
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with recovery to pre-operative values within 3 months.43 In agreement with the results of Perez-Santonja et al.,42 they found LASIK had a more profound effect on corneal sensitivity, with recovery beginning around 3 months post-procedure. However, over the short duration of this study (3 months) these workers found that sensitivity after LASIK failed to reached baseline levels.43 In vivo confocal microscopy has provided excellent information about the regeneration of corneal nerves after laser refractive surgery. Kauffmann et al. compared the regeneration of corneal nerves after PRK and LASIK.44 In PRK, recovery of subepithelial re-innervation started from the margin of the ablated zone towards the centre of the cornea. At 8 weeks post-operatively, rarefied subepithelial nerve fibres were visible at the edges, and after 3 months single non-branched nerve fibres were present at the centre of the ablation zone. By 6–8 months after PRK, subepithelial nerve regeneration seemed to be complete; however, abnormal branching and thin accessory nerve fibres were present without exception. After LASIK, corneal nerve-fibre regeneration followed the same course as described for PRK, except that the regenerated subepithelial nerve fibres were barely visible in the central cornea after 6 months. Further changes in nerve structure were visible for up to 12 months postoperatively. These observations correlate well with clinical data obtained on the return of corneal sensitivity.
Corneal transparency If each corneal layer has the same refractive index, then the transparency of the cornea is explained easily. However, this is not the case. The refractive index of the epithelium is quoted to range from 1.375 to 1.543,45 while that of the stroma is typically around 1.55 in the dry state. It is probably fair to say that the exact reasons for corneal transparency are still poorly understood. Maurice has offered an explanation of the transparency of the stroma.46,47 His theory embraces light of all incidences and explains how transparency is lost in various circumstances. According to Maurice, six neighbouring fibrils surround each collagen fibril in a regular, hexagonal array or lattice. The fibrils are arranged so because they act as a series of diffraction gratings that permit transmission through the liquid ground substance, which has a lower refractive index of 1.34. As the fibrils in adjacent regions of the stroma have
very regular diameters and spacing, a duplication of the diffraction grating exists in any plane. Spacing between adjacent fibrils is approximately one-tenth the wavelength of visible light.48 Normally, this spacing is quite regular. The incident light that impinges on the collagen fibrils either passes through or reflects off the fibril, such that light scatter cancels by destructive interference. As a result, the matrix can transmit visible light with an efficacy of 90–98 per cent. Supporting this hexagonal theory is that the collagen fibrils of the sclera have a larger diameter and are spaced more irregularly than those in the cornea. When the cornea is oedematous its transparency is reduced. This may be explained in terms of the lattice theory in that the excessive fluid disturbs the regularity of the fibrillar spacing, so the efficacy of the fibrils as grating elements is lost. Alternatively, transparency loss with oedema may be explained by the formation of spaces within stroma. A similar explanation can be applied when loss of stromal transparency occurs as an adverse response to surgery.
Corneal wound healing In the context of laser refractive surgery, corneal wound healing is best described in terms of epithelial and stromal healing. Neither is exclusive and now a strong body of evidence indicates that there is interaction between these organs as part of the normal wound-healing process. This is thought to occur via a number of chemotactic factors. Endothelial wound healing is a less important feature in the context of laser refractive surgery, but is discussed briefly here. Epithelial wound healing The pattern of epithelial wound healing is generally size dependent. Small, central wounds tend to recover more slowly than larger more peripheral ones. The rate of corneal wound healing is also dependent on the presence or absence of the epithelial basement membrane. When present, re-epithelialization takes a shorter period of time, typically 2–3 days, but when absent the same process can take longer, normally 5–7 days.49 Epithelial wound healing is described in four distinct stages: the latent phase, cell migration, cell proliferation and adhesion. Latent phase Extensive reorganization occurs at both cellular and subcellular levels as a result of wounding. Initially, PMNs from the tear
layer and limbus appear in the basal layer at the edge of the wound to remove dead cells and debris. After this, cells at the leading edge of the undamaged epithelium lose their surface microvilli and subsequently flatten and separate. These flattened cells develop surface ruffles and filopodia at their free edges.50 Concurrently, hemidesmosomes are broken between basal cells and the basal lamina, which allows the cells to slide. The ruffles and long, fine filopodia extend to form attachments to the basal lamina, which gives the impression of a capacity to draw cells forwards into the area of the defect. Cell migration Between 4 and 6 hours after wounding, epithelial cells migrate across the wounded area. This migration results, initially, in a monolayer of cells that plug the wound. Consequently, this accounts for the disappearance of symptoms 4–6 hours post-wounding. As a result of numerous chemical changes in the basal lamina, the formation of hemidesmosomes is suspended. Consequently, sliding cells are supported by actin filaments, located in the filopodia, which act as a cytoskeleton. Cell migration occurs in a centripetal manner and as a continuous sheet. It is rare for cells to migrate independently. Individual cells generally maintain the same position within a sheet. Sheet migration occurs from several directions, which meet at a junction as the wound closes. Cell proliferation After cell coverage of the wound with a monolayer of epithelial cells, the corneal epithelium undergoes stratification. This is facilitated by mitosis of the corneal epithelial stem cells located in the basal layer of the limbal conjunctiva. In the normal ocular surface, stem cells are relatively quiescent and rarely undergo mitosis. However, in response to wounding these cells readily divide. Division of a stem cell produces two offspring, one a stem cell and the other a TAC. The newly produced TACs migrate centripetally from the basal layer of the limbus, where they are generated originally, to the basal layer of the cornea. It is principally the rapid division of a TAC that results in stratification of the corneal epithelium. As these cells mature and become more differentiated they move away from the basal lamina to become post-mitotic cells (PMCs). Near the corneal surface, PMCs become fully differentiated into terminally differentiated cells that are eventually sloughed off into the tear film.
Corneal anatomy, physiology and response to wounding
Adhesion The final stage of epithelial healing involves reconstruction of the normal epithelium adhesion structures to Bowman’s layer. Intraepithelial attachments also form and can take up to 8 weeks to become complete. Prior to this the attachments are relatively weak. The speed of hemidesmosomal attachment to the basement membrane is dependent on whether the latter is intact. A more rapid regeneration occurs when the basal lamina is undisturbed. Stromal wound healing As epithelial wound healing begins, stromal keratocytes disappear. This is a rapid process and can begin as soon as 30 minutes after wounding. Within 15 hours of the initial injury almost 40% of the anterior stroma is void of keratocytes.51 Reduction of keratocyte numbers occurs by apoptosis. Helena et al. suggested that in refractive surgery this was because these cells became redundant.52 A benefit of apoptosis is that minimal damage occurs to the surrounding tissue, as a consequence of which corneal clarity is preserved. Another advantage of apoptosis is that a potential vehicle for infection is closed down. As time passes stromal keratocytes undergo migration and proliferation to regenerate a normal stroma. Stromal keratocytes migrate from the posterior stroma to the surface and expand the cell numbers by undergoing mitosis. Keratocyte reproduction begins after the wound has been covered by new epithelial cells and, typically, reaches a peak 3–6 days later. The newly generated keratocytes synthesize collagens, glycoproteins and proteoglycans (Figure 3.5). After corneal injury, this effect is intense for the first 3 months and tapers off at 6–15 months.
Endothelial wound healing Any significant decrease in endothelial cell density or a change to the cell mosaic results in corneal decompensation. Additionally, it is well established that human corneal endothelium has a low capacity for regeneration. Both of these observations have stimulated research into the effect of laser refractive surgery on this layer of the cornea. It appears that the majority of work has found no significant change in either property in PRK or LASIK. Stulting et al. noticed, however, an increase in central cell density and a correlating decrease in the periphery.53 This was attributed to recovery of the central area through a migration of some of the cell mass from the periphery after discontinuation of contact lens wear.
Confocal microscopy after refractive surgery The advent of in vivo confocal microscopy has furnished the clinician with a way to improve imaging of the living cornea. This facility has been used to study the cornea at the cellular level, to describe normal morphology, keratitis and other corneal pathologies, and lately the effects of refractive surgery. Moreover, confocal microscopy has furnished the clinician with highly accurate morphometric data. In normal human corneas, Patel et al. demonstrated that the full thickness density of corneal keratocytes was 20,522 ± 2981 cells/mm3.54 This study investigated 70 subjects who did not wear contact lenses and had normal corneas, with ages that ranged from 12 to 80 years. These workers found that keratocyte density was highest in the anterior 10% of the stroma and that density decreased with age at a rate of 0.45% per year. Using a continu-
Figure 3.5 Corneal fibroblasts (keratocytes) viewed topographically
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ous through-focus method (CTFM), they found that the normal central corneal thickness was 563.0 ± 31.1μm and the central epithelial thickness was 48.6 ± 5.1μm. Erie et al. investigated the affect of LASIK on epithelial and stromal thickness.55 They found a similar epithelial thickness (46 ± 5μm) to Patel et al. (48.6 ± 5.1μm) before treatment.54,55 After surgery, epithelial thickness had increased 22% by 1 month. Thereafter, epithelial thickness did not change, but remained thicker at 12 months after LASIK (54 ± 8μm) than before. Post-operative epithelial hyperplasia has also been linked to refractive regression after myopic LASIK.56 Spadea et al. demonstrated that epithelial thickness increased as early as 1 week after LASIK, reached maximum thickness between 1 and 3 months, and then remained stable for up to 1 year.57 An increase in epithelial thickness has also been demonstrated after myopic PRK treatments.58,59 However, unlike in LASIK, the epithelium continues to thicken for up to 1 year after surgery. Normally, LASIK does not disrupt the corneal epithelium or Bowman’s layer and, consequently, it should affect anterior corneal homeostasis less than PRK. Erie et al. suggested, however, that although preservation of the anterior corneal layers does not prevent thickening, it does seem to allow the earlier establishment of a stable epithelium compared with that in PRK.55 The cause of epithelial remodelling after LASIK is unclear. Epithelial hyperplasia is noted frequently in corneal diseases associated with stromal loss as the epithelium attempts to fill and restore a smooth corneal surface. Dierick and Missotten suggested a tension model in which the epithelium attempts to restore the original curvature of the cornea.60 Reinstein et al. used high-frequency ultrasound to demonstrate that the epithelium varies in thickness after LASIK and appears to possess the ability to remodel itself to compensate for underlying stromal surface anomalies.61 The origin of this property is thought to be the semirigid concave tarsus of the upper eyelid, which polishes and remodels the epithelial surface during blinks. Erie et al. found no significant change in the thickness of the total stroma, flap stroma or base stroma between 1 and 12 months after LASIK.55 However, rather than being relatively quiescent, as this observation would suggest, Vesaluoma et al. demonstrated marked cellular activity near the ablation zone.62 They found activated keratocytes near the interface. The
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initial response to LASIK is the creation of a thin keratocyte-free zone on both sides of the lamellar cut. Apoptosis is thought to be the mechanism that underlies the disappearance of keratocytes. Activated keratocytes have been indicated as part of the healing process after PRK,63 and have been implicated in the stromal thickening shown to occur after this procedure.64 The duration of activated keratocytes in LASIK was shown to diminish after 1–2 weeks by Vesaluoma’s group,62 whereas it typically peaks 3 weeks to 4 months after PRK.65 Keratocyte activation after PRK is thought to be caused by an epithelial–stromal interaction mediated by the release of cytokines during epithelial removal. However, as the epithelium and Bowman’s layer are left intact on LASIK, it is possible that keratocyte activation and subsequent stromal regeneration are less than that observed in PRK, which may in part explain the lack of stromal thickening found after LASIK. Studies in rabbits have shown that the wound-healing process occurs only in the periphery of the corneal flap and in relative proximity to the epithelium,66 which further supports the theory that epithelial–stromal interaction mediates keratocyte activation. A surprising and novel finding after LASIK is the apparent loss of cells in the most anterior keratocyte layers, beginning at 6 months after surgery.62 The exact reason for this is poorly understood. It is now thought that there is a direct innervation of keratocytes by stromal nerve fibres.67 During LASIK, most of the stromal nerve trunks are cut – only those at the hinge are spared. Consequently, most of the keratocytes in the flap zone probably lose their neural input. Lack of communication with the sensory nerves may be the reason for the loss of the anterior-most keratocytes. However, it is important to remember that inconsistencies in this theory exist. Keratocyte loss is not observed until after 6 months, and innervation in LASIK is on the whole restored after 6 months.
In addition to morphometric data, confocal microscopy allows the clinician to view the morphological changes induced in the cornea as a result of refractive surgery. A common feature after LASIK, present in almost every eye, are microfolds.62 These typically appear in two forms, as a wavy unevenness in Bowman’s layer or as more prominent folds that extend into the anterior stroma. The latter variety might affect topography and result in irregular astigmatism. Flap particles are readily visible by confocal microscopy. The potential origin of the material in the flap interface includes metal from the microkeratome blade, fibres from swabs, lipids or inflammatory cells from the tear film, or epithelial particles carried into the interface by the microkeratome. Post-interface keratocyte morphology has been shown to be profoundly different between LASIK and PRK. Variations occur in extent and onset.64 In LASIK, the first change in keratocyte morphology is the presence of oval, brightly reflective keratocyte nuclei and thick cell processes behind the flap interface by 3 days. These changes are still present at 1–2 weeks, although the processes became thinner with time. Similar cells are present in the mid-stroma 1 month after PRK.64 Vesaluoma et al. suggested these might represent activated keratocytes.62 Abnormal reflective bodies were reported in all layers of the stroma after PRK.68 These are confined to ablated areas only. Two distinct forms are found, neither of which are visually significant. The first, the so-called rods, are long (≥50μm), slender (2–8μm in diameter) and dimly reflective. Rods sometimes contain bright, punctate, crystal-like inclusions, arranged linearly and at irregular intervals. The second variety, needles, are shorter (<25μm) and more slender (<1μm in diameter), but are highly reflective. Needles are composed of crystal-like granules in linear array, with an individual appearance similar to the bright punctate inclusions seen in rods, but more densely packed.
The incidence of needles and rods does not correlate to either the volume of tissue ablated or the length of post-operative interval. Although they are present throughout the stroma, there is a predominance of these entities in the anterior layers. Interestingly, in contact lens wearers, highly reflective granules, reminiscent of those from which needles are composed, are found scattered as isolated entities throughout the entire depth of the corneal stroma, but rods and needles are not encountered. Rods and needles are probably manifest some months after surgery and are not thought to be present in the early postoperative phase. Böhnke et al. suggested that rods represented the processes of keratocytes that have undergone chronic change during wound healing, whereby their light-scattering properties are enhanced.68 Such a modification could be attributed to an augmented synthetic activity, which indicates that stromal wound healing continues for a long time after the initial ablation (up to 3 years in Böhnke’s et al.’s study68). The highly reflective, crystal-like granules encountered sporadically within rods may represent lipofuscin granules. During the course of keratocyte degeneration and necrosis, the processes that are thought to constitute the rods may shrivel, shrink backwards to the perikaryon and eventually disappear, during which sequence of events the granules may be shunted against one another and thereby condense into a shorter length. This could account for the existence and morphological features of the needles. As part of this theory, rods and needles could be indicative of apoptotic activity. Another possibility is that rods could represent pathological collagen synthesized in response to corneal inflammation after surgery. Alternatively, rods and needles may represent accumulations of reflective material deposited in the corneal matrix, alongside the surfaces of collagen fibres.
References
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in the human cornea. Invest Ophthalmol Vis Sci. 28, 1719–1728. 7 Doshi S (1998). The Limbal Palisades of Vogt. PhD Thesis. (London: City University). 8 Schermer A, Galvin S and Sun T-T (1986). Differentiation-related expression of a major 64K corneal keratin in vivo and in culture suggests a limbal location of corneal epithelial stem cells. J Cell Biol. 103, 49–62.
1 Gipson IK, Spurr-Michaud S, Tisdale A and Keough M. (1989). Reassembly of the anchoring structures of the corneal epithelium during wound repair in the rabbit. Invest Ophthalmol Vis Sci. 30, 425–434. 2 Bergmanson JPG (1989). CCC or continuously changing cornea. Contact Lens J. 17, 10–14.
Corneal anatomy, physiology and response to wounding 9 Cotsarelis G, Cheng SZ, Dong G, Sun T-T and Lavker RM (1989). Existence of slowcycling limbal epithelial basal cells that can be preferentially stimulated to proliferate – implications on epithelial stem cells. Cell 57, 201–209. 10 Davanger M and Evensen A (1971). Role of the pericorneal papillary structure in the renewal of the corneal epithelium. Nature 229, 560–561. 11 Thoft RA, Wiley LA and Sundraj N (1989). The multipotential of cells at the limbus. Eye 3, 109–113. 12 Lauweryns B, Vandenoord JJ and Missotten L (1993). A new epithelial cell type in the human cornea. Invest Ophthalmol Vis Sci. 34, 1983–1990. 13 Alexander RA and Garner A (1983). Elastic and precursor fibres in the normal human cornea. Exp Eye Res. 36, 305–315. 14 Jakus MA (1961). The fine structure of the human cornea. In: The Structure of the Eye, Ed. Smelser GK (New York: Academic Press). 15 Chusid MJ, Nelson DB and Meyer LA (1986). The role of the polymorphonuclear leukocyte in the induction of corneal edema. Invest Ophthalmol Vis Sci. 66, 192–198. 16 Jakus MA (1964). Ocular fine structure. Selected electron micrographs. In Institute of Biology and Medical Science Monographs and Conferences, Vol. 1, Ed. Retina Foundation (London: Churchill). 17 Hirsch M, Nicolas G and Pouliquen Y (1989). Interfibrillary structures in fastfrozen deep-etched and rotary-shadowed extracellular matrix of rabbit corneal stroma. Exp Eye Res. 49, 311–315. 18 Scott JE (1992). Morphometry of cupromeronic blue-stained proteoglycan in animal corneas, versus that of purified proteoglycans stained in vitro, implies that tertiary structures contribute to corneal ultrastructure. J Anat. 180, 155–164. 19 Kangas TA, Edelhauser HF, Twining SS and O’Brien WJ (1990). Loss of stromal glycosaminoglycans during corneal edema. Invest Ophthalmol Vis Sci. 31, 1994–2002. 20 Hanna C and O’Brien JE (1961). Thymidine-tritium labelling of the cellular elements of the corneal stroma. Arch Ophthalmol. 31, 29–33 21 Murphy C, Alvarado J and Juster R (1984). Prenatal and postnatal growth of the human Descemet’s membrane. Invest Ophthalmol Vis Sci. 25, 1402–1415. 22 Tseng S, Smuckler D and Stern R (1982). Comparison of collagen types in the adult and fetal bovine corneas. J Biol Chem. 257, 2627–2633. 23 Fitch J, Gibney E, Sanderson R and Lisenmayer T (1982). Domain and basement membrane specificity of a monoclonal antibody against chick type IV collagen. J Cell Biol. 95, 641–647. 24 Von der Mark K, Von der Mark A, Timpl R and Trelstad R (1977). Immunofluorescent localisation of collagen type I, II and III in the embryonic chick eye. Dev Biol. 59, 75–85 25 Lisenmayer T, Fitch J and Mayne R (1984). Extracellular matrices in the developing avian eye. Type V collagen in corneal and non-corneal tissues. Invest Ophthalmol Vis Sci. 25, 41–47.
26 Svedbergh B and Bill A (1972). Scanning electron microscopic studies of the corneal endothelium in man and monkeys. Acta Ophthalmol. 50, 321–335. 27 Treffers W (1982). Corneal Endothelial Wound Healing (Nijmegen: Janssen Print). 28 Laing R, Neubauer L, Leibowitz H and Oak S (1983). Coalescence of endothelial cells in the traumatised cornea II. Clinical observations. Arch Ophthalmol. 101, 1712–1715. 29 Yue B, Sugar J, Gilboy J and Elvart J (1989). Growth of human corneal endothelial cells in culture. Invest Ophthalmol Vis Sci. 30, 248–253. 30 Hyldahl L (1986). Control of proliferation in the human embryonic cornea: An autoradiographic analysis of growth factors on DNA synthesis in endothelial and stromal cells in organ culture after explantation in vitro. J Cell Sci. 83, 1–21. 31 Ikebe H, Takamatsu T, Itol M and Fujita S (1984). Cytofluorometric nuclear DNA determination on human corneal endothelial cells. Exp Eye Res. 39, 497–504. 32 Lim CH and Ruskell GL (1978). Corneal nerve access in monkey. Graefe’s Klin Exp Arch Ophthalmol. 208, 15–23. 33 Matsuda H (1968). Electron microscopic study of the corneal nerve with special reference to its endings. J Physiol. 122, 367–391. 34 Stone RA and McGlinn AM (1988). Calcitonin gene-related peptide immunoreactive nerves in human and rhesus monkey eyes. Invest Ophthalmol Vis Sci. 29, 857–863. 35 Boberg-Ans J (1956). On the corneal sensitivity. Acta Ophthalmol. 35, 149–162. 36 Jalavisto E, Orma E and Tawast M (1951). Ageing and relation between stimulus intensity and duration in corneal sensibility. Acta Physiol Scand. 23, 224–233. 37 Sédan J, Farnarier G and Ferrand G (1958). Contribution à l’ etude de la keraesthésie. Ann Oculist. 191, 736–751. 38 Millodot M and Lamont A (1974). Influence of menstruation on corneal sensitivity. Br J Ophthalmol. 58, 752–756. 39 Millodot M (1972). Diurnal variation of corneal sensitivity. Br J Ophthalmol. 56, 844–877. 40 Millodot M (1975). Do blue-eyed people have more sensitive corneas than browneyed people? Nature 255, 151–152. 41 Yang B, Chen J and Wang Z (1998). Changes in corneal sensitivity after excimer laser corneal refractive surgeries. Chung Hua Yen Ko Tsa Chih 34, 50–52. 42 Perez-Santonja JJ, Sakla HF, Cardona C, Chipont E and Alio JL (1999). Corneal sensitivity after photorefractive keratectomy and laser in situ keratomileusis for low myopia. Am J Ophthalmol. 127, 497–504. 43 Matsui H, Kumano Y, Zushi I, Yamada T, Matsui T and Nishida T (2001). Corneal sensation after correction of myopia by photorefractive keratectomy and laser in situ keratomileusis. J Cataract Refract Surg. 27, 370–373. 44 Kauffmann T, Bodanowitz S, Hesse L and Kroll P (1996). Corneal reinnervation after photorefractive keratectomy and laser in situ keratomileusis: an in vivo study with a confocal videomicroscope. Ger J Ophthalmol. 5. 508–512.
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45 Clark BAJ and Carney LG (1971). Refractive index and reflectance of the anterior surface of the cornea. Am J Optom. 48, 333–338. 46 Maurice DM (1957). The structure and transparency of the cornea. J Physiol. 136, 263–286. 47 Maurice DM (1962). Clinical physiology of the cornea. Int Ophthalmol Clin. 2, 561–572. 48 Gyi TJ, Meek KM and Elliott GF (1988). Collagen interfibrillar distances in corneal stroma using synchrotron X-ray diffraction: A species study. Int J Biol Macromol. 10, 265–269. 49 Dayhaw-Barker P (1995). Corneal wound healing: II. The process. Int Contact Lens Clin. 22, 110–116. 50 Pfister RR (1975). The healing of corneal epithelial abrasions in the rabbit: A scanning electron microscopic study. Invest Ophthalmol Vis Sci. 14, 648–641. 51 Nassaralla BA, Szerenyi K and Pinheiro MN (1995). Prevention of keratocyte loss after corneal de-epithelialization in rabbits. Arch Ophthalmol. 113, 506–511. 52 Helena MC, Baerveldt F, Kim WJ and Wilson SE (1998). Keratocyte apoptosis after corneal surgery. Invest Ophthalmol Vis Sci. 39, 276–283. 53 Stulting RD, Thompson KP, Waring GO and Lynn M (1996). The effect of photorefractive keratectomy on the corneal endothelium. Ophthalmology 103, 1357–1365. 54 Patel SV, McLaren JW, Hodge DO and Bourne WM (2001). Normal human keratocyte density and corneal thickness measurement by using confocal microscopy in vivo. Invest Ophthalmol Vis Sci. 42, 333–339. 55 Erie JC, Patel SV, McLaren JW, et al. (2002). Effect of myopic laser in situ keratomileusis on epithelial and stromal thickness: A confocal microscopic study. Ophthalmology 109, 1447–1452. 56 Lohmann CP and Güell JL (1998). Regression after Lasik for the treatment of myopia: The role of the corneal epithelium. Semin Ophthalmol. 13, 79–82. 57 Spadea L, Fasciana R, Necozione S and Balestrazzi E (2000). Role of the corneal epithelium in refractive changes following laser in situ keratomileusis for high myopia. J Refract Surg. 16, 133–139. 58 Gartry DS, Kerr-Muir MG and Marshall J (1992). Excimer laser photokeratectomy. 18-month follow-up. J Cataract Refract Surg. 99, 1209–1219. 59 Gauthier CA, Epstein D and Holden BA (1995). Epithelial alterations following photorefractive keratectomy for myopia. J Refract Surg. 11, 113–118. 60 Dierick HG and Missotten L (1992). Is corneal contour influenced by a tension of the superficial epithelial cells? A new hypothesis. Refract Corneal Surg. 8, 54–59. 61 Reinstein DZ, Silverman RH, Sutton HFS and Coleman DJ (1999). Very high frequency ultrasound corneal analysis identifies anatomic correlates of optical complications of lamellar refractive surgery. Anatomic diagnosis in lamellar surgery. Ophthalmology 106, 474–482. 62 Vesaluoma M, Perez-Santonja J, Petroll WM, Linna T, Alió J and Trevo T (2000). Corneal stromal changes induced by myopic Lasik. Invest Ophthalmol Vis Sci. 41, 1447–1452.
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63 Del Pero RA, Gigstad JE and Roberts AD (1990). A refractive and histopathologic study of excimer laser keratectomy in primates. Am J Ophthalmol. 109, 419–429. 64 Møller-Pedersen T, Cavanagh HD, Petroll WM and Jester JV (2000). Stromal healing explains refractive instability after photorefractive keratectomy: A 1-year confocal microscopic study. Ophthalmology 107, 1235–1245.
65 Frueh BE, Cadez R and Böhnke M (1998). In vivo confocal microscopy after photorefractive surgery in humans. A prospective long-term study. Arch Ophthalmol. 116, 1425–1431. 66 Perez-Santonja JJ, Linna TU, Trevo KM, Sakla HF, Alio y Sanz JL, Trevo TM, et al. (1998). Corneal wound healing after laser in situ keratomileusis. J Refract Surg. 14, 602–609.
67 Müller L, Pels L and Vrensen GFJM (1996). Ultrastuctural organisation of human corneal nerves. Invest Ophthalmol Vis Sci. 37, 476–488. 68 Böhnke M, Thaer A and Schipper I (1998). Confocal microscopy reveals persisting stromal changes after nyopic photorefractive keratectomy in zero-haze corneas. Br J Ophthalmol. 82, 1393–1400.
4 Surgical procedures Sunil Shah, Mohammad Laiquzzaman and Stephen J Doyle
Although the idea that the ocular power of the human eye could be changed to correct ametropia has been around since ancient times, the modern concepts of refractive surgery were devised in Europe during the early part of 19th century and developed to its modern status in Japan and Russia.1–3 These concepts were based on the idea that modification of the corneal curvature could alter the refractive power of the eye and thereby help millions of people to be able to see without any visual aids. Radial keratometry (RK) has played a critical role in the development of refractive surgery. RK opened the window to the surgical correction of common refractive disorders. However, the relative safety and efficacy of excimer laser refractive surgery has brought this new field into the realm of everyday practice and made refractive surgery acceptable to the general public as well as to the ophthalmic profession. As about one-quarter of the world’s population have refractive errors,4 the potential population for treatment is huge.
History In the late 19th century, Lans showed experimentally that non-perforating radial incisions caused central corneal flattening accompanied by peripheral steepening.5 Greater central flattening was noted with deeper incisions. In the 1930s, Sato of Japan noted corneal flattening in several patients with keratoconus after spontaneous ruptures in Descemet’s membranes.6 Based on this concept, Sato performed RK in patients with keratoconus and successfully induced central corneal flattening.7 Enhanced flattening was achieved in the late 1940s by adding anterior radial incisions.7 Sato’s tech-
nique was modified by several Soviet ophthalmologists during the 1970s. They placed radial incisions in the anterior peripheral cornea only. Multifactorial formulas that incorporated patient and surgical variables were devised by Fyodorov to improve predictability.8,9 Millions of Americans underwent this procedure in the subsequent years. However, the procedure did not become popular in the UK or in many parts of Europe, which may, in part, be because in Europe excimer lasers were approved for refractive surgery many years before the Food and Drugs Administration (FDA) in the USA gave similar approval. In 1983, Trokel et al. discovered a new form of tissue–laser interaction. 10,11 Srinivasan, an engineer, was studying the far ultraviolet (193nm), argon fluoride (ARF) excimer laser for computer-chip photo-etching applications, when Trokel, an ophthalmologist, observed that corneal tissue could also be removed discretely and precisely with minimal damage to the adjacent corneal tissue. Trokel recognized the potential of the excimer laser to offer a new sculpting approach to corneal surgery. Photoablation occurs because the cornea has an extremely high absorption coefficient at 193nm, such that the 193nm photon has sufficient energy to break directly carbon–carbon and carbon–nitrogen bonds that form the peptide backbone of the corneal collagen molecule. Consequently, excimer laser radiation ruptures the collagen polymer into small fragments and a discrete volume of corneal tissue is removed with each pulse of the laser.12,13 The depth of the ablation per pulse is dependent on the radiant exposure, typically within the range 0.1–0.5μm per pulse at a radiant exposure of 50–250mJ/cm2.14,15
Table 4.1 Currently available refractive procedures Refractive keratotomy Arcuate or astigmatic keratotomy (AK) Photorefractive keratectomy (PRK) Laser epithelial keratectomy (LASEK) Laser in situ keratomileusis (LASIK) Intracorneal ring segment (ICRS) Corneal inlay lenses (CIL) Phakic intraocular lenses (phakic IOL) Clear lens extraction (CLE) Presbyopic surgery
During the early years, it was suggested that the excimer laser could be used as a ‘laser scalpel’ for corneal surgery in procedures such as RK.16 However, the excimer laser is a poor replacement for a cutting scalpel, because the laser removes tissue rather than incising it.17 The more promising application of the excimer laser is to re-shape the corneal curvature and thereby alter its refractive power.18 This new technique was termed photorefractive keratectomy (PRK) by Marshall et al. and Liu et al.18,19 McDonald et al. treated the first sighted human eye in 1989.20 The currently available refractive procedures are listed in Table 4.1.
Decision making for appropriate surgical procedures About 25% of the adult Caucasian population are myopic and 90% of these are –6D or less. As a rough guide, most low myopes (less than –6D) achieve within 0.5D of the goal and most higher myopes
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(–6 to –10D) achieve within 1D. Although surgery is less accurate for the higher myopes, the patients are often even more pleased, as they are effectively blind without glasses or contact lenses. The end results of PRK and laser in situ keratomileusis (LASIK) are the same in low prescription ranges.21 It is expected that the results from laser epithelial keratectomy (LASEK) will be similar. LASIK ‘gets there’ faster and with less patient discomfort than PRK or LASEK, whereas PRK and LASEK are essentially safer. Which procedure to choose depends on each patient’s attitude to risk versus convenience. Neither procedure should be used for myopia greater than about –10 D, as the optical zones carved on the cornea are too small for low light vision. One eventually simply runs out of cornea! If the cornea is thicker than average, more treatment is possible and, correspondingly, if it is thinner then less is viable. LASIK is better for the high myopes because of the speed of visual recovery and predictability. There is an ‘overlap’ area between –2 to –3D for which the pros and cons are about even. LASEK is a recent modification of PRK and can be the treatment of choice for patients with high myopia and a thin steep cornea, and in patients for whom LASIK is contraindicated.22
Excimer laser technology The term excimer comes from ‘excited dimer’ – a mixture of two inert gases that bind together to produce an unstable diatomic gas halide. The gases involved are from the halogen and noble gas groups. Krypton fluoride (KrF) lasers use an ultraviolet wavelength of 248nm and ArF lasers use an ultraviolet wavelength
>+4.5
+4.5 to +1
+1 to –2
CLE
LASIK
LASEK
of 193nm. Ultraviolet light is strongly absorbed by most biomaterials. At 193nm the laser-head photon energy is around 6.4 electron volts (eV), sufficient to break the corneal intermolecular bonds, which are about 3.6eV, without causing any thermal effects. The remaining energy is used to expel particles from the surface at supersonic speeds, but with no significant heating of the adjacent tissues. At wavelengths greater than 200nm, the thermal effects become more marked locally. Investigations of a range of excimer lasers have shown the ArF laser to produce the smoothest ablations of the corneal tissue, with minimal collateral damage from thermal diffusion. However, even at 248nm, the photons still cannot penetrate more than a few microns.23,24
Treatment plan Figure 4.1 gives a proposed plan for surgery favoured by the authors.
Individual surgical procedures Refractive keratectomy and PRK are not discussed separately as the authors feel these are essentially outdated procedures. PRK versus LASEK versus LASIK Shah et al. carried out a prospective, nonrandomized, comparative, paired-eye trial that comprised 72 eyes of 36 patients, using a Nidek EC-5000 excimer laser.25 The eyes were divided into two groups. The first eye of each patient was treated with 20% ethanol debridement and the second eye with an epithelial flap, which was replaced after treatment. After a mean follow up of 62.6 weeks, the final mean
–2 to –3
LASEK
spherical equivalent (MSE) was +0.07 ± 0.61D in the debridement group and –0.24 ± 0.43D in the epithelial flap group. There was no statistically significant difference between the two groups in the post-operative MSE. The best-corrected visual acuity (BCVA) was better in the epithelial flap group at all visits, a difference that was statistically significant (p < 0.05). The corneal haze was less in the epithelial flap group, and again the difference was statistically significant (p < 0.05). In another study, Anderson et al. also found better and quicker post-operative results, and most patients achieved a better correction for myopia and myopic astigmatism than achieved with LASIK, quicker epithelial healing and no or fewer complaints of pain.26 Serrati concluded that LASEK may prove superior to LASIK.27 Shahinian reported no serious or vision-threatening complications with LASEK,22 for a wider range of patients and with the elimination of stromal flap complications. LASEK Indications, absolute contraindications and relative contraindications for LASEK are given in Table 4.2. Overview LASEK is a relatively new technique that combines particular advantages of LASIK and of PRK, and is slowly gaining popularity. The technique is safe, the epithelial healing is faster with reduced stromal haze, and it has quicker post-operative recovery and minimum post-operative pain compared with PRK.22,25–29 The main rationale behind LASEK is to keep the corneal epithelium alive to prevent biochemical changes in the cornea, which can lead to haze formation. It is inherently safer than LASIK and so patients are attracted to this treatment.
–3 to –6
LASIK
LASIK
>–6
Thick cornea, consider for LASIK
LASEK + mitomycin 0.02%
Figure 4.1 Author’s treatment plan for an individual patient
Thin cornea unsuitable for LASIK
Phakic IOL >35 years
CLE >35 years
Surgical procedures
Surgical procedure The cornea is anaesthetized by topical anaesthetics. Usually, the non-operated eye is covered with an eye pad. The patient is made to lie on a couch and asked to focus on a flashing light. A lid speculum is inserted in the eye to be treated. A LASEK 8.0mm corneal trephine is used to create an epithelial incision. The circular blade is designed to perform a 270° incision with a blunt section at the 12 o’clock position for a hinge. A 9mm corneal ring is applied, which acts as a cup and is filled with 18% ethanol and left for 30 seconds. This 9mm corneal ring allows a 7.5mm treatment zone to be achieved, as the epithelium at the edges is still adherent. A flap can be raised in most eyes 20–25 seconds after the application of ethanol, but in some patients the epithelium is more adherent and needs more time. The ethanol is soaked up with a mercel sponge and the cornea washed with a topical nonsteroidal anti-inflammatory agent applied (diclofenac). An epithelial flap is fashioned by lifting (not debriding and not damaging the stromal bed) the edge of the loosened epithelium with a sharp beaver blade. The flap can be created horizontally or vertically, or the epithelium is cut in the centre and a flap is created in all four directions. Once the epithelial flap has been created, the
Table 4.2 Indications, absolute contraindications and relative contraindications for LASEK Indications Age 21 years and above Stable refraction Adequate central corneal thickness Myopia –3.00D to –6.00D Hyperopia up to +4.00D Astigmatism up to 4.00D Absolute contraindications Keratoconus Herpes virus infection of the cornea Deep corneal dystrophy Grossly amblyopic eye Corneal melt Unstable refraction Relative contraindications Significant cataract Certain occupations (pilots, computer programmers and heavy goods vehicle drivers, because contrast sensitivity and glare can be a handicap among these groups of patients) Patients with obsessive personality
corneal stroma is bare and laser is applied without delay, before the stroma dehydrates, as this might lead to overcorrection. The patient must be warned that the ablation usually produces a burning smell. After laser ablation the flap is replaced onto the cornea. A contact lens is then placed on the eye and removed after 4 days. This results in less pain and quicker visual recovery than for standard PRK. This procedure is especially beneficial for patients with small palpebral apertures, deep-set eyes, extremely flat or steep corneas, thin corneas or high myopia, as well as for patients who may not qualify for refractive surgery.28 Post-operative care Post-operative care includes topical antibiotics for 1 week. The patients are told to avoid swimming, contact sports, dust and smoke for about a month. Reviews of the patients are usually after 1 week, 6 weeks and 6 months. The vision gradually improves over a few days to a few weeks (at most) depending on the size of the ablation. Complications of LASEK As LASEK and PRK are essentially same procedures, the potential complications are the same.26 However, in the authors’ experience since using this technique, the incidence of complications is very low. The authors have not seen haze that affects visual acuity. The complications after LASEK can be classified into two broad groups: • Refractive; and • Miscellaneous. These are summarized in Table 4.3.
Table 4.3 Complications after LASEK Intra-operative Intra-operative loss of epithelial flap25 Refractive Early: • Induced irregular astigmatism • Primary undercorrection • Primary overcorrection Late: • Regression • Undercorrection • Overcorrection • Miscellaneous • Decentred ablation • Glare • Haloes • Ptosis • Infectious keratitis
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LASIK Overview LASIK was first performed by Pallikaris et al. in 1990,30 and is a combination of excimer laser with lamellar corneal surgery for the correction of refractive errors. LASIK is mainly carried out to correct myopia, but it is also used to correct astigmatism and hyperopia. Most refractive surgery in the USA is now LASIK. To achieve the desired refractive power the corneal thickness and shape are altered. The excimer laser is used to ablate the corneal stromal tissue to achieve the desired refractive change.30,31 Indications, absolute contraindications and relative contraindications for LASIK are given in Table 4.4. Surgical procedure The patient lies on a couch with the excimer laser delivery system above the patient’s head. The cornea of the eye to be operated is anaesthetized with topical anaesthetic drops. A lid speculum is inserted after instilling topical anaesthesia. The patient is asked to fixate on the laser bream and the cornea is marked with gentian violet to help realign the flap. A suction ring is applied to the limbus and the pressure increased to more than 65mmHg to
Table 4.4 Indications, absolute contraindications and relative contraindications for LASIK Indications Stable refraction (no change over a period of 2 years) Age ≥21 years Adequate central corneal thickness Myopia ≤–10.00D Hyperopia ≤+4.00 to 5.00D Astigmatism ≤6D Absolute contraindications Keratoconus Central corneal thickness <410μm Unstable refraction Deep corneal dystrophy Previous corneal melt (or systemic conditions predisposing to corneal melt) History of herpetic keratitis Amblyopia Relative contraindications Cataract Selected occupations (e.g., commercial pilots) Obsessive personality
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ensure a regular cut. This is confirmed using an applanation tonometer. The patient may feel a transient loss of vision because of increased intraocular pressure. An automated microkeratome is fitted on the track and activated to pass across the cornea to create stromal flap. The vacuum is released and the epithelial flap is reflected back to expose the stromal bed. The hinge of the flap is made, either nasally or at the 12 o’clock position. Pachymetry is repeated to ensure adequate residual tissue, and excimer laser ablation is carried out on the corneal stroma. The patients are warned that they might experience a pungent smell during laser ablation. The ablation usually takes less than 90 seconds. The flap is washed with balanced salt solution and replaced. Centration is checked and the edges are smoothed down. After checking the adhesion, the speculum is removed. Topical antibiotics and topical corticosteroid are prescribed for 1 week. Post-operative care The patient is directed to avoid swimming, dust or smoke and any contact sport for about 1 month after the surgery. A clear eye shield is worn during sleep for 2 weeks to avoid trauma while sleeping. The patient is examined after 1 day, 1 week, 1 month, 3 months, 6 months and 1 year. Complications Complications of LASIK can be divided into two broad groups, intra-operative and post-operative, which can be further subdivided as early and late. Flap related complications The intra-operative flap complications include incomplete or free (completely cut) flap, lost flap, decentred flap, irregular flap and flap stria.32 However, Jacobs and Taravella, in a study on 84,711 eyes, conclude that overall flap complications are very low (0.3%).33 Late complications are epithelial ingrowth (epithelium within the stromal interface, one of the most common causes of reduced visual acuity),32 wrinkles or striae, interface infection and flap dislocations.34,35 Refractive complications These include under- or overcorrection, regression, decentred ablation and induced irregular astigmatism caused by folds or microstriae of flaps.32 This is often difficult to correct and results in decrease in visual acuity and/or quality of vision. Other complications are given in Table 4.5.
Clinical outcomes LASEK Predictability Claringbold conducted a study in 222 eyes with myopia that ranged from –1.25 to –11.25D and astigmatism up to 2.25D.29 Of these, 84 eyes had a 1 year follow-up, of which 82.0% had an uncorrected visual acuity (UCVA) of 20/20 or better and 100% had an UCVA of 20/25 or better. In another study, 343 eyes with refractive errors that ranged from –1.00 to –14.00D and astigmatism up to +4.75D were followed up for 6 months. Of these patients, 98% had unaided visual acuity of 20/40 or better.26 Shahinian reported, in a study of 146 eyes with myopia that ranged from –1.00 to –14.38D, that the UCVA was 20/40 or better in 96% of the eyes after a 12 month follow-up.22 Stability O’Bart, in a study on 105 eyes, reported that refractive stability was rapid with a mean refractive change between 1 week and 6 months post-operatively of ±0.34D.37 Claringbold reported that all eyes achieved ±0.75D of the intended correction and more than 96% of the eyes were within ±0.5D after 12 months.29 Loss of uncorrected visual acuity In a study of 222 eyes with myopia that ranged from –1.25 to –11.5D, UCVA was 20/40 or better after 4 days in more than 80% and 20/20 or better in 75% after 2 weeks.29 Loss of best-corrected visual acuity Claringbold in a study on 222 eyes reported no loss of BCVA.29
Table 4.5 Non-refractive complications after LASIK Central islands36 Interface debris32 Haze Glare and haloes34 Infectious keratitis (rare)32,34 Diffuse interstitial keratitis (sands of Sahara)32 Dry eye34 Endothelial cell loss34 Night-vision problems36 Reduction in corneal sensitivity34 Posterior ectasia29
LASIK for myopia Predictability Pop and Payette reported a study of 107 LASIK-treated myopic eyes with refractive error that ranged from –1.00D to –9.00D.21 Of these eyes, 70% (77 eyes) were evaluated 12 months post-operatively, of which 100% had UCVA of 20/40 or better and 83% achieved UCVA of 20/20 or better. In another study of 290 highly myopic eyes (range from –9.00 to –22.00D), the UCVA was 20/40 or better in 73.3% after 1 month.38 Stability In 131 eyes with high myopia (range from –9.00 to –22.00D), overall most scores were stable or improved between early and late follow-ups. In 88% of the eyes, UCVA was stable or improved after 1 month and in 95% of the eyes BCVA was stable or improved after 1 month.38 Loss of best-corrected visual acuity Pop and Payette reported that after 1 month of surgery 90% of the eyes were within ±1.00D and 64% of the eyes were within ±0.5D of BCVA.21 After 12 months, 99% were within ±1.00D and 78% were within ±0.5D. No eye lost two Snellen lines of BCVA. LASIK for hyperopia Predictability LASIK can be used reasonably successfully to treat low hyperopia. Cobo-Soriano et al. conducted a study of 376 hyperopic eyes (range from +1.00D to +8.50D), for which a mean post-operative refraction of +0.46 ± 0.8D was achieved after a follow up of 8.2 months.39 In eyes with an error of ≤+4.00D, the final UCVA was 20/40 in 96%, and 88% in patients with >+4.00D. In another study on 54 hyperopic eyes (range +1.00D to +6.00D), Lian et al. reported that predictability was good after 12 months: 83% eyes were within +1.00D and 66% achieved +0.5D.40 Loss of uncorrected visual acuity Lian et al. also reported that 92.6% of the eyes had UCVA of 20/40 or better and 63% had 20/20 or better.40 One eye lost two lines of BCVA and two eyes gained two or more lines. LASIK versus LASEK This topic is covered in more detail in Chapter 8, but here it is sufficient to say that in some prescription groups the end results of LASIK and LASIK are the similar. Claringbold suggests that LASEK appears to be safe and more effective than
Surgical procedures
LASIK in that complications related to the stromal flap are eliminated and it can be performed in patients for whom LASIK may be contraindicated (e.g., deep-set eyes, thin corneas, etc.).29 However, LASEK has some disadvantages with respect to LASIK: • Patients experience varying degrees of pain during the first 2 days after surgery; • Recovery of vision is slower, as vision is somewhat blurred for the first week after LASEK surgery; and • Patients may have mild recurrent epithelial erosion and so require postoperative corticosteroid for a longer period than required after LASIK. Intracorneal ring segments Overview Intracorneal ring segments (ICRS) is a procedure based on the assumption that the refractive error can be corrected by flattening the cornea using tissue added to the outer two-thirds of cornea. This extra tissue in the peripheral cornea distends the cornea, which in turn flattens the central cornea.41 This technique is used to correct low myopia and astigmatism. In this procedure half-ring segments of Perspex are inserted into channels created in the corneal stroma, which results in a flattening of cornea. The advantage is that the central cornea is not involved and the ring is positioned outside the pupillary margin. This process is easily reversible and the corneal shape remains intact. The indications for ICRS are low grade myopia <4.5D and keratoconus. Surgical procedure The operation is carried out under sterile conditions. The geometrical centre of the cornea is marked and intra-operative ultrasonic pachymetry carried out at the sight of incision. The diamond blade is set at 70% of the measured corneal thickness to create a single radial incision that is less than 2.0mm at the steepest meridian. A stromal pocket is dissected on both sides of the incision using a modified spatula. The intrastromal dissection is created to the full depth of the incision. Either a suction device is used to dissect a stromal plane to create semicircular lamellar pockets or this can be carried out manually.42 After removal of the suction device, two intracorneal rings of different thicknesses are inserted into each semicircular channel. Selection of the rings is based on the refractive error. Finally, the radial incision is sutured with nylon sutures. Post-operative antibiotics and hydrocortisone are
given to minimize the risk of keratitis. The advantages and disadvantages are given in Table 4.6, but the procedure is not popular among UK surgeons, Holmium laser thermokeratoplasty and diode thermokeratoplasty Holmium laser thermokeratoplasty (LTK) and diode thermokeratoplasty (DTK) are used to correct hypermetropia. In both of these procedures an infra-red laser is used to coagulate the cornea. Spots are arranged in a ring 6–9mm from the centre of cornea, and as the scar tissue forms the central cornea steepens. Charpentier et al. reported that the stability of refractive outcome is poor.44 Phakic intraocular lenses Overview The word ‘phakic’ is derived from the Greek phakos, which means lens. Phakic intraocular lenses (PIOLs) are artificial lenses placed inside the eye to correct refractive error such as myopia and hyperopia. For years, intraocular lenses (IOLs) have been used during cataract surgery after removal of the natural crystalline lens. More recently, IOLs were designed to be placed in the eye to correct refractive error without removal of the natural lens. Additional IOLs are used to treat refractive errors. This surgical procedure is generally carried out to treat high refractive errors for which corneal surgery cannot be performed. In this technique, lenses made of polymethylmethacrylate or colorate (which is a soft lens made of collagen, water and polymers) are placed inside the eye. The lens is wedged between the posterior surface of the cornea and the anterior surface of the iris, or is attached to the
Table 4.6 Advantages and disadvantages of intracorneal ring segments Advantages Corneal shape is not disturbed Centre of the cornea is not touched Process is reversible Surgical procedure is safe Adjustment can be performed using thinner or thicker rings41 Useful in keratoconus eyes Predictability of surgical outcome is good44 Disadvantage Can only be used in low myopia (e.g., <4.50D)
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anterior surface of the iris by a clip, or is placed in the space between the posterior surface of the iris and the anterior surface of the natural lens of the eye. The indications are: • Myopia >–5.00D;45 • Hyperopia >+5.00 to +15.00D;45 • Thin corneas; • Previous refractive keratotomy surgery. Surgical procedures Anterior chamber lens implantation The surgery is carried out under sterile conditions to avoid intraocular infection. The pupil is dilated with mydriatics and anaesthesia, either topically or with peribulbar anaesthetics. A temporal corneal incision of about 3–3.5mm is made with the diamond blade. Sodium hyaluronate is injected into the anterior chamber to deepen it. The lens is implanted into the anterior chamber, the haptic ends are placed under the iris with a spatula and the lens is centred. Peripheral iridectomy is performed to avoid blockage by the peripheral haptic. The viscoelastic material is removed by either irrigation or aspiration with balanced solution. Postoperative antibiotic and corticosteroid drops are given for 5–7 days. Posterior chamber lens implantation The pupil is dilated with mydriatics and the eye to be treated is anaesthetized with peribulbar anaesthetics. A temporal or nasal corneal incision of about 3–3.5mm is made with the diamond blade. The silicone IOL is implanted in front of the natural crystalline lens, under the protection of a viscoelastic substance. No suture is necessary. A peripheral iridectomy is performed, either intra-operatively or by laser after surgery. At the end of the surgical procedure, gentamicin and corticosteroid are given topically or both topically and subconjunctivally. The advantages and disadvantages of PIOL procedures are given in Table 4.7. Surgical outcome, anterior chamber lens implant Loss of best-corrected visual acuity Hoyos et al. reported for anterior chamber lens implantation a mean BCVA in myopic eyes of 20/35 and in hyperopic eyes of 20/23 after 1 year in a study on 31 eyes (17 myopic and 14 hyperopic, myopia ranged from –11.8 to –26.00D and hyperopia from +5.25 to +11.00D).45 In myopic eyes, no eye lost lines of acuity, and in hyperopic eyes one eye gained one line of BCVA and one eye lost one line.
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Table 4.7 Advantages and complications of phakic intraocular lenses Advantages Preservation of accommodation Compatibility with proved cataract and phakic IOL implantation procedures Correction of higher levels of myopic and hyperopic refractive errors Reversibility46–48 Complications Post-surgical astigmatism Secondary glaucoma (major complication of the anterior chamber lens) Chronic intraocular inflammation Pigment dispersion Uveitis Endothelial cell damage Cataract formation Endophthalmitis Glare and poor-quality vision at night with a wider pupil
Predictability After 1 year follow-up, the MSE of refraction was –0.22 ± 0.87D in myopic eyes, with 87% within the desired refraction of ±1.00D; in hyperopic eyes the MSE was +0.38 ± 0.82D, with 79% within the desired refraction of ±1.00D.45 Surgical outcome, posterior chamber lens implant Loss of best-corrected visual acuity Brauweiler et al. evaluated 18 eyes with high myopia (pre-operative MSE –14.58 ± 3.04D).49 BCVA remained unchanged in one eye or improved by two lines or better, and three eyes lost one line of BCVA. Predictability After 2 years follow-up the MSE was –1.33 ± 0.71D. Clear lens extraction Overview Various treatments for patients with high refractive errors have been used in the past (e.g., glass spectacles, contact lenses, etc.), but the higher the refractive error the higher the dissatisfaction with these traditional treatment methods. During the past two decades refractive surgery has made much progress and become popular. More and more patients with refractive error seek life without these traditional
methods of treatment (e.g., glass and contact lenses), and refractive surgery results are promising in terms of rapid recovery and safety.29 The indications are: • High myopia >6.00D; and • Hyperopia. Surgical procedure This surgical procedure is similar to a cataract operation, the only difference being that the natural crystalline lens is removed even though it is not opaque, and an artificial lens is implanted. The IOL’s strength is calculated such that when it replaces the crystalline natural lens the required refractive power is achieved. The complications of clear lens extraction are given in Table 4.8. Results Loss of best-corrected visual acuity Usitalo et al. reported that, for 38 eyes, 71.9% gained one or more lines and 40.6% gained two or more lines in their study of highly myopic eyes (range from –7.75D to –29.00D), and 6.2% lost one line of BCVA after 1 year.50 Accuracy In the same 38 eyes, the spherical equivalent refraction was within ±1.00D in 81.6% and within ±0.5D in 71.1%, and in eyes with myopia <–18.0D refraction of within ±1.00D was achieved in 96.4% and within ±0.5D in 85.7%.50 In another study by Pop et al. of 65 eyes with hyperopia up to +12.25D, 1 month after clear lens extraction the BCVA was 20/40 or better in 95% of eyes and 20/20 or better in 38.5%.51 Long-term safety The short-term results are very promising, and long-term safety is as for cataract surgery. Presbyopic surgery Overview Accommodation is the mechanism by which the curvature of the anterior surface of a crystalline lens increases, and it produces the optical power of the lens.52 In a relaxed state the suspensory ligament, which is attached to the lens and ciliary muscle, is in tension and so stretches the lens and keeps it flatter. However, during accommodation the ciliary muscle contracts, which in turn reduces the tension of the suspensory ligament and allows the anterior surface of the lens to move towards the cornea. The change in the curvature of lens
Table 4.8 Complications of clear lens extraction Post-surgical astigmatism Chronic intraocular inflammation Posterior capsular opacity Endothelial cell damage Uveitis Endophthalmitis Glare
occurs in the centre, and the peripheral part flattens. With advancing age the lens material reduces or loses its elasticity, which results in a reduction or loss of forwards movement of the lens and finally in loss of accommodation. This condition is known as presbyopia. Surgery for presbyopia is in its infancy. It can be either corneal, scleral or an IOL implant using a multifocal or accommodative lens. The lens is implanted after cataract surgery, on the assumption that movement of the vitreous gel behind the lens will create the desired refraction.53 Surgical procedure Corneal surgery A multifocal cornea is created under a LASIK flap by steepening the cornea inferior or by implanting a multifocal intracorneal inlay. Scleral surgery Scleral surgery can be carried out by surgical incision or laser. The required result is expected to be achieved by creating a multifocal cornea. Intraocular An intraocular procedure was first described in 1997, and is called presbyopic lens exchange (PRELEX).53 IOLs are used to restore accommodation at the time of cataract surgery.52 Two types of lenses are used mainly: • Accommodative lenses are singlepower optic lenses. The theory is that this will mimic the natural physiology of the eye, whereby relaxation and contraction of the ciliary muscle will result in a change in the power of the lens. • Multifocal lenses comprise two main types, refractive and defractive multifocal IOLs. Use of these lenses to treat presbyopia has been approved by the FDA. The complications of presbyopic surgery are given in Table 4.9.
Surgical procedures
Table 4.9 Complications of presbyopic surgery Dislocation of lens Long-term refractive stability Lens decentration Fibrosis of the lens capsule that resultsin loss of forwards movement of the implanted lens Glare Haloes Post-operative refractive errors Surgically induced astigmatism
References 1
2
3
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5
6 7 8
9 10 11 12
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Akiyama K, Shibata H, Kanal A, et al. (1992). Development of radial keratotomy in Japan, 1939–1960. In Refractive Keratotomy for Myopia and Astigmatism, p. 179–220, Ed. Waring GO III. (St Louis: Mosby–Yearbook Inc). Schimmelpfennig BH and Waring GO (1992). Development of refractive keratotomy in the nineteenth century. In Refractive Keratotomy for Myopia and Astigmatism, p. 171–178, Ed. Waring GO III. (St Louis: Mosby–Yearbook Inc). Waring GO (1992). Development of radial keratotomy in the Soviet Union, 1960–1990. In Refractive Keratotomy for Myopia and Astigmatism, p. 221–236, Ed. Waring GO III. (St Louis: Mosby–Yearbook Inc). Spertduto RD, Seigel D, Roberts J and Rowland M (1983). Prevalence of myopia in the United States. Arch Ophthalmol. 101, 405–407. Lans W (1898). Experimentelle Untersuchungen uber Entstehung von Astigmatismus durch nicht-perforirende corneawunden. Graefes Arch Clin Exp Ophthalmol. 45, 117–152. Sato T (1939). Treatment of conical cornea (incision of Descemet’s membrane). Acta Soc Ophthalmol Jpn. 43, 544–555. Sato T, Akiyama K and Shimbata H (1953). A new surgical approach to myopia. Am J Ophthalmol. 36, 823–829. Fyodorov SN and Durnev VV (1979). Operation of dosaged dissection of corneal circular ligament in cases of myopia of mild degree. Ann Ophthalmol. 11, 1885–1890. Enaliev FS (1978). Experience in surgical treatment of myopia. Vestn Oftalmol. 3, 52–55. Trokel SL, Srinivasan R and Braren B (1983). Excimer laser surgery of the cornea. Am J Ophthalmol. 96, 710–715. Srinivasan R (1986). Ablation of polymers and biological tissue by ultraviolet lasers. Science 234, 559–565. Puliafito CA, Wong K and Steinert RF (1987). Quantitative and ultrastructural studies of excimer laser ablation of the cornea at 193 and 248nm. Lasers Surg Med. 7, 155–159. Srinivasan R and Sutcliffe E (1987). Dynamics of the ultraviolet laser ablation of corneal tissue. Am J Ophthalmol. 103, 470–471.
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Arcuate surgery Arcuate keratotomy can be used to treat corneal astigmatism before and after cataract surgery. Several nomograms are available for the incisional keratotomy to
correct naturally occurring astigmatism. However, this is not the case for eyes with secondary astigmatism. Oshika et al. designed a prospective, multicentre study that involved 104 pseudophakic eyes with a corneal astigmatism of 1.50D or more.54 All these patients were treated with arcuate keratotomy incisions. The parameter of predictability (35%) was lower than that reported for congenital astigmatism (56%).54
14 Amano S and Shimizu K (1995). Excimer laser photorefractive keratectomy for myopia – two years follow-up. J Refract Surg. 11, S253–S260. 15 Krueger RR, Trokel SL and Schubert HD (1985). Interaction of ultraviolet laser light with the cornea. Invest Ophthalmol Vis Sci. 26, 1455–1464. 16 Cotliar AM, Schubert HD, Mandel ER and Trokel SL (1985). Excimer laser radial keratotomy. Ophthalmology 92, 206–208. 17 Marshall J, Trokel SL, Rothery S and Schubert H (1985). An ultrastructural study of corneal incisions induced by an excimer laser at 193 nm. Ophthalmology 92, 749–758. 18 Marshall J, Trokel SL and Rothery S (1986). Photoablative reprofiling of the cornea using an excimer laser–photorefractive keratectomy. Lasers Ophthalmol. 1, 21–48. 19 Liu JC, McDonald MB, Varnell R and Andrade HA (1990). Myopic excimer laser photorefractive keratectomy: An analysis of clinical correlations. Refract Corneal Surg. 6, 321–328. 20 McDonald MB, Kaufman HE and Frank JM (1989). Excimer laser ablation in the human eye. Arch Ophthalmol. 107, 641–642. 21 Pop M and Payette Y (2000). Photorefractive keratectomy versus laser in-situ keratomileusis: A controlmatched study. Ophthalmology 107, 251–257. 22 Shahinian L (2002). Laser-assisted subepithelial keratectomy for low to high myopia and astigmatism. J Cataract Refract Surg. 28, 1334–1342. 23 Dagenhardt AH (1976). Light coagulation of the eye. Br J Physiol Opt. 31, 11–18. 24 Kerr-Muir MG, Trokel SL, Marshall J and Rothery S (1987). Ultrastructural comparison of conventional surgical and argon fluoride excimer laser keratectomy. Am J Ophthalmol. 103, 448–453. 25 Shah S, Sarhan AS, Doyle SJ, Pillai CT and Dua HS (2001). The epithelial flap for photorefractive keratectomy. Br J Ophthalmol. 85, 393–396. 26 Anderson NJ, Beran RF and Schneider TL (2002). Epi-LASEK for the correction of myopia and myopic astigmatism. J Cataract Refract Surg. 28, 1343–1347.
27 Scerrati E (2001). Laser in situ keratomileusis versus laser epithelial keratomileusis (LASIK vs LASEK). J Refract Surg. 17, S219–S221. 28 Dastjerdi MH and Soong HK (2002). LASEK (laser subepithelial keratomileusis). Curr Opin Ophthalmol. 13, 261–263. 29 Claringbold II TV (2002). Laser assisted subepithelial keratectomy for the correction of myopia. J Cataract Refract Surg. 28, 18–22. 30 Pallikaris IG, Papatzanaki ME, Siganos DS and Tsillimbaris MK (1991). A corneal flap technique for laser in situ keratomileusis. Human study. Arch Ophthalmol. 109, 1699–1702. 31 Burrato I and Ferrari M (1992). Excimer laser intrastromal keratomileusis; Case reports. J Cataract Refract Surg. 18, 37–41. 32 Stephenson C (2002). Complications of PRK, LASIK and LASEK: Diagnosis and treatment. Refract Eye News 1, 6–11. 33 Jacobs JM and Taravella MJ (2002). Incidence of intra-operative flap complications in laser in-situ keratomileusis. J Cataract Refract Surg. 28, 23–28. 34 Oliveira-Soto L and Charman WN (2002). Some possible longer-term ocular changes following excimer laser refractive surgery. Ophthalmic Physiol Opt. 22, 274–288. 35 Sachdev N, McGhee CN, Craig JP, Weed KH and McGhee JJ (2002). Epithelial defect, diffuse lamellar keratitis, and epithelial ingrowth following post-LASIK epithelial toxicity. J Cataract Refract Surg. 28, 1463–1466. 36 Farah SG, Azar DT, Gurdal C and Wong J (1998). Laser in situ keratomileusis: Literature review of a developing technique. J Cataract Refract Surg. 24, 989–1006. 37 O’Bart D (2002). Laser epithelial keratomileusis (LASEK). Refract Eye News 1, 12–15. 38 Kawesch GM and Kezirian GM (2000). Laser in situ keratomileusis for high myopia with VISX star laser. Ophthalmology 107, 653–661. 39 Cobo-Soriano R, Llovet F, González-Lopez F, Domingo B, Gomez-Sanz F and Baviera J (2002). Factors that influence outcomes of hyperopic laser in situ keratomileusis. J Cataract Refract Surg. 28, 1530–1538.
Surgical outcome A study carried out on 456 patients treated with bilateral multifocal lens implantation reported 81% could function without glasses.52
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40 Lian J, Ye W, Zhou D and Wang K (2002). Laser in situ keratomileusis for correction of hyperopia and hyperopic astigmatism with the Technolas 117C. J Refract Surg. 18, 435–438. 41 Alio JL, Salem TF, Artola A and Osman AA (2002). Intracorneal rings to correct corneal ectasia after laser in situ keratomileusis. J Cataract Refract Surg. 28, 1568–1574. 42 Siganos D, Ferrara P, Chatzinikolas K, Bessis N and Papastergiou G (2002). Ferrara intrastromal corneal rings for the correction of keratoconus. J Cataract Refract Surg. 28, 1947–1951. 43 Asbell PA and Ucakhan OO (2001). Long term follow up of Intacs from a single center. J Cataract Refract Surg. 27, 1456–1468. 44 Charpentier DY, Nguyenkhoa JL, Duplessix M, Colin J and Denis P (1995). Intrastromal thermokeratoplasty for correction of spherical hyperopia – one year
45
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47
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prospective-study. J Fr Ophtalmol. 18, 200–206. Hoyos JE, Dementiev DD, Cigales M, Hoyos-Chacon J and Hoffer KJ (2002). Phakic refractive lens experience in Spain. J Cataract Refract Surg. 28, 1939–1946. Baikoff G, Arne JL, Bokobza Y et al. (1998). Angle-fixated anterior chamber phakic intraocular lens for myopia of –7 to –19 diopters. J Refract Surg. 14, 282–293. Rosen E and Gore C (1998). Staar Collamer posterior chamber phakic intraocular lens to correct myopia and hyperopia. J Cataract Refract Surg. 24, 596–606. Landesz M, Worst JGF, Siertsema JV and van Rij G (1995). Correction of high myopia with the Worst claw intraocular lens. J Refract Surg. 11, 16–25. Brauweiler PH, Wehler T and Busin M (1999). High incidence of cataract formation after implantation of a silicone posterior chamber lens in
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phakic, highly myopic eyes. Ophthalmology 106, 1651–1655. Uusitalo RJ, Aine E, Sen NH and Laatikainen L (2002). Implantable contact lens for high myopia. J Cataract Refract Surg. 28, 29–36. Pop M, Payette Y and Amyot M (2001). Clear lens extraction with intraocular lens followed by photorefractive keratectomy or laser in situ keratomileusis. Ophthalmology 108, 104–111. Hope-Ross M. (2002). Lens surgery and presbyopia: Refract Eye News 1, 11–18. Chisholm C (2002). Report on the Current Status of Refractive Surgery. (Birmingham: British Society of Refractive Surgery). Oshika T, Shimazaki J, Yoshitomi F et al. (1998). Arcuate keratotomy to treat corneal astigmatism after cataract surgery: A prospective evaluation of predictability and effectiveness. Ophthalmology 105, 2012–2016.
5 Post-operative follow-up of the refractive surgery patient Catharine Chisholm
Patients who have undergone any form of refractive surgery procedure require careful follow-up, particularly during the first year. In some clinics the operating surgeon undertakes all such examinations, but it is increasing likely that optometrists will be called upon to share the ever-increasing workload. The optometrist may be involved merely in refraction and topography measurements, or may have to undertake the full examination, particularly in cases for which the surgeon is not on site. Opinions vary considerably as to the point at which care of a patient can be passed from the surgeon to an optometrist, although 3 months appears to be a common handover point to general optometrists who do not specialize in postrefractive surgery patient care. It is important to clarify with whom the responsibility for the patient lies – this will vary depending on the co-management set up, which is discussed in Chapter 7. If time permitted, many surgeons would prefer to see the patient right up until the point of discharge to maintain continuity of care and collect outcome data. This allows surgeons to audit their own performance and modify their techniques accordingly. For this reason, it is important that optometrists involved in the assessment of refractive surgery patients provide feedback after each assessment, which should include details such as residual refractive error, uncorrected and best-correction vision, slit-lamp findings, symptoms, etc. The surgeon or a designated ophthalmological colleague must be contactable at all times in case problems are detected during a followup examination. Referral to the Hospital Eye Service should remain the last resort. Once the refractive error and topography have stabilized, the cornea is quiet and any visual problems have been dealt with, the patient can be discharged back to their
own optometrist with a letter that provides details of the surgery and outcome. Optometrists involved in refractive surgery co-management are responsible for educating the patients on the importance of regular eye examinations. Patients will still need reading glasses when they reach presbyopia and the health of their eyes should be checked at least every 2 years, as for any other patient. Occasionally, non-presbyopic patients require a small residual correction for certain critical tasks, which again can be provided by the optometrist.
Initial post-operative period The primary purpose of follow-up examinations during the early post-operative period is to recognize and manage acute problems, such as infections, slipped corneal flaps, etc. Over the longer term, examinations should include the investigation of refractive and topographical stability, address any visual problems and refer patients back for enhancements where necessary. Table 5.1 summarizes time scales for follow-up examinations.
Table 5.1 Suggested time scale for follow-up examinations Follow-up time
Personnel
Primary purpose
Immediately post-operative
Surgeon
Check flap position and integrity (LASIK and LASEK)
1 week
Surgeon and optometrist
Check flap (LASIK and LASEK); check for epithelial closure (PRK)
1 month
Surgeon and optometrist
Look for epithelial ingrowth (LASIK) Haze maximum after PRK Assess initial refractive outcome
3 months
Optometrist (and surgeon?)
Full examination Consider enhancement post-LASIK if required and refraction stable
6 months
Optometrist (and surgeon?)
Full examination Consider enhancement post-PRK if stable
12 months
Optometrist (and surgeon?)
Full examination Discharge to general optometric practice if no problems Information letter to patient’s own optometrist
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Photorefractive keratectomy Some degree of aqueous flare is present in a large proportion of eyes during the first 24–48 hours.1 Patients can suffer quite severe pain and photophobia caused by the large epithelial wound and will need to use systemic painkillers during the first 24 hours, in addition to topical medication [antibiotics and non-steroidal antiinflammatory drugs (NSAIDs)]. Bandage contact lenses can be fitted to manage pain, but are rarely used after photorefractive keratectomy (PRK). At 1 week Re-epithelialization occurs within 4.6 ± 0.2 days of PRK (range 3–6 days).2 Epithelial cells from the margin of the wound migrate and proliferate to form a single layer of cells across the central cornea. Once this stage has been completed, mitosis gradually increases the number of layers to form a stratified epithelium. Functional vision returns upon re-epithelialization, with 83% of low myopes (–1.00 to –5.99D) achieving an unaided vision of 6/12 by 1 week.3 The initial refraction tends to be slightly hyperopic followed by a gradual drift towards emmetropia or myopia. Irregularities of both the refraction and topography are common at this stage. A significant epithelial defect is visible without fluorescein staining, but if this has to be used the eye should be irrigated thoroughly afterwards to remove any remaining dye. Slow re-epithelialization tends to be associated with greater haze and regression in the longer term. By 1 week, there should be no pain, although mild grittiness may persist. If corticosteroids have been prescribed and the epithelium is intact, careful tonometry can be undertaken to detect steroid responders. At 1 month Subepithelial haze develops during the first month, and reaches a peak in intensity between 6 and 12 weeks.4,5 The formation of haze is a process of tissue remodelling that involves corneal basal epithelial cells,
activated keratocytes and the deposition of type III collagen within the stroma.6 Haze is visible because the cornea both reflects and scatters light back towards the observer. Haze should be graded as shown in Table 5.2. Grade 0.5–1.5 haze is not uncommon at 1 month and may be associated with a reduction in low-contrast acuity. Haze does not influence Snellen acuity unless it reaches grade 2 or more, but it can affect post-operative Orbscan data corneal thickness measurements should be interpreted with caution in the presence of significant haze.8 At 3 months There should be little if any stromal haze by 3 months and best-corrected visual acuity (BCVA) should have returned to pre-operative levels. The refractive error should have regressed from low hyperopia to near emmetropia and some individuals may even show signs of refractive and topographical stability. If significant myopic regression is going to occur, it is generally evident by 3 months post-PRK. At 6 months and beyond Since PRK is now reserved for treatments of –4.00D or less, refractive stability is generally achieved within 6 months of surgery. Cellular activity ceases at around 3 months, but long-term healing processes continue for up to 18 months post-operatively. The time course of this activity correlates with an initial reduction in visual performance, associated with changes in the number, size and density of the stromal keratocytes.9,10 Stromal haze rarely persists beyond 12 months.5,7 Refractive outcome By 12 months, 87–99% of low and medium myopes (<–6.00D) are within ±1D of emmetropia. Enhancement procedures can be performed to correct residual refractive error, but predictability is not as good as for the initial procedure. Diurnal variations that result from PRK are clinically insignificant and any shift tends to be in the hyperopic
Table 5.2 Grading of haze post-PRK7 Grade of haze
Description
0 0.5 1 1.5 2 3 4 5
Clear Haze barely detectable Mild haze, refraction unaffected Mild haze that affects refraction Moderate haze, refraction difficult, high-contrast vision affected Opacity prevents refraction, vision impaired, anterior chamber visible Opacity impairs view of anterior chamber Unable to see anterior chamber
direction, and therefore has little or no impact on vision.11,12 The reduction in atmospheric pressure and reduced oxygen levels found at high altitude have been shown to result in temporary, but significant, peripheral corneal thickening in PRK subjects. This does not appear to be associated with any refractive shift.13 Complications specific to PRK Persistent haze Up to grade 1.5 haze (on a 0–4 scale) is expected during the first 2–3 months postPRK, but more significant haze may develop in those treated for higher refractive errors and in those with darker irides.14 Intense and persistent haze may require the use of topical corticosteroids. Research into wound healing post-PRK suggests that anti-transforming growth factor-B (TGFB) and mitomycin may help to prevent and treat stromal haze in the future.15 Anisometropia Most clinics and surgeons perform PRK on patient’s eyes unilaterally. It is usual to have an interval of a few months between the PRK for each eye, during which time some patients find the level of anisometropia between the eyes uncomfortable. This is especially difficult immediately prior the second operation, as the patient will have to cease contact lens use in preparation for the next operation. Epithelium irregularity PRK involves removal of the corneal epithelium pre-operatively and subsequent re-epithelialization of the cornea, and some patients have reported recurrent corneal erosions. This may be troublesome for patients with seasonal ocular allergies and many patients feel the need for occasional ocular lubrication. Laser in-situ keratomileusis Immediately post-surgery Immediately after laser in situ keratomileusis (LASIK), a slit lamp should be used to check the flap position and look for wrinkles, striae (Figure 5.1) or significant interface debris that may require the flap to be re-floated by the surgeon.16 Failure to do so could result in a compromised visual performance because of significant corneal irregularity over the pupil. No fluorescein staining should be visible, other than a little around the flap margins, but some degree of anterior chamber activity is relatively common.1,17 If fluorescein reveals an epithelial defect (Figure 5.2), corticosteroids can be used to prevent or at least limit interface inflammation. In the majority of cases, functional vision returns within a few hours of surgery, with 80% achieving
Post-operative follow-up of the refractive surgery patient
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increase in myopia and an associated reduction in vision have been reported at high altitude after LASIK.31
Figure 5.1 Striae post-LASIK. (Courtesy of Michelle Hanratty)
Figure 5.2 Flap-edge defect post-LASIK. (Courtesy of Michelle Hanratty)
a level of vision within one line of their preoperative BCVA by 3 days after LASIK.18 Patients may experience grittiness, photophobia and perhaps burning for the first 24 hours until the epithelium around the flap margin has healed, but discomfort can generally be controlled using anti-inflammatory drops (e.g., 0.5% diclofenac sodium, extended release) rather than a bandage contact lens19 Eye rubbing or squeezing can dislodge or distort the flap during the early post-operative period, so the patient is usually fitted with a transparent eye shield to minimize the risk of this happening.
decide whether the location and extent of the ingrowth warrant intervention.
At 1 week The epithelium should fully cover the flap margin by 1 week post-LASIK. The margins can be quite difficult to detect at this stage, as fibrosis has yet to take place. Epithelial defects should be monitored carefully, since they increase the risk of epithelial ingrowth and diffuse lamellar keratitis (DLK). Interface debris should also be watched as it may lead to focal infiltrates that require flap re-floatation. Topographical irregularities and sub-clinical flap oedema complicate objective and subjective refraction and limit visual quality in the early post-operative period. At 1 month By this stage, the vision tends to be very good. However, as the novelty of clear vision without glasses begins to wear off, some patients start to notice visual problems such as reduced-quality night vision and haloes around lights. The refractive error may have stabilized in those treated for lower degrees of myopia (<6.00DS), although regression of approximately 15% of the pre-operative error is not uncommon20 (e.g., –0.25D after LASIK for –1.50D, and –1.50D after LASIK for –10.00D), and is associated with an increase in corneal thickness and central corneal steepening.21 If epithelial ingrowth is going to develop it tends to do so within the first month. The clinician must then
At 3 months After LASIK, healing is limited to the region around the lamellar interface and haze occurs around the flap margin only. Histological investigations show a regular stromal architecture, in contrast to the obvious anterior stromal disorganization seen after PRK.22 Most LASIK patients demonstrate a stable refractive error by 3 months, with the exception of those treated for very high myopia.23,24 The possibility of an enhancement procedure can be discussed if the refractive outcome is poor, but few surgeons consider an enhancement unless the residual error is greater than 1.00DS. LASIK enhancements are usually performed between 3 and 6 months after the first procedure. At this stage, the flap can still be lifted after removal of the epithelium from around the margin. Flaps in some patients can be lifted more than 18 months post-surgery.25 Late enhancements require a second lamellar cut, which increases the risk of complications.26 If an enhancement procedure is to be considered, both the refraction and corneal topography must be stable and there must be sufficient residual corneal thickness. As with the pre-operative examination, a cycloplegic refraction is essential to minimize the risk of overcorrection. Any enhancement obviously requires the follow-up period to begin again. At 6 months and beyond The percentage of eyes that achieve within ±1.00D of emmetropia has been quoted as 88–100% at 6 months post-LASIK for corrections of –8.00D or less.27,28 To correct low hypermetropia, hyperopic LASIK has proved slightly more successful than hyperopic PRK,29 but the stabilization rate is approximately four times longer than for myopic treatments.30 As with PRK, there is no evidence of a diurnal variation in vision, although a temporary
Complications specific to LASIK Complications can arise from either the flap or, less commonly, the laser ablation.32 Flap complications include those that occur at the time of surgery (in approximately 0.3% of cases), such as an incomplete or decentred flap,33 and complications that present after surgery, such as flap striae and epithelial ingrowth.32,34 The vast majority of complications manifest themselves within 6–8 weeks of LASIK surgery. Most can be treated and have a minimal effect on the final outcome after surgery, if managed properly.35 Serious adverse complications that lead to a significant permanent visual loss, such as infections and corneal ectasia, are very rare, but side effects such as dry eyes, night-time starbursts and reduced contrast sensitivity are relatively common for the first few months.36 Surgeon experience is a key factor in the initial outcome. Epithelial ingrowth Epithelial ingrowth occurs when nests of epithelial cells trapped beneath the flap begin to proliferate (Figure 5.3). Ingrowth presents as a milky deposit in the interface (Figure 5.4) and is more common after
Figure 5.3 Nests of proliferating epithelial cells trapped beneath the flap can result in epithelial ingrowth. (Courtesy of Michelle Hanratty)
Figure 5.4 Ingrowth often presents as a milky deposit in the interface. (Courtesy of Michelle Hanratty)
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Refractive surgery: a guide to assessment and management
enhancement than after the initial procedure. The extent should be measured since growth less than 1.0mm from the flap margin is acceptable, as it is usually self-limiting. Ingrowth greater than 1.0mm, invading the visual axis or progressing rapidly requires surgical management, particularly if the flap margin is rolled or eroded, as it can lead to significant irregularity and flap melt. Although a small degree of ingrowth is common (approximately 15% of eyes), few cases require management. Untreated ingrowth can lead to corneal irregularity and glare, and very occasionally to corneal melt. Microstriae Fine grey lines that are related to crinkles in Bowman’s membrane are not uncommon in those treated for moderate or high myopia, as the flap does not fit the remodelled stromal bed. Such cases are difficult to manage and are usually left alone unless vision is compromised. Interface debris Some debris is seen in virtually all eyes postLASIK. Sources include dust from the atmosphere, meibomian secretions (Figure 5.5), metallic deposits and oils from the microkeratome blade and fibres (Figure 5.6). Debris is usually inert and causes no problems, but it can be associated with stromal infiltrates or DLK, in which case it requires treatment with topical corticosteroids.
Diffuse lamellar keratitis (Sands of the Sahara) DLK is a sterile, diffuse inflammation at the level of the interface that may be accompanied by anterior chamber activity (Figure 5.7).37 It looks a little like post-PRK haze, but is very obviously confined to the interface. It is thought to be an immune response to interface debris or perhaps bacterial toxins. The onset tends to occur within a day or two of the LASIK procedure, with symptoms such as pain and photophobia, and additional signs of ciliary hyperaemia and lacrimation. Visual quality may be reduced because of the increase in forward light scatter, although Snellen acuity is unaffected generally. Referral back to the operating surgeon is required for treatment with topical corticosteroids such as fluorometholone, antibiotics and cycloplegics. The flap may be lifted and irrigated in some cases. A number of systems are used to grade DLK, including one that divides cases into one of four categories (Table 5.3). A cluster is defined as a group of DLK cases that occur in patients treated on the same day. One study found that DLK occurred in 1.3% of eyes treated, with 58% and 42% showing type I and type II, respectively. Cases with central involvement (type II) took significantly longer (12.1 days) to resolve than cases with central sparing (type I – 3.5 days). Not surprisingly, central involvement carries a much higher risk of a reduction in BCVA. The majority of cases were sporadic rather than part of
Figure 5.5 Post-LASIK interference debris. (Courtesy of Michelle Hanratty)
Figure 5.7 DLK is a sterile, diffuse inflammation at the level of the interface. (Courtesy of Michelle Hanratty)
a cluster.38 DLK can also present many months after LASIK in association with an epithelial defect.39 White blood cells migrate from the limbal blood vessels into the interface, since it is the easiest path for them to take. Central corneal sparing is much more likely if the DLK is related to an epithelial defect. There is also a reported case of DLK that occurred 10 months post-LASIK in association with acute iritis,40 which suggests that DLK is a nonspecific corneal inflammatory response rather than a condition caused by a particular agent. Appropriate management of patients with DLK generally results in complete resolution of the condition. Corneal integrity Concern has been raised as to the integrity of the globe post-LASIK, since healing does not appear to lead to the growth of collagen fibres between the corneal flap and the ablated stromal bed. The flap is attached to the underlying cornea only at its margins, by the corneal epithelium, and therefore does not contribute significantly to the strength of the cornea. However, a study that examined the integrity of the globe after a range of different refractive surgery procedures concluded that, although LASIK eyes required slightly less energy to rupture than control eyes, the difference was not significant.41 LASIK eyes ruptured either at the flap margin or at the edge of the limbus. Other studies have also concluded that ocular integrity is not compromised by LASIK.42,43 The risk of the flap being dislodged is very low, with one study on rabbit eyes showing no flap damage even at 1 week post-LASIK, when an airgun was fired at the edge of the flap.44 This can be attributed to the endothelial pump and the multiple layers of corneal epithelium that cover the flap margin. In a few isolated reports of flap damage, this occurred with 2 months of the procedure.45,46 However, one study reported flap dislocation 6 months postLASIK after focal trauma from a tree branch.47 This suggests that flap dislocation can occur at any time if the trauma is discrete and from such an angle that it catches the edge of the flap. Patient’s who report with flap dislocation should be referred urgently to the operating surgeon
Table 5.3 Classification of DLK38 Figure 5.6 Fibres trapped in the interface post-LASIK. (Courtesy of Michelle Hanratty)
Sporadic case Case part of a cluster
No central involvement
Central involvement
Type IA Type IB
Type IIA Type IIB
Post-operative follow-up of the refractive surgery patient
for irrigation and refloating of the flap, followed by a course of topical antibiotics and corticosteroids, since DLK and epithelial ingrowth are common after such an occurrence. Keratectasia Keratectasia is a rare condition in which surgically induced corneal thinning leads to protrusion of the corneal tissue, an increase in myopia and irregular astigmatism, and consequently to a reduction in visual performance.48 Some cases require a corneal graft to achieve functional vision. This is a severe complication that may not present for a year or more post-surgery (mean of 1 ± 0.3 years).49,50 Most cases of keratectasia can be attributed to miscalculation of the remaining corneal thickness. The general consensus is that keratectasia can be avoided by ensuring that the residual stromal bed after creation of the flap is at least 250μm in thickness. Unless the thickness of the stromal bed is measured intra-operatively, it is not always possible to ensure that adequate thickness remains because of the limited accuracy of microkeratomes, (standard deviation of ±30μm). Iatrogenic ectasia is most commonly associated with the treatment of high myopia (>–15.00DS),24,49,51 since a deeper ablation is required and residual corneal thickness calculations become much more critical. A recent study of 2873 eyes reported ectasia in 0.66%.52 The authors noted that ectasia did not occur in those treated for less than –8.00DS or those with a residual corneal bed thickness of 325μm or more.52 Studies have suggested that there maybe more to ectasia than simply inadequate residual corneal thickness. The anterior 100–120μm of the corneal stroma is known to have a more tightly interwoven anterior lamellae than the underlying stroma,53 which makes this part of the stroma stronger and more resistant to swelling than the deeper layers. Differences between individuals in their stromal structure may mean some corneas are innately susceptible to developing ectasia. Examination of the biomechanics of the cornea after severance of anterior lamellae during the creation of the flap and the reshaping of the underlying stroma suggests that the whole cornea, including the posterior surface, bows forwards as a result of surgery.54,55 This movement, which has also been implicated in the refractive regression seen post-LASIK, suggests that the anterior lamellae play an important structural role.
Retinal complications The risk of retinal detachment increases with increasing myopia above –3.00D, and highly myopic eyes (greater than –10D) also have an increased risk of primary open angle glaucoma, pigment dispersion syndrome, cataracts and myopic maculopathy.56–59 In theory, creation of the corneal flap could lead to retinal complications, such as retinal tears or rhegmatogenous retinal detachment, particularly in susceptible individuals. A large study of almost 30,000 eyes reported vitreopathologic conditions in only 0.06% of eyes post-LASIK.60 Since the average onset was 13.9 months post-surgery, these cases might have been unrelated to the surgery and simply the result of myopic retinal degeneration. This highlights the importance both of a thorough retinal examination with scleral indentation (to allow the identification and treatment of retinal lesions prior to surgery) and of the education of all patients in the importance of regular eye examinations post-surgery. Laser subepithelial keratectomy Immediately post-surgery After laser subepithelial keratectomy (LASEK), the epithelial flap should be examined to ensure that it is as smooth as possible. A bandage contact lens is often fitted over the flap to hold it in place. Plano silicon hydrogel lenses (e.g., Ciba Night and Day or Bausch and Lomb Purevision), or medium water content, non-ionic lenses (e.g., Bausch and Lomb Soflens 66) are popular options. All topical medication instilled into an eye that has a bandage lens should be preservative free (e.g., Minims chloramphenicol). Bandage lenses are associated with an increased risk of infection and infiltrates and therefore eyes fitted with a lens should be monitored carefully. Lens removal on day three or four should be accompanied by copious irrigation to prevent damage to the fragile epithelium. If flap damage occurs at any point during the procedure and the epithelial layer cannot be saved, the patient should be managed as if he or she had undergone a PRK procedure. At 1 week The epithelium should be examined to ensure that it is intact, but by 1 week the flap should have been replaced by new epithelial cells that migrate from the limbus. There is a rapid recovery of vision following LASEK – in one recent study of 222 eyes (range from –1D to –11D), 98% of the eyes achieved 6/12 unaided vision at the 2 week examination.61
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At 1 month LASEK produces less haze than does PRK,62,63 and therefore there is little to see at 1 month. The cornea should be checked for fluorescein staining. At 3 months For the treatment of low and medium myopia (<–6.50DS), differences in unaided vision and refractive outcome between LASEK and PRK are insignificant by 3 months post-surgery.63 Refractive outcome Of 222 eyes, 63% achieved 6/6 unaided vision at 1 year.61 The procedure appears to be safe, since no eyes showed a reduction in BCVA despite the wide range of preoperative myopia. Complications common to all forms of excimer laser surgery Undercorrection Residual myopia is usually the result of an inaccurate pre-operative refraction or an insufficient period free of contact lenses prior to surgery. Enhancement can be considered once the refraction has stabilized. Overcorrection An initial hyperopic result is to be expected after PRK, but if hyperopia greater than 1.00D with minimal haze formation is still present 6 weeks post-surgery, the patient may be an ‘under-healer’64 and require a hyperopic enhancement. Hyperopic treatments are not as successful as myopic procedures, with a relatively high risk of regression, irregularity and a long stabilization period. Regression Regression is the loss of refractive effect over time and is more common following larger refractive corrections, particularly after PRK. A degree of regression is expected during the first 6 weeks post-PRK and the first 3 weeks post-LASIK, and is associated with stromal remodelling, thickening of the epithelium and corneal biomechanics.21,65 Severe regression associated with intense haze is very rare now that PRK is limited to the treatment of low myopia. The risk of regression is much higher in all people exposed to high levels of ultraviolet radiation (natural sunlight and sun beds), and in females who take oral contraceptives.66 Dry eye Grittiness and asthenopia associated with dry eye are relatively common during the first 6 months post-excimer laser surgery. A number of possible causes include damage to the
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conjunctival goblet cells by the lid speculum and impaired corneal sensitivity.67–69 Preservative-free ocular lubricants throughout the day (e.g., carmellose) and an ointment at night (e.g., liquid paraffin) normally suffice. Punctal plugs can be useful in more severe cases and lid hygiene to maximize meibomian gland function is useful. Intraocular pressure elevation If corticosteroids are used to treat the intense haze of DLK, for example, a small proportion of patients will demonstrate a significant rise in intraocular pressure (IOP). Steroid responders require immediate referral for cessation of topical corticosteroids and possible beta-blocker treatment. When assessing IOP post-surgery, clinicians should note that all excimer laser techniques lead to an artificially low IOP reading,70 by about 2mmHg, which is related to the reduced thickness of the central cornea. Stromal infiltrates Infiltrates, both sterile and infectious, can occur in the presence of a bandage contact lens (post-PRK or -LASEK) or interface debris (post-LASIK). Sterile infiltrates are also associated with the use of nonsteroidal anti-inflammatory eye drops.71 These must be assumed to be infectious until proved otherwise and the patient referred back to the surgeon for topical antibiotics (infectious) or topical corticosteroids (sterile). Corneal infections Cases of infectious keratitis are rare, but both fungal and bacterial infections have been reported in the early post-operative period.72,73 These can take the form of a corneal ulcer with epithelial staining, infiltrates and stromal oedema, or be confined to the interface (LASIK). Rapid referral is necessary to identify the cultures and for intensive treatment, but a penetrating keratoplasty may be the only solution. Excimer laser procedures have also been known to reactivate the herpes simplex virus, of which the classic dendritic pattern should be a warning. Those at risk should have been screened out prior to surgery.74,75
Visual outcome Unaided vision PRK For low and medium degrees of myopia (<–6.00D), 88–99% achieve 6/12 or better (uncorrected vision), and 58–78% achieve 6/6 or better by 12 months postPRK.76–79
LASIK For eyes treated for –9.50D or less, the percentage of eyes that achieve 6/6 or better has been quoted as 83%, with 6/12 vision or better achieved by 86–100% at 6 months post-LASIK.27,28,80 LASEK For a range of myopia up to –11.25D, an unaided vision of 6/4.5 was achieved by 19% of eyes, 6/6 by 63% of eyes and 6/7.5 by 18% of eyes.61 Visual complications Poor unaided vision is a common reason for dissatisfaction post-surgery,81 particularly if the patient’s expectations are unrealistic. However, these cases can be managed with an enhancement procedure, spectacles or contact lenses to correct the residual error. Corneal refractive surgery procedures are designed to minimize refractive error, but in modifying the shape of the cornea, they also tend to alter the optical quality of the eye. In the majority of post-surgery patients, these changes are clinically insignificant, and result in no apparent loss of visual performance. One indicator of the safety of a refractive surgery procedure is the percentage of eyes that lose two or more lines of BCVA. Recent studies on myopes being treated for <–6.00D suggest that 0–1.8% of eyes lose two or more lines of BCVA after PRK, compared to 0–1.2% post-LASIK.27,80 Further work is needed to determine the corresponding percentage for LASEK, but initial studies suggest that the level of risk is similar.61 For all excimer laser procedures, the percentage of eyes that exhibit a reduction in visual acuity increases with the degree of pre-operative myopia. An unacceptably high percentage of patients (7.3%) treated for hypermetropia >+4.00D were found to lose two or more lines of best-corrected acuity,82,83 and therefore the majority of surgeons do not consider medium and high hypermetropes for corneal refractive procedures. High-contrast acuity provides limited insight into visual quality in the real world, and a loss of high-contrast visual acuity tends to indicate a significant loss of visual quality. Of the many patients who exhibit normal levels of high-contrast BCVA post-surgery, a proportion complain of poor visual performance, particularly under low illumination. Also, a minority of patients have compromised vision and yet are unaware of it because they rarely find themselves in visually demanding environments. Reasons for patients refusing treatment to their second eye include glare, haloes
and poor-quality night vision.84 A study of 690 patients who had undergone PRK reported that 92% of patients were satisfied with the surgical outcome, with the degree of satisfaction closely related to the post-operative uncorrected vision in the better eye. Approximately 30% of patients reported some problems with their night vision.81 The Refractive Status and Vision Profile (RSVP) questionnaire has established itself as a useful tool with which to assess patient views on visual outcome. The overall RSVP score has been shown to correlate with changes in patient satisfaction.85 The reduction in visual performance that can occur post-refractive surgery has been attributed to an increase in forwards scattered light within the eye and increased aberrations (optical imperfections). Active keratocytes and disorganized collagen fibrils within the post-operative cornea act as scatter sources, scattering light both forwards (towards the retina) and backwards (towards the observer – e.g., stromal haze). The stray light is superimposed over the retinal image, which reduces its contrast. A reduction in the contrast of a high-contrast image, such as a Snellen letter, has limited impact on the ability of the eye to discriminate it – the letter will still be visible, just slightly fainter. Reducing the contrast of a low-contrast image is likely to result in the image contrast falling below the threshold for discrimination, that is the detail of the object will no longer be visible. Scattered light can cause disability glare (image degradation) in all individuals in the presence of a significant glare source, such as car headlights, but those who have raised levels of intraocular light scatter suffer reduced vision, even when there is no bright glare source, because light is scattered from one part of the retinal image to another. To date, most refractive procedures have concentrated on correcting spherical and cylindrical refractive errors, which constitute approximately 97% of all aberrations. The eye naturally possesses higher order aberrations, which are known to increase with age.86 Axial aberrations are known to be the most problematic in terms of visual performance, particularly spherical aberration and coma. Both are highly dependent on pupil size and, on average, there is between a five- and seven-fold increase in total aberrations as the pupil dilates from 3 to 7mm.87,88 The problem for most patients is not that traditional excimer laser procedures cannot correct these aberrations, but that both PRK and LASIK actually induce a significant and permanent increase in the aber-
Post-operative follow-up of the refractive surgery patient
rations.89–93 An increase in total aberrations of between 25 and 300 times has been reported for a 7mm pupil.87,88 Coma is associated with decentration of the ablation zone in relation to the pupil centre and increases with increasing preoperative refractive error. The degree to which the aberrations of the eye increase varies considerably between individuals. Previous studies reported a high incidence of night-vision problems (such as haloes, starbursts and poor-quality night vision) after laser surgery.94–96 These problems were associated with high levels of stromal haze, which caused stray light, and with treatment zones significantly smaller than the average pupil, which led to extreme aberrations. Nowadays, haze is less common and much less severe because high myopes are no longer treated with PRK. LASIK and LASEK cause little or no haze in the majority of cases and ablation zone diameters for all techniques have increased from around 4 or 5mm up to 6 or 6.5mm, which makes them larger than or the same size as the average pupil under low illumination. PRK Forward light scatter is known to increase during the first 2 weeks postPRK, peaking at 3 months and returning to normal levels comparable to those of spectacle wearers and soft contact lens wearers by 12 months. 97,98 However, evidence suggests that the distribution of light scatter around the retinal image is permanently modified by PRK, with an increase in the spread of stray light leading to a reduction in retinal image contrast. 99 PRK has also been shown to induce higher order aberrations.88–100 Since both forwards scatter and aberrations cause a reduction in the retinal image contrast, low-contrast acuity and contrast sensitivity are affected for the first 3 months,101,102 with permanent changes in a minority of cases that exhibit large aberrations or persistent scatter.103 High-contrast acuity is only affected in severe cases. Studies indicate that visual performance under dilated pupil conditions (low illumination) may be compromised for a year or more, particularly for low-contrast acuity tasks.104,105 LASIK Forward light scatter does not appear to increase significantly following LASIK,99 unless the patient suffers from DLK. Higher-order aberrations are known to increase following LASIK,87,106 and spher-
ical aberration is thought to be greater after LASIK than after PRK, since the ablation zone is often smaller87 and creation of the flap leads to an increase in aberrations independently of the ablation profile. Limited study has been made of the effects of LASIK on visual performance, but there are suggestions that problems are less common and less severe than those that result from PRK. Some studies suggest that contrast sensitivity for high and medium spatial frequencies is reduced for the first 3 months,107–109 although some spatial frequencies do not appear to fully recover before 6 months.110 There is evidence that mesopic contrast sensitivity is reduced once photopic sensitivity has returned to normal.105 Contrast discrimination thresholds are persistently raised for many LASIK subjects compared to untreated subjects, but the ‘real-world’ significance of such findings is difficult to predict.111 LASEK Since LASEK is a relatively new development, its impact on scattered light, aberrations and visual performance has yet to be considered. However, the procedure is very similar to that of PRK and therefore the outcome is likely to be similar to PRK and LASIK. Less forward light scatter would be expected than is seen after PRK because of the limited stromal haze, but aberrations are likely to be similar.
Managing patients with visual symptoms Patients with visual problems should be questioned carefully about their symptoms. There is a tendency to gloss over problems that do not significantly impact on high-contrast acuity or cannot be attributed to slit-lamp or topography findings. It is essential that practitioners incorporate suitable tests into their assessment to obtain a full picture of any visual problems and hence select the correct management strategy. The percentage of patients who suffer from a significant reduction in visual performance may be as high as 10–15%,85,111 but this is likely to reduce in the future as wavefront technology improves and becomes more readily available. Although such technology is unlikely to live up to the initial expectations that it would create ‘super’ vision in a high proportion of patients, it should reduce levels of induced aberrations and provide some hope for those who already suffer from high levels of surgically induced aberrations.
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Modification of standard assessment techniques Vision and visual acuity measurement Most optometrists and ophthalmologists rely on the Snellen chart, but the benefits of a logMAR chart are considerable if the data are to be analyzed and/or published.112 Bailey–Lovie logMAR charts, for example, use equal numbers of letters per line and have equal increments of change of letter size between the lines. Further information can be obtained from patients, where necessary, by employing high- and low-contrast logMAR charts, as outlined below. Refraction Autorefractors often give a poor measure of refractive error post-corneal surgery, as a result of the significant changes in corneal profile.113 Retinoscopy can also prove difficult because of irregularity of the reflex, particularly during the early post-operative period or when the ablation zone is decentred or very small compared to the pupil. A 3–4mm ‘pinhole’ can be helpful in such circumstances. The retinoscopy reflex can also detect some cases of corneal ectasia should it develop at a later stage (swirling reflex). Subjective refraction is also complicated by both any irregularity and the multifocal nature of the post-operative cornea. The increase in spherical aberration associated with treatment for myopia alters the equivalent defocus by 0.25D or more for large pupils (≥6mm) in 27% of eyes. The advantage for the patients is that they tend to see better than would be expected for their apparent refractive error, and the onset of presbyopia may be delayed. The disadvantage is a reduction in the quality of vision and a shift towards myopia with pupil dilation. As with contact lens patients, do not assume that visual symptoms are necessarily associated with the surgery. Assessment of corneal profile Keratometers are poorly suited to refractive surgery work because of the small area of the cornea from which the curvature is calculated, which does not enable sufficient information regarding the regularity of the cornea to be gathered. The strange shape of the post-operative cornea also means that the keratometry readings are inaccurate and useless for contact lens fitting or intraocular lens calculations. It is essential that clinicians involved in the management of refractive surgery have access to topographic equipment and
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are able to understand and assess the plots produced. On occasions the topography plot is influenced by artefacts such as the lids or a disrupted tear film, so it is worth viewing the Placido disc image prior to processing the image (not relevant for Orbscan). The choice of scale is critical, since treatment for myopia produces a generalized flattening of the central cornea and the absolute scale tends to obscure useful detail because of the large intervals. Although a normalized plot may not allow comparison with other eyes, it generally better reveals the ablation zone margin and irregularities of the scale that affect vision. A good post-PRK or -LASEK plot (myopia) shows a large central flattened area with smooth contours if an axial algorithm is employed. Tangential maps of the same eye reveal a smaller flat zone surrounded by a steep ring. Post-LASIK topographies tend to be less regular with discontinuous contours and localized areas of flattening. Often, a crescentshaped region that relates to the flap margin is visible. The corneal profile becomes smoother as the epithelium is modified post-surgery. Some systems can give an estimate of the potential acuity based on the irregularity of the cornea after compensation for any residual sphere or regular astigmatism, but they do not consider whole eye aberrations. Particular topographical features that may signal problems include central islands, decentred ablations and irregularities. A central island is defined as a 2–4mm area with 1.5–3.5D of corneal steepening associated with undercorrection, more common after treatment with a broad-beam laser.114 They tend to subside over the first year.115 Decentrations of the ablation in the region of 0.5mm are very common and tend to cause a slight reduction in visual quality, related to an increase in higher order aberrations (coma), but rarely a reduction in high-contrast acuity. Larger decentrations can cause monocular diplopia, irregular astigmatism and a loss of BCVA. Such cases require retreatment, ideally using a laser with a topographic or wavefront link. Decentrations may be symptom free if the pupil itself is slightly decentred. Slit-lamp examination A detailed examination of the anterior segment, including the anterior chamber, is essential on every follow-up visit. By varying the magnification and the illumination technique employed, complications can be identified quickly. Retroillumination is particularly useful in revealing complications such as flap microstriae, interface debris and ectasia.
Assessment of visual quality High-contrast acuity alone is inadequate to assess the visual outcome of refractive surgery and patient satisfaction correlates poorly with visual acuity.116 Any residual refractive error must be corrected when assessing visual quality, as optical defocus attenuates high spatial frequencies. All tests of contrast vision have their limitations and it is difficult to compare the results of different tests, since they employ different stimuli, measurement techniques and, often, different light levels. The full contrast-sensitivity function can be determined by finding the contrast at which an eye can detect a series of sine wave gratings of different spatial frequencies. However, this is very time consuming and is generally reserved for the field of research. The Contrast Acuity Assessment (CAA) test is a computer-based psychophysical test that measures contrast acuity over the central ±5° field and is able to determine whether visual performance falls within the normal range under both photopic and mesopic conditions. Thresholds for those with an increase in forward light scatter and aberrations are elevated.117 In practice, one simple method is to compare high- and low-contrast acuity measurements, but it is important that both letter charts are logMAR rather than Snellen, to allow accurate scoring and to overcome other drawbacks of the Snellen chart, such as the non-geometric letter size progression between lines and the variation in the number of letters per line.118 Normal subjects show a difference of approximately one line of letters between high- and low-contrast acuity (10% contrast), but those who suffer from an increase in forward light scatter and/or aberrations will have reduced low-contrast acuity, which results in a larger difference between the two. The Pelli–Robson chart is easy to use and shows good repeatability if each letter is scored individually.119 The chart examines mid spatial frequencies close to the peak of the contrast-sensitivity curve, which are known to be affected by refractive surgery.110 Normal scores vary with age,120 and are around 1.84 for those between 20 and 40 years, reducing to around 1.68 for those over 60 years. An assessment of performance under dim illumination can provide additional information, particularly in those who complain of night-vision problems. Measurements of pupil diameter should ideally be made using an infrared pupillometer that allows assessments at very low light levels.
Glare testing is an indirect way to assess intraocular light scatter. Like contrast-sensitivity testing, glare tests vary considerably and result in a wide range of outcomes. In general, they are not particularly useful since the increased retinal illumination that results from the glare source tends to cause pupil constriction and hence an improvement in visual performance.121 The City University Light Scatter Program is a computer-based psychophysical technique that can measure forward light scatter directly.122 However, it is rather time consuming and therefore maybe not currently suitable for clinical use.
Specific visual symptoms Poor-quality vision If a patient is symptomatic under daylight conditions, he or she is also very likely to experience problems at night. Poor contrast vision under photopic conditions when the pupil is relatively small can often indicate poor central optical quality within the pupil area. This may relate to an increase in aberrations or an irregularity caused by flap striae, epithelial ingrowth, and so on. Examination of the topography plot may help identify the problem, although such irregularities are often too subtle to detect. Rigid contact lenses may improve visual quality and, in future, wavefront technology may allow induced irregular aberrations to be corrected. The cornea should also be examined for haze, although the quantity of forward light scatter cannot be directly predicted from backscatter.123 Poor-quality night vision Night vision in the normal population is relatively poor compared to vision under good illumination. Firstly, the dark-adapted retina relies on the rod receptors, which have poor resolution compared to cone receptors and are more sensitive to scattered light within the eye. Secondly, pupil dilation is associated with an increase in aberrations. The peripheral cornea is known to scatter more light than the central cornea, so pupil dilation also increases the stray light within the eye.124 Thirdly, the contrast of an object against its background tends to be much lower at night and so any reduction in image contrast as a result of scatter or aberrations is more likely to render the object invisible. Visual difficulties at night may also relate to the presence of intense glare sources, such as car headlights, which further reduce contrast. If the ablation zone is larger than the pupil, but well
Post-operative follow-up of the refractive surgery patient
centred, visual problems are likely to be related to aberrations (including irregularities) and scattered light within the ablated area. A rigid contact lens may help, as it provides a smooth refracting surface. Ideally, an enhancement procedure is needed to correct induced aberrations, but wavefront technology is still in its infancy and the ability to correct aberrations is not as advanced as the ability to measure them; the healing response of the eye adds further unpredictability. A decentred zone gives rise to visual problems that relate to coma and, if severe, the patient may complain of monocular diplopia or polyopia. Haloes Individuals with particularly large pupils under low illumination may suffer from an extreme version of positive spherical aberration. Peripheral light rays pass through the untreated cornea and superimpose an unfocussed image over the clear retinal image to give the appearance of haloes around lights at night.94,125 Haloes are more common in eyes that have undergone small-diameter ablations, and in patients with naturally large pupils (6.5–7.0mm in diameter). Ablation diameters of 6.0mm or more have significantly reduced halo problems in the majority of patients, although Roberts and Koester126 suggested the use of even larger diameter ablations for ‘at risk’ groups (i.e., young patients with large pupils, those with deep anterior chambers and patients in occupations for which glare is of serious concern). Some laser software quotes the complete zone size, including the transition zone, rather than the optic zone size. Those with significant haloes may benefit from a further excimer treatment to increase the ablation zone, which has been shown to reduce symptoms in seven out of ten cases without any significant change in refraction.127 Topical miotic drops, such as 0.25% or 0.5% pilocarpine, can provide temporary relief, although pilocarpine can cause headaches and blurred vision.128 Carbachol is an alternative that has fewer side effects. Patients can be comforted by the fact that the pupil diameter does decrease with age.
Starburst effects Starburst effects are related to forward light scatter and high cellular activity postPRK. They tend to subside over time as haze resolves. Many patients confuse starburst effects, haloes, glare and even photophobia. In such cases more details may need to be obtained through subjective questioning, as well as testing methods. Contact lens fitting post-refractive surgery In some cases, the only solution to visual problems may be to fit the patient with contact lenses. However, it should be realized by the contact lens clinician that many patients opt for refractive surgery because of the inconvenience perceived with contact lens wear. It is, therefore, advisable that negative commentary by the contact lens clinician be avoided. Soft contact lenses are suitable for those with a simple under- or overcorrection associated with a regular cornea and, as long as an ultrathin lens is chosen, there is no need to use a specialist lens designed for oblate corneas.129 Aspheric soft lenses designed to minimize aberrations (e.g., Ultravision) can reduce symptoms in those with severe night-vision complaints and may provide a reasonable alternative to miotics. Soft lenses should not be fitted after radial keratotomy (RK) because of the high risk that neovascularization will be induced along the radial incisions. Some post-surgery patients require a contact lens to correct surgically induced irregularities. In such cases a rigid lens provides a smooth refracting surface, but fitting is complicated by the change in corneal profile from prolate to oblate, which necessitates a reverse geometry lens or, at least, a back surface aspheric. Materials should be of mid-to-high Dk, but the most important factor is good material stability. Trial lenses are essential to determine lens power, back optic zone radius (BOZR; Table 5.4) and back optic zone diameter (BOZD). When fitting a back surface aspheric lens, the lens should be centrally positioned with a degree of clearance over the flattened ablation zone, a 1–2mm ring of light touch in the mid-periphery and an
Table 5.4 Selection of BOZR for first trial lens post-refractive surgery Source of curvature information
First BOZR selected
Pre-operative K readings (or from second untreated eye)
0.1 to 0.2mm flatter than flattest K
Post-operative K readings
0.2 to 0.3mm steeper than flattest K
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edge lift of approximately 0.3–0.6mm. The central clearance results in a positive tear lens that necessitates compensation of the lens power. Reverse geometry lenses have a second curve that is steeper than the BOZR by 0.6–1.2mm, to achieve midperipheral alignment without excessive central pooling. Topography can be used to improve fitting success, but careful selection of the most informative plot is essential. Binocular vision problems In a few susceptible individuals, surgical reduction of the refractive error can affect their binocular status. Spectacles for myopia may effectively control an exophoric deviation and, likewise, hypermetropic corrections can control esodeviations. Changes in the accommodative convergence:accommodation (AC:A) ratio are adapted to easily by the majority of patients, but a careful pre-operative assessment is essential to identify those who might suffer from binocular vision problems post-surgery; a prismatic correction is difficult to incorporate in an effectively emmetropic correction. Non-tolerance of monovision A number of patients opt for monovision to delay their need for reading glasses once they reach presbyopia. In general, monovision is well tolerated, and only about 4% of patients require an enhancement procedure to correct both eyes for distance in cases of non-tolerance.130 The majority of monovision patients achieve good binocular visual acuity and adapt quickly. Binocular summation and a degree of stereopsis (40–800 seconds of arc) remain as long as the difference between the two eyes is no more than about 1.50D.131 Non-tolerant patients may complain of disorientation and blurred distance vision. Those who suffer from motion sickness are less likely to adapt to monovision, but this should be revealed by the pre-operative monovision contact lens trial. Non-tolerance is more common if the dominant eye has been corrected for near vision rather than for distance, although many individuals can tolerate this situation given time.132 Some patients report haloes around lights at night as the brain attempts to fuse the blurred retinal image in the near eye with the clear image in the eye corrected for distance. Patients must be made aware that the advantages of a monovision correction reduce as accommodative power reduces with age, although a +1.50D addition should allow functional vision for most basic day-to-day tasks. Around 20% of patients benefit
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from a balancing spectacle overcorrection for detailed tasks such as driving at night or computer use.
Long-term implications of refractive surgery It is not uncommon to hear concerns voiced about the possible long-term implications of PRK and LASIK, since routine excimer laser surgery has only been available for around 15 years. However, it should be stressed that eye care clinicians who continue to inform their patients that excimer laser surgery is still experimental or in its infancy are not only misinformed, but also passing on poor advice to their patients. Commonly raised concerns about long-term implications are outlined below. Endothelial damage The corneal endothelium does not appear to be affected adversely, with no significant alteration in central cell density,133 although an increase in the coefficient of variation of cell area and a decrease in the percentage of hexagonal cells was reported in one study.134 Cancer risk Examination of unscheduled DNA synthesis, a measure of the tissue repair mechanism, revealed no difference between the effects of an excimer laser and a diamond knife.135 Extensive animal studies have failed to establish a link between epithelial or connective tissue neoplasms and excimer laser exposure. Risk of cataract Short-term changes in the aqueous humour and prolonged biochemical changes in rabbit crystalline lenses in the form of free radicals have been detected after photoablation. These changes are thought to be the earliest signs of cataractogenic changes.136 However, no significant elevation of malondialdehyde (MDA) levels, a possible indicator of oxidative effects, has been found after PRK on rabbits,137 and to date there has been no increase in the incidence of cataract. Clinical implications of refractive surgery The increase in aberrations as a result of surgery can make refraction rather difficult to perform, particularly if the patient has large pupils. A large 3–4mm pinhole can prove useful during retinoscopy. When the aberrations are large, it is difficult to find a definite end point for the subjective refraction.
The change in corneal thickness after corneal refractive surgery has implications for the measurement of IOP. A thinner cornea causes underestimation of the IOP, since the resistance of the cornea to indentation is altered. The average surgical refractive correction reduces the central corneal thickness (CCT) by 10–15% (20% in a few high myopes), which leads to a 1–2mmHg decrease in measured pressure. Examination of a wide range of studies led Doughty to conclude that 2mmHg should be added to IOP measurements for each 10% reduction in CCT.70 Modification of the corneal profile also has implications for the calculation of a suitable intraocular lens power if the patient should undergo cataract surgery in the future. Ideally, surgical records should include a note of the patient’s pre-operative keratometry readings, since post-operative keratometry readings misrepresent the power of the cornea,138 with the possibility of a significant refractive error postcataract surgery.
Radial keratotomy Clinicians are unlikely to encounter new patients who have undergone RK, as the technique is now obsolete. However, problems can develop many years after incisional surgery and clinicians should be aware of the these and how to manage them. Refractive stability The long-term stability of the refractive result has been questioned following the detection of a drift towards hyperopia in up to one-third of patients after 4 years.139 The 10 year follow-up of the Prospective Evaluation of Radial Keratotomy (PERK) study revealed an alarming 43% of eyes with a hyperopic shift greater than 1.00D from the refractive result at 6 months postRK.139,140 In theory, patients who exhibit a hyperopic shift could be referred to a surgeon for hyperopic excimer laser treatment – both PRK and LASIK procedures have been undertaken successfully after unsuccessful RK.141 However, hyperopic refractive errors, unless large, are usually tolerated well by younger patients and, since hyperopic treatments are less predictable than myopic corrections, a retreatment is probably best avoided. Exposure to high-altitude conditions has also been shown to cause a significant hyperopic shift (up to +1.50 ± 1.01D by day three) in those who have undergone RK, but the effect is reversible13
A relatively common complication of RK is a diurnal fluctuation in refractive error and hence visual acuity, associated with a structurally weakened cornea.142–145 The PERK study encountered an increase in myopia greater than –0.50D in 30% of eyes116 A study of firefighter applicants who had undergone RK found a myopic shift between morning and afternoon of –0.41 ± 0.33D compared to +0.06 ± 0.42D in the control group. The refractive change caused three out of 10 subjects to fail the unaided vision standard in the afternoon, despite passing in the morning.146 Unfortunately, patients who suffer from a significant diurnal variation have few options. Spectacles can be provided to correct the refractive error at its most myopic, but this is generally a poor compromise. Patients who cannot tolerate the fluctuations in vision have few options other than a corneal graft. Ocular integrity Corneal perforation is a serious, but very rare, complication of RK.147 The continuity of collagen fibrils is not restored after RK, and so the tensile strength of the cornea is reduced. A reduction in ocular integrity is not surprising, considering that micro-perforations of the cornea have been shown to occur in about 18% of eyes at the time of surgery; these induce significant alterations in the blood–aqueous barrier for the first week post-RK.1 The formation of an epithelial plug in an otherwise fully healed RK incision concentrates stress at the incision site, which may predispose the cornea to rupture. The variability in strength measurements between post-RK corneas indicates that the increase in rupture susceptibility is hard to predict, since it is dependent on the size of any epithelial plug and the strength of the wound collagen.148 One study reported three cases of corneal rupture more than 10 years after RK; these resulted from an assault, a sports injury and ‘daily living’.149 Another study considered 28 eyes that had ruptured some years after RK and attributed seven cases to an assault, four to sports injuries, five to car crashes and 12 to ‘daily living’.150 Wound leakage that resulted from blunt trauma has been reported 8 years after RK,151 and incision rupture has occurred during routine cataract surgery more than 11 years after RK.152 Following suturing of the wound, BCVA can return to normal levels, but a corneal graft may be the only answer for some patients. Clinicians are unlikely to encounter such unfortunate individuals
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at the acute stage, but might see them some time after the incident if they experience problems associated with an irregular cornea. Corneal infection that involves the incisions has been reported many years after surgery. Such patients are likely to be in some discomfort and are more likely to present to casualty. Clinicians should look out for infiltrates, oedema and fluorescein staining (rather than pooling) of incisions. The need for routine yearly or two-yearly eye examinations should be stressed to
patients, who should also be given an indication of what to look out for and who to contact. Visual performance In common with excimer laser techniques, RK can cause a reduction in visual performance associated with an increase in forward light scatter and corneal aberrations.153 This is not surprising considering that the clear optical zone may be as small as 2.0mm in diameter. Corneal aberrations are more of a problem than forward light
scatter, as the discrete corneal scars scatter little light.153,154 Data on the average effect of RK on visual performance are inconclusive because of inconsistencies between commercially available contrastsensitivity tests and the use of high luminance conditions.155–157 A reduction in visual performance in the presence of a glare source has been reported,146,158 although glare sources under clinical conditions often cause pupil constriction that masks some of the effects of increased corneal aberrations.121
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(letter). J Refract Corneal Surg. 10, 587–588. Kouyoumdjian GA, Forstot SL, Durairaj VD, et al. (2001). Infectious keratitis after laser refractive surgery. Ophthalmology 108, 1266–1268. Levartovsky S, Rosenwasser GOD and Goodman DF (2001). Bacterial keratitis following laser in situ keratomileusis. Ophthalmology 108, 321–325. Pepose JS, Laylock KA and Miller JK (1992). Reactivation of herpes simplex virus by excimer laser photokeratectomy. Am J Ophthalmol. 114, 45–50. Taravella MJ, Weinberg A, May M, et al. (1999). Live virus survives excimer laser ablation. Ophthalmology 106, 1498–1499. Tuunanen TH and Tervo TT (1998). Results of photorefractive keratectomy for low, moderate and high myopia. J Refract Surg. 14, 437–446. McDonald MB, Deitz MR, Frantz JM, et al. (1999). Photorefractive keratectomy for low-to-moderate myopia and astigmatism with a small-beam, trackerdirected excimer laser. Ophthalmology 106, 1481–1488. Pallikaris IG, Koufala KI, Siganos DS, et al. (1999). Photorefractive keratectomy with a small spot laser and tracker. J Refract Surg. 15, 137–144. Nagy ZZ, Fekete O and Suveges I (2002). Photorefractive keratectomy for myopia with the meditec MEL 70(GScan) flying spot laser. J Refract Surg. 17, 319–326. Pop M and Payette Y (2000). Photorefractive keratectomy versus laser in situ keratomileusis: A control-matched study. Ophthalmology 107, 251–257. Brunette I, Gresset J, Boivin JF, et al. (2000). Functional outcome and satisfaction after photorefractive keratectomy – Part 2, Survey of 690 patients. Ophthalmology 107, 1790–1796. Ditzen K, Huschka H and Pieger S (1998). Laser in situ keratomileusis for hyperopia. J Cataract Refract Surg. 24, 42–47. Goker S, Er H and Kahvecioglu C (1998). Laser in situ keratomileusis to correct hyperopia from +4.25 to +8.00 diopters. J Refract Surg. 14, 26–30. Quah BL, Wong EY, Tseng PS, et al. (1996). Analysis of photorefractive keratectomy patients who have not had PRK in their second eye. Ophthalmic Surg Lasers 27, S429–S434. Schein OD, Vitale S, Cassard SD, et al. (2001). Patient outcomes of refractive surgery – The Refractive Status and Vision Profile. J Cataract Refract Surg. 27, 665–673. Oshika T, Klyce SD, Applegate RA, et al. (1999). Changes in corneal wavefront aberrations with aging. Invest Ophthalmol Vis Sci. 40, 1351–1355. Oshika T, Klyce SD, Applegate RA, et al. (1999). Comparison of corneal wavefront aberrations after photorefractive keratectomy and laser in situ keratomileusis. Am J Ophthalmol. 127, 1–7. Martinez CE, Applegate RA, Klyce SD, et al. (1998). Effect of pupillary dilation on corneal optical aberrations after photorefractive keratectomy. Arch Ophthalmol. 116, 1053–1062.
Post-operative follow-up of the refractive surgery patient 89 Martinez CE, Applegate RA, Howland HC, et al. (1996). Changes in corneal aberration structure after photorefractive keratectomy. Invest Ophthalmol Vis Sci. 37, S933. 90 Oliver KM, O’Brart DPS, Stevenson CS, et al. (1997). Corneal aberrations and visual performance following photorefractive keratectomy (PRK) for hyperopia. Invest Ophthalmol Vis Sci. 38, S531. 91 Seiler T, Kaemmerer M, Mierdel P, et al. (2000). Ocular optical aberrations after photorefractive keratectomy for myopia and myopic astigmatism. Arch Ophthalmol. 118, 17–21. 92 Marcos S, Barbero S, Moreno-Barriuso E, et al. (2001). Total and corneal aberrations before and after standard LASIK refractive surgery. Invest Ophthalmol Vis Sci. 42, S2843. 93 Marcos S, Barbero S, Llorente L, et al. (2001). Optical response to LASIK surgery for myopia from total and corneal aberration measurements. Invest Ophthalmol Vis Sci. 42, 3349–3356. 94 O’Brart DPS, Lohmann CP, Fitzke FW, et al. (1994). Disturbances in night vision after excimer laser photorefractive keratectomy. Eye 8, 46–51. 95 Dello Russo J (1993). Night glare and excimer laser ablation diameter. J Cataract Refract Surg. 19, 565. 96 Kriegerowski M, Schlote T, Derse M, et al. (1997). Mesopic vision in correction of myopia: Soft contact lenses, spectacles and photorefractive keratectomy. Invest Ophthalmol Vis Sci. 38, S2458. 97 Miller WL and Schoessler JP (1995). Comparison of forward and backward scattered light in pre and post-surgical photorefractive keratectomy. Invest Ophthalmol Vis Sci. 36, S709. 98 Veraart HGN, Van Den Berg TJTP, Hennekes R, et al. (1995). Stray light in photorefractive keratectomy for myopia. Doc Ophthalmol. 90, 35–42. 99 Chisholm CM (2002). Assessment of Visual Performance: Comparison of Normal Subjects and Post-refractive Surgery Patients. PhD Thesis. (London: City University). 100 Oliver KM, Hemenger RP, Corbett MC, et al. (1997). Corneal optical aberrations induced by photorefractive keratectomy. J Refract Surg. 13, 246–254. 101 Esente S, Passarelli N, Falco L, et al. (1993). Contrast sensitivity under photopic conditions in photorefractive keratectomy: A preliminary study. J Refract Corneal Surg. 9, S70–S72. 102 Pallikaris IG, McDonald MB, Siganos D, et al. (1996). Tracker-assisted photorefractive keratectomy for myopia of –1 to –6 diopters. J Refract Surg. 12, 240–247. 103 Chisholm CM, Barbur JL, Edgar DF, et al. (2000). The effect of excimer laser refractive surgery on visual performance. Invest Ophthalmol Vis Sci. 41, S462. 104 Strolenberg UA, Jackson WB, Mintsioulis G, et al. (1996). Visual performance under dilated and non-dilated conditions following PRK: One year results. Invest Ophthalmol Vis Sci. 37, S566. 105 Montes-Mico R and Charman WN (2002). Mesopic contrast sensitivity function after excimer laser
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photorefractive keratectomy. J Refract Surg. 18, 9–13. Marcos S (2001). Refractive surgery and optical aberrations. Optics Photonics News 12, 22–25. Alanis L, Ramirez R, Suarez R, et al. (1996). Spatial contrast sensitivity in pre- and post-operative LASIK for high myopia patients. Invest Ophthalmol Vis Sci. 37, S570. Perez-Santonja JJ, Sakla HF and Alio JL (1998). Contrast sensitivity after laser in situ keratomileusis. J Cataract Refract Surg. 24, 183–189. Mutyala S, McDonald MB, Scheinblum KA, et al. (2000). Contrast sensitivity evaluation after laser in situ keratomileusis. Ophthalmology 107, 1864–1867. Montes-Mico R and Charman WN (2001). Choice of spatial frequency for contrast sensitivity evaluation after corneal refractive surgery. J Refract Surg. 17, 646–651. Chisholm CM, Barbur JL, Edgar DF, et al. (2000). The effect of refractive surgery on visual performance. Ophthalmic Physiol Opt. 20, 415–416. Waring GO (1989). Conventional standards for reporting results of refractive surgery. Refract Corneal Surg. 5, 285–287. Salchow DJ, Zirm ME, Stieldorf C, et al. (1999). Comparison of objective and subjective refraction before and after laser in situ keratomileusis. J Cataract Refract Surg. 25, 827–835. Hersh PS and Schwartz-Goldstein BH (1995). Corneal topography of phase III excimer laser photorefractive keratectomy;: Characterization and clinical effects. Ophthalmology 102, 963–978. McGhee CN and Bryce IG (1996). Natural history of central topographic islands following excimer laser photorefractive keratectomy. J Cataract Refract Surg. 22, 1151–1158. Bourque LB, Cosand BB and Drews C (1986). Reported satisfaction, fluctuation of vision and glare among patients one year after surgery in the Prospective Evaluation of Radial Keratotomy (PERK) Study. Arch Ophthalmol. 104, 356–363. Chisholm CM, Evans ADB, Harlow AJ, et al. (2003). New test assesses pilots’ vision following refractive surgery. Aviation Space Environ Med. 74, 551–559. Lovie-Kitchin JE (1998). Validity and reliability of visual acuity measurements. Ophthalmic Physiol Opt. 8, 363–370. Pelli DG, Robson JG and Wilkins AJ (1988). The design of a new letter chart for measuring contrast sensitivity. Clin Vis Sci. 2, 187–199. Mantyjarvi M and Laitinen T (2001). Normal values for the Pelli–Robson contrast sensitivity test. J Cataract Refract Surg. 27, 261–266. Boxer-Wachler BS, Durrie DS, Assil KK, et al. (1999). Improvement of visual function with glare testing after photorefractive keratectomy and radial keratotomy. Am J Ophthalmol. 128, 582–587. Barbur JL, Edgar DF and Woodward EG (1995). Measurement of the scattering
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characteristics of the eye in relation to pupil size. In Non-invasive Assessment of the Visual System (Technical Digest Series), Vol. 1, p. 250–253. (Washington, DC, USA: Optical Society of America). Allen MJ and Vos JJ (1967). Ocular scattered light and visual performance as a function of age. Am J Optom. 44, 717–727. Edgar DF, Barbur JL and Woodward EG (1995). Pupil size measurements in relation to light scatter in the eye. Invest Ophthalmol Vis Sci. 36, S938. O’Brart DPS, Lohmann CP, Fitzke FW, et al. (1994). Night vision after excimer laser photorefractive keratectomy: Haze and halos. Eur J Ophthalmol. 4, 43–51. Roberts CW and Koester CJ (1993). Optical zone diameters for photorefractive corneal surgery. Invest Ophthalmol Vis Sci. 34, 2275–2281. Lafond G (1997). Treatment of halos after photorefractive keratectomy. J Refract Surg. 13, 83–88. Alster Y, Loewenstein A, Baumwald T, et al. (1996). Dapiprazole for patients with night haloes after excimer keratectomy. Graefes Arch Clin Exp Ophthalmol. 234, S139–S141. Szczotka LB and Aronsky M (1998). Contact lenses after LASIK. J Am Optom Assoc. 69, 775–784. Goldberg DB (2001). Laser in situ keratomileusis monovision. J Cataract Refract Surg. 27, 1449–1455. Wright KW, Guemes A, Kapadia MS, et al. (1999). Binocular function and patient satisfaction after monovision induced by myopic photorefractive keratectomy. J Cataract Refract Surg. 25, 177–182. Jain S, Ou RJ and Azar DT (2001). Monovision outcomes in presbyopic individuals after refractive surgery. Ophthalmology 108, 1430–1433. Carones F, Brancato R, Venturi E, et al. (1994). The corneal endothelium after myopic excimer laser photorefractive keratectomy. Arch Ophthalmol. 112, 920–924. Venturi E, Carones F and Brancato R (1995). Evaluation of the corneal endothelium immediately after myopic excimer laser photorefractive keratectomy. Invest Ophthalmol Vis Sci. 36, S1062. Nuss R, Puliafito CA and Dehm EJ (1987). Unscheduled DNA synthesis following excimer laser ablation of the cornea in vivo. Invest Ophthalmol Vis Sci. 28, 287–294. Wachtlin J, Blasig IE, Schrunder S, et al. (2000). PRK and LASIK – their potential risk of cataractogenesis: Lipid peroxidation changes in the aqueous humor and crystalline lens of rabbits. Cornea 19, 75–79. Costagliola C, Balestrieri P, Fioretti F, et al. (1994). ArF excimer laser corneal surgery as a possible risk factor in cataractogenesis. Exp Eye Res. 58, 453–457. Sun R, Gimbel H and Penno EE (2000). Intraocular lens power calculation after corneal refractive surgery remains challenging. Ophthalmology 107, 226–208.
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139 Waring GO, Lynn MJ and Strahlman ER (1991). Stability of refraction 4 years after radial keratotomy. Am J Ophthalmol. 111, 133–144. 140 Waring GO, Lynn MJ and McDonnell PJ (1994). Results of the Prospective Evaluation of Radial Keratotomy (PERK) study at ten years after surgery. Arch Ophthalmol. 112, 1298–1308. 141 Meza J, Perezsantonja JJ, Moreno E, et al. (1994). Photorefractive keratectomy after radial keratotomy. J Cataract Refract Surg. 20, 485–489. 142 Bores LD, Myers W and Cowden J (1981). Radial keratotomy: An analysis of the American experience. Ann Ophthalmol. 13, 941–948. 143 Cowden JW and Bores LD (1981). A clinical investigation of the surgical correction of myopia by the method of Fyodorov. Ophthalmology 88, 737–741. 144 Hoffer KJ, Darin JJ, Pettit TH, et al. (1981). UCLA clinical trial of radial keratotomy. Preliminary report. Ophthalmology 88, 729–736. 145 Kwitko ML, Gritz DC, Garbus JJ, et al. (1992). Diurnal variation in corneal topography after radial keratotomy. Arch Ophthalmol. 110, 351–356.
146 Bullimore MA, Sheedy JE and Owen D (1994). Diurnal visual changes in radial keratotomy – implications for visual standards. Optom Vis Sci. 71, 516–521. 147 Waring GO, Lynn MJ, Gelender H, et al. (1985). Results of the Prospective Evaluation of Radial Keratotomy (PERK) study one year after surgery. Ophthalmology 92, 177–198. 148 Bryant MR, Szerenyi K, Schmotzer H, et al. (1994). Corneal tensile strength in fully healed radial keratotomy wounds. Invest Ophthalmol Vis Sci. 35, 3022–3031. 149 Panda A, Sharma N and Kumar A (1999). Ruptured globe 10 years after radial keratotomy. J Refract Surg. 15, 64–65. 150 Vinger PF, Mieler WF, Oestreicher JH, et al. (1996). Ruptured globes following radial and hexagonal keratotomy surgery. Arch Ophthalmol. 114, 129–134. 151 Lee BL, Manche EE and Glasgow BJ (1995). Rupture of radial and arcuate keratotomy scars by blunt trauma 91 months after incisional keratotomy. Am J Ophthalmol. 120, 108–110. 152 Behl S and Kothari K (2001). Rupture of a radial keratotomy incision after 11
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years during clear corneal phacoemulsification. J Cataract Refract Surg. 27, 1132–1134. Applegate RA, Howland HC, Sharp RP, et al. (1998). Corneal aberrations and visual performance after radial keratotomy. J Refract Surg. 14, 397–407. Applegate RA, Hilmantel G and Howland HC (1996). Corneal aberrations increase with the magnitude of radial keratotomy refractive correction. Optom Vis Sci. 73, 585–589. Krasnov MM, Avetisov SE, Makashova NV, et al. (1988). The effect of radial keratotomy on contrast sensitivity. Am J Ophthalmol. 105, 651–655. McDonald MB, Haik M and Kaufman HE (1983). Colour vision and contrast sensitivity testing after radial keratotomy. Am J Ophthalmol. 103, 468. Olsen H and Andersen J (1991). Contrast sensitivity in radial keratotomy. Acta Ophthalmol. 69, 654–658. Corbe C, Jacquelin P, Pedeprat P, et al. (1993). Aircrew fitness decisions and advances in refractive surgery techniques. German J Ophthalmol. 2, 146–149.
6 Case reports Sunil Shah, Stephen J Doyle and Paul Cherry
In this chapter we look at various interesting and complicated patient outcomes from refractive surgery. We begin, however, with a brief guide to the video clips on the CD that accompanies this book.
Videos on CD Video 1 – Orbscan Video 1 (courtesy of Bausch and Lomb) shows the two scanning slits of the Orbscan anterior segment analysis unit in operation. In real time each scan takes 0.7 seconds. The scans are also combined with Placido information.
Video 2 – Superior flap Video 2 (courtesy of Bausch and Lomb) shows the Bausch and Lomb Hansatome microkeratome cutting a corneal flap in a laser in-situ keratomileusis (LASIK) procedure. This is a two-piece microkeratome and the cutting device can be seen being placed onto the base plate. Note that the flap is hinged superiorly (seen from the surgeon’s
persepctive), which some surgeons argue is better for the healing of the cornea, although others suggest that a nasal hinge leads to less corneal nerve disturbance. Video 3 – Amadeus Video 3 (courtesy of AMO) shows the Amadeus one-piece microkeratome by Advanced Medical Optics (AMO). Note that the corneal flap is hinged nasally and that the flap thickness in this particular case is slightly thicker than that in Video 2.
Video 4 – Laser subepithelial keratectomy Video 4 starts with the alcohol well in place filled with a low percentage ethanol solution. This is absorbed with a Merocel surgical spear. The cornea is washed and dried and the surgeon (Sunil Shah) then breaks the epithelium. The epithelium is moved away from the central area and the excimer laser beam is applied to Bowman’s membrane to create the new corneal profile, in accordance with the refractive error. The epithelium is
replaced. Note that the epithelium does not lift in one thick slice, but rather a thin sheet is kept intact and removed and subsequently repositioned. The thickness of the epithelium is around 50μm, as opposed to a LASIK flap of around 160μm. Video 5 – WAVE CL system Video 5 (courtesy of Northern Lenses) shows the Keratron SCOUT topography unit, from Carlton Ophthalmic. This portable device can be handheld, table mounted or slit-lamp mounted. The system is based on a Placido cone and able to use its radius of curvature data from the anterior cornea to design a gas permeable contact lens for that particular cornea. The data are used by Northern Lenses in their WAVE system contact lenses. This system is particularly useful when a corneal lens is required for an irregular corneal profile.
Flap transaction • •
Corneal consultant, Sunil Shah; Urgent patient referral to hospital corneal consultant; • 0/–1.5 refraction in affected eye, –2D in other eye; • Age 55 years, male. The patient had planned to have surgery on the left eye (LE) only, but on the day of surgery (when he saw the surgeon at the
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high-street clinic for the first time), he was persuaded to have surgery on the right eye (RE) also. What persuaded the patient to have surgery in the RE also is unclear, as he stated that he could read reasonably well with the RE and his personal thinking previously had been that he could always have the RE treated at a later date. The Hansatome was apparently normal (nothing unusual with the K-readings, corneal topography or corneal examination), as no abnormality was noted by the surgeon or scrub nurse. The pressure readings were good. The LE was not treated. When the microkeratome was removed and the flap lifted, there was a horizontal transaction of the flap (superior hinge with half of the flap attached), a 9 o’clock to 3 o’clock cut through the flap, with that portion being a free flap.
Figure 6.1 Superior portion of flap in situ, with an epithelial and stromal deficit inferiorly
The inferior free-flap portion could not be found on the eye or on the microkeratome. Examination of the blade was unremarkable. No abnormality could be found in the microkeratome set up. The patient was seen urgently by the ophthalmological consultant at the hospital that evening. Obviously extremely anxious, he had been speaking to his solicitor in the car while being transferred from the clinic. Examination confirmed the superior portion of the flap in situ, with an epithelial and stromal deficit inferiorly (Figure 6.1). Options were discussed with patient and conservative management suggested for the time being. He was started on guttae hyaluronic acid 0.18% hourly, ocular liquid paraffin six times daily, and both chloramphenicol and dexamethasone four times daily. The patient was reviewed at 1 day, 1 week, 1 month and 3 months post-incident: • 1 day review: no significant change, with little epithelial healing. • 1 week review: full epithelial healing, and the epithelium looked quite healthy. There was a clear stromal deficit. Retinoscopy confirmed a mild myopic astigmatism in the superior portion and high hyperopia in the lower portion. • 1 month review: epithelium had filled in the stromal deficit completely, with refraction now stable across whole cornea at pre-surgery levels. • 3 month review: stable, no haze and best-corrected visual acuity (BCVA) back to 6/5 (Figure 6.2).
Epithelial remodelling following laser subepithelial keratectomy that caused an increase in prescription A patient persuaded the surgeon (Sunil Shah) to perform laser subepithelial keratectomy (LASEK) on her. She was very eager to have surgery as a number of her family members had previously undergone successful refractive surgery by the same surgeon. Initially, surgery was refused as the prescription was very mild (0/–1.5D × 115 and 0/–1.5D × 80) with an unaided visual acuity (VA) of 6/12 in each eye, improving to 6/9 binocularly unaided. The prospective patient was adamant that she wanted refractive surgery and clearly stated that she was aware that she may gain little benefit from laser refractive surgery, but she wanted to take the chance. She was particularly aware that there was very little risk from LASEK for this sort of prescription and agreed that LASIK was an unnecessary risk in her case. Surgery was uneventful using the butterfly LASEK technique. Recovery was unremarkable in that the epithelial healing occurred within 3 days. A review appointment was given for 1 week. She telephoned a few days later and said she felt that her uncorrected visual acuity (UCVA) was worse than it had been before surgery and she had returned to wearing her glasses. At the 2 week review, her refraction was –1/–1 × 90 and –0.75/–1.25 × 65. This refraction was maintained over the next 3 months (subjective and objective), which was presumed to result from epithelial hyperplasia, and it played a significant role as the original prescription was so small.
Bilateral flap infection masquerading as diffuse lamellar keratitis
Figure 6.2 Patient in Figure 6.1 at 3 month review: stable, no haze, BCVA back to 6/5
The patient underwent routine uncomplicated LASIK in a full ophthalmic operating theatre (not just a clean laser room). The surgery (Sunil Shah) was uncomplicated, in the middle of a laser list and all the other patients were uncomplicated. Review at 1 day was unremarkable, with UCVA 6/9 in each eye. An urgent review was carried out at 4 days, as the patient was worried. UCVA was 6/60 RE and 6/24 LE. The appearance was of grade 1 diffuse lamellar keratitis (DLK) with some small focal opacities in the RE only. The patient was asked to continue topical chloramphenicol four times daily, and
Corneal anatomy, physiology and response to wounding
to increase the prednisolone acetate 1% from four times a day to hourly in each eye. On review 1 day later (at 5 days), the DLK had substantially resolved, but one focal abnormality in the RE was unchanged and the LE had multiple tiny focal abnormalities that appeared to be settling. UCVA had not improved and the treatment was continued. The eyes remained white throughout. On review 2 days later (at 7 days), the UCVA was unchanged. The RE focal abnormality was larger in size and had slightly fluffy edges, the LE focal abnormalities were unchanged and there was no further evidence of DLK. A diagnosis of bilateral flap infection was made. Treatment was changed to prednisolone acetate 1% four times daily, chloramphenicol once an hour and ofloxacin once an hour. On review 1 day later (at 8 days), there was no deterioration, but no improvement, in the LE; in the RE, the size was unchanged, but there were satellite lesions around the original lesion. A decision was made to collect samples from the RE with a flap washout and a corneal scrape. The LE was felt safe to leave. The RE flap was lifted and a corneal scrape performed. The opacity was not impressive in terms of infection: it felt like a string of mucus. There was no corneal melt around the opacity. A washout was then performed. Urgent microscopy revealed a staphylococcus as the probable organism. The treatment was felt to be adequate and therefore continued. Culture of the organism confirmed Staphylococcus aureus, sensitive to both chloramphenicol and ofloxacin. It is worth noting that a typical DLK is probably an infection and an indication for an early flap lift. Slow resolution followed and left some minor stromal scarring. Refraction improved from –1.5/–2 × 90 RE and –1.25/–0.5 × 110 at 2 weeks to 0.75/–0.5 × 90 and –0.75/–0.25 × 100 at 3 months. BCVA was initially 6/20 in the RE, which improved to 6/7.5 at 3 months, but it remained at 6/5 in the LE throughout.
Flap melt after treatment for epithelial ingrowth The patient was referred from a ‘highstreet clinic’ to local corneal consultant (Sunil Shah). Apparently an uncomplicated LASIK originally, but recurrent epithelial ingrowth had been treated twice already. Further recurrence occurred with poor BCVA. Examination revealed 30% epithelial ingrowth that encroached on the visual axis. Pre-operative refraction was –3DS,
and the refraction on presentation to the corneal consultant was +3/–2 × 85. The other eye was UCVA 6/5, but for the ingrowth eye BCVA was 6/15. At surgery, the flap was lifted very easily. A notch in the flap found in an area of ingrowth was assumed to be the cause of the ingrowth. Flap and base were cleaned and treated gently with absolute ethanol and then copious irrigation. It was decided not to suture the flap at this stage as, despite the notch, the flap was sitting nicely and the extent of the original treatment was unclear because the original treating doctor was no longer in the country and records were sketchy. Recovery was unremarkable. UCVA improved to 6/12 and BCVA to 6/6, with +0.75/+0.75 × 180. Over the next 6 months, the peripheral flap melted in an area of previous epithelial ingrowth. The central flap was not affected visibly, but the subjective cylinder increased to 1.75D and the BCVA dropped to 6/9. The flap melt remained stable. Therefore, a rigid contact lens was tried, which improved the BCVA to 6/5–. A custom-fit contact lens was ordered, based on the topography, which gave an excellent fit and visual outcome.
LASEK in one eye with LASIK in the other eye because of unilateral von Hippel–Lindau lesion This case is an example of both LASEK and LASIK used to give the same end result for each eye. The patient’s RE had a unilateral von Hippel–Lindau lesion (i.e., an angiomatous lesion), which was at risk of bleeding and had been treated with laser previously by a medical retina surgeon in an attempt to stabilize it. The patient was –5D in each eye, and the risks and benefits were discussed with the surgeon (Sunil Shah). LASIK was too risky in the RE as increased intraocular pressure from the suction ring may cause the angiomatous lesion to bleed. The recommendation was to have LASEK in the RE and LASIK in the LE simultaneously. The surgery was uncomplicated: • 1 day post-operatively, UCVA 6/12 and 6/5. • 1 week post-operatively, UCVA 6/6 and 6/5, refraction +0.5DS RE, 0 LE. • 1 month post-operatively, UCVA 6/5 and 6/5, refraction plano both eyes. The patient’s comments were that 3 days of discomfort were well worth it, there
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was no significant pain and with visual recovery within 1 week she had no particular problem. When asked whether she preferred LASIK or LASEK, she said she was petrified by the cut of LASIK and so, if she had to do it again, she would probably consider bilateral LASEK.
Keratectasia case The patient was a 24-year-old male: • RE: –6.25/–2.25 × 15, best-corrected spectacle visual acuity (BCSVA) 6/7.5; • LE: –12.25/–2.0 × 15, BCSVA 6/10; • Pupils were 6mm diameter; • Pachymetry: RE 540 and LE 508. LE LASIK was undertaken by the surgeon (Doyle) with the aim of only –3.25 because of the depth problems (the patient was happy with this outcome as then he could wear normal best-form glasses even if the RE was not good enough unaided). The optical zone (OZ) was 5.5mm and the treatment zone (TZ) was 7mm plus an elliptical cylinder to save depth (Nidek EC 5000). The flap was 130μm (Moria ‘one’). The predicted ablation depth was 130μm and the predicted bed was 248μm. Intraoperative pachymetry was not carried out in this case, in either eye. After 2 weeks RE LASIK was undertaken, with 6mm OZ, 7.25mm TZ and a predicted ablation depth of 131μm (Nidek EC 5000). The flap was 160μm (Moria ‘one’), with a predicted bed of 249μm. Post-operative pachymetry at 1 month was 400μm in both eyes, at 6 months it was RE 375μm and LE 400μm and at 1 year it was RE 385μm and LE 412μm. Refraction at 2 months post-operatively was: • RE +1/–1 × 180 (VA 6/12 unaided and 6/7.5 with glasses); • LE –2/–1.5 × 30 (VA 6/15 unaided and 6/10 with glasses). At 30 weeks refraction was: • RE –1/–0.75 × 140 (VA 6/12 unaided and 6/7.5 with glasses); • LE –3/–0.75 × 15 (VA 6/60 unaided and 6/10 with glasses). He developed keratectasia in the RE, and serial topography showed that it started over the area of an old small scar. This scar was caused by a thorn when a child (thickness over this area post-operatively was about 475μm). Whether the scar was a relevant factor in this case is not known. What made the surgeon discount this initially was that surgeons carry out LASIK after penetrating keratoplasty, with a full-thickness 360° scar and the patients appear to have no problems. The initial topography was entirely
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regular and the patient had worn glasses from the age of 4 years. It appears that the depth of the ablation and/or the flap was more than expected in the RE. The surgeon would have expected a greater likelihood of such a problem in the LE because of the initial thinness. The patient was referred to the local University contact lens clinic for a gas permeable contact lens fitting, which was accomplished with great skill, although not without difficulty.
Late flap lift post-LASIK The patient was first examined by the surgeon (Cherry) on 29 April 1997. He was a high myope who successfully wore disposable soft contact lenses that had last been used 2 weeks prior to the examination. Presenting glasses prescription was RE –9.00, LE –8.00. Cycloplegic refraction was RE –8.50, –0.50 × 180, 6/7–, and LE –8.00, 6/6–. Keratometry was recorded as RE 43.87, 44.12 × 86, and LE 43.62, 44.00 × 98. He elected for a monovision LASIK treatment because he was a current successful wearer of monovision contact lenses. His LE was dominant. Treatment day was scheduled for 16 May 1997 at The Toronto Laser Sight Centre. EyeSys topography was performed prior to the procedure and showed no abnormalities. Central pachymetry was RE 584, LE 580. Bilateral, simultaneous LASIK was undertaken, using the VISX 20/20 excimer laser (software version 4.02c) and the ACS keratome, which produced 8.5mm flaps with nasal hinges. Two treatments were undertaken in each eye, in accordance with multi-zone technology. Details were as follows (corneal plane): • RE –2.31, –0.52 × 180, zone size an ellipse 6.00mm × 5.4mm, ablation 36μm; • RE –3.00, zone size of 6.00mm, ablation 45μm;
•
LE –3.43, zone size of 6.00mm, ablation 52μm; • LE –3.50, zone size of 6.00mm, ablation 53μm; • The flaps were 160μm in thickness, so stromal beds of 343μm in the RE and 315μm in the LE remained. Follow-up was uneventful. However, on 22 September 1997, the patient expressed some dissatisfaction in that the reading distance was too close with his RE. At that time, manifest refraction was RE –3.75, 6/7– and LE plano, 6/7–. He elected to have his RE retreated, which was accomplished by relifting the flap on 3 October 1997 at The Toronto Laser Sight Centre. Topography prior to retreatment showed central treatments with no complications. Pre-enhancement pachymetry was RE 541μm, somewhat thicker than the predicted thickness of 343 + 160 = 503μm. The laser used was the Chiron Keracor 116. Treatment for the RE, was –1.58, zone size of 6.00mm and ablation 27μm to leave a theoretical stromal bed of 316μm in the RE. He was extremely happy after the retreatment, and did not quite complete his scheduled follow-up visits, having last attended for follow-up on 17 January 1998, when the uncorrected distance vision was RE 6/60, LE 6/9+. Manifest refraction was RE –1.50, –0.50 × 180, and he was delighted with the reading distance that this remaining myopia produced. His opinion about monovision slowly changed, however. When he finally came back for a further examination on 27 November 2001, he was still happy with both his reading and distance vision, but was considering having the RE enhanced for distance in Arizona, where he was planning to go to escape the Canadian winter! At that time, cycloplegic refraction was LE +1.50, –0.50 × 90, 6/7 but, despite this overcorrection, he had no problems with distance vision.
He returned for re-examination on 28 November 2002, having not had any surgery in Arizona. Uncorrected distance vision was LE 6/12–. He was still happy with both his distance and reading vision, but now expressed a definite wish to rid himself of monovision and, further, to have the procedure done in Toronto. Keratometry was RE 39.37, 39.25 × 98. Cycloplegic refraction was RE –1.50, 6/7–. RE enhancement was performed at the Sacor On-Site Laser, Toronto, on 3 January 2003, 5 years and 3 months after the previous enhancement. Pre-operative topography again showed no abnormalities. Pachymetry was RE 528μm compared with the theoretical corneal thickness of 316 + 160 = 476μm. The right scotopic pupil measured 5.5mm. The flap was lifted at the operating microscope using a Sinskey hook. The correct site of the flap edge in the temporal, peripheral cornea was first identified by gentle pressure on the cornea. Flap lifting was completed with a blunt spatula. It was noted that the flap lifted very easily; it certainly did not give the impression that it had last been lifted 5 years and 3 months previously. The excimer laser used to do the treatment was the Laser Sight LSX with an Accutrack eye tracker. The treatment at the corneal plane was RE –1.47, zone size of 6.00mm with a 1mm blend zone outside the optical zone, ablation of 24μm to leave a theoretical stromal bed of RE 316 – 24 = 292μm. A bandage soft contact lens did not have to be used post-operatively because the flap junction had not been disrupted unduly. The initial result of this second RE enhancement on 10 January 2003 was uncorrected RE vision of 6/7– with a plano refraction. There was a small, horizontal microstria just below the pupil margin at slit-lamp examination; this was asymptomatic. The patient is currently lost to follow-up because he has escaped the Canadian winter again!
7 Co-management issues Shehzad A Naroo, Baldev K Ubhi and W Neil Charman
Introduction It is clear that the development of refractive surgery offers both a challenge and an opportunity to ophthalmic practice. Many patients, having been examined by an optometrist, express an interest in the possibilities of refractive surgery and require advice on what it involves and its likely success rate. Such patients, if they opt for surgery, represent lost short-term dispensing revenue. Thus, maybe optometrists should explore possible formal links with their local refractive surgery centres, so that not only are they better informed of the facilities these offer, but also some integration of the examination and advice offered in the practice can occur with the work of the centre. It may be sensible to make available to patients in the practice written information on the pros and cons of refractive surgery. A second, growing and possibly more significant class of patients are those who arrive at the practice and already have had refractive surgery at some earlier date. It is important that the optometrist be alert to this possibility, since it may, for example, affect the results of tonometry and be the cause of dry eye and other problems (as detailed in Chapter 5). Even though many of these patients may consider that their surgery was successful, they may still need a spectacle or other form of correction, for example to raise their acuity to the standard required for driving, or to correct presbyopia. There is little information on the long-term enthusiasm of patients for ‘enhancement’ procedures to correct the slow drifts in refraction that occur throughout adult life in all eyes,1 but it seems unlikely that the majority of patients will wish to (or be able to) rely exclusively on their surgical correction. Many may be resentful
that having “thrown away their glasses and contact lenses”, they now need an additional optical aid. It is, then, important that the optometrist be able not only to recognize the possible symptoms of post-surgical patients, but also to respond sympathetically to any psychological difficulties that they may be experiencing. Undoubtedly, as the first generation of refractive surgery patients age, new problems may emerge, and probably the primary care optometrist is best placed to detect these and alert the general ophthalmic community. Patients who discuss their desire to undergo refractive surgery procedures with their own optometrist often report comments from their optometrist such as “laser surgery is still experimental” or “the results are not stable”. This often leads to patients losing faith in their optometrist and possibly seeking primary eye care from another optometrist for themselves and, perhaps, for their family. Usually, the optometrists who dismiss refractive surgery with such blasé comments are those who are illinformed about current techniques. At the very least, optometrists should be aware of the type of surgery that is available in their local vicinity and should inform themselves of the results being achieved, especially now that refractive surgery has established a firm foothold as an alternative to optical aids. Those optometrists with more of an interest might want to consider a ‘sharedcare’ scheme with a local refractive surgeon or refractive surgery centre, although it would be wise first to establish with whom the responsibility for the patient ultimately lies. In this type of scheme the role of the optometrist could also involve counselling the potential patient, although ultimately the treating surgeon will decide on the patient’s suitability for surgery.
As newer techniques and instruments are developed and complication rates are minimized, refractive surgery is expected to grow further. The need for all eye-care professionals to be well equipped in their knowledge of refractive surgery is becoming increasingly important, as they are the source of professional guidance for patients who wish to undergo refractive surgery.
Optometrists and comanagement in refractive surgery Recent market research from a Mintel report on Optical Goods & Eyecare suggests that in the UK approximately 75,000 procedures were carried out in 2001 on around 41,000 consumers and that this figure probably doubled in 2002.2,3 The question arises, what is the appropriate level of involvement that an optometrist should have with refractive surgery? Certain optical companies are now able to provide some refractive surgery procedures and to use their existing network of practices to recruit potential patients, as well as for the pre- and post-operative assessment of patients. Certainly, for these latter tasks an optometrist is qualified appropriately.4 Many independent practices enter agreements with external laser eye clinics, under which the patients are seen initially at the optometric practice and are then referred to the laser eye clinic for surgery, with some of the post-operative assessment carried out at the initial optometric practice. Some of these agreements involve the laser eye clinic making a payment to the referring optometrist, in return for the optometrist making the initial assessment and providing some of the post-operative
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aftercare. This can be a delicate issue, as the optometrist should not feel that the payment is a ‘referral fee’. However, this fee is often more than the optometrist would charge his or her own routine patients for a regular eye examination. Of course, it could be argued that during a pre-operative consultation for refractive surgery the optometrist performs additional tests (such as pachymetry, pupillometry and corneal topography). It could be further argued that a typical regular eye examination fee is actually less than the realistic fee calculated on the chair time for that appointment, and that the fee is subsidized by the sale of optical appliances, which would not be the case in a pre- or post-operative refractive surgery appointment. For example, let us say that a co-management fee of £250 is paid to an optometrist for referring a patient to a laser eye clinic. For this fee the optometrist is expected to perform a pre-operative consultation and post-operative assessments at 1 week, 1 month, 3 months and 1 year. This fee may be hard to justify if a routine eye examination with that optometrist is charged at £20; the excessive fee may be seen as an inducement for referral. Furthermore, the optometrist should not feel any obligation or limitation to restrict himor herself to being able to advise patients to go to one particular laser eye clinic. Currently, the General Optical Council (GOC) is examining the issue of co-management fees, and recently it made a proposal to ban optometrists and opticians from receiving co-management fees or inducements.5,6 The GOC published a statement on the professional conduct of optometrists regarding photorefractive keratectomy (PRK) in 1995, although currently there are no other regulations on the more recent techniques, such as laser in situ keratomileusis (LASIK) and laser subepithelial keratectomy (LASEK). The statement advised that no optometrist should accept any fee or other inducement for referring a patient to a particular clinic. Nor should any agreement be made whereby the optometrist is restricted to referring a patient to a particular clinic. In addition, any work done prior to a referral must be paid by the patient to the optometrist and not by the clinic. However, any work carried out after treatment can be paid by a clinic on the patient’s behalf and with the patient’s consent to avoid any risk of bias or unethical behaviour. Refractive surgery is considered a private treatment and currently is not provided under the NHS. The necessary interaction between general practitioners (GPs), optometrists and refractive surgeons and/or ophthalmologists is somewhat dif-
ferent to that which exists in the provision of general ophthalmic services (GOS). The sharing of care for a patient is often termed ‘co-management’, whereby the patient is co-managed between the ophthalmologist at the clinics and/or hospitals and the optometrist. Post-surgery, the ophthalmologist refers only non-complicated cases back to the optometrist involved, and complicated cases remain under the care of the surgeon until he or she feels that they are ready to return back into the optometrist’s care. However, although the management may be passed on to the optometrist under the co-management scheme, the ultimate duty of care remains with the surgeon.7 The Royal College of Ophthalmologists does not have specific guidelines for the involvement of optometrists in the comanagement of LASIK patients, although general guidelines on co-management schemes were issued in 1996, in conjunction with the Royal College of General Practitioners and College of Optometrists. These general guidelines advise that every scheme should be set out in a locally agreed formal protocol that should specify overall clinical responsibility at any one time. However, it has been claimed by some optometric authorities in refractive surgery that the reluctance of some clinics to have an ophthalmologist on site full time has meant that optometrists are being forced to make clinical decisions that lie outside the scope of their responsibility. An example is the ‘prescribing’ of topical ocular therapeutic agents, including topical corticosteroids, as a result of the surgeon’s absence, which is not only against the advise from the College of Ophthalmologists, but also against the Opticians Act 1989.8
Training Many laser clinics provide training to practitioners who sign up for a co-management agreement with that clinic. The level of training varies from clinic to clinic; some offer only a few hours, which may largely consist of a visit to the laser clinic, whereas other clinics insist on regular attendance for training. There are also many other courses in the management of refractive surgery patients, some offered by the clinics themselves and others by universities or learned societies, such as the British Society for Refractive Surgery (BSRS). Much of the continued education and training (CET) available is approved by the Directorate for Optometric Continued Education and Training (DOCET). However, there is no requirement for those who elect to be
involved in co-management schemes to undergo any CET.
Professional relationships and responsibilities It is worth taking a step back and looking at the traditional relationships between optometrist, GP and ophthalmologist under the GOS system. A patient seen by an optometrist and deemed to have a pathology or abnormality that requires medical intervention or observation is referred, under the GOS terms and conditions, via the GP to the ophthalmologist to be seen under the Hospital Eye Service (HES). The urgency of the referral is suggested by the optometrist and the GP, but ultimately the ophthalmologist decides on how soon the patient is seen. The situation is slightly different if the patient’s problem is considered to be an emergency. In such a case the optometrist and/or the GP may arrange for an immediate appointment with the ophthalmologist if this is deemed to be in the patient’s best interest. When the patient is sent by the optometrist directly to be seen by an ophthalmologist, the GP is notified. Let us also consider the role of these three clinicians. The GP’s role A GP is described as a physician who does not have a sub-speciality, but who has a medical practice in which he or she investigates and treats illnesses. Complex problems or acute illnesses, however, are referred for secondary care at the hospital and, in terms of ophthalmological care, to the HES. The GP may also refer a patient with visual symptoms to the optometrist. GPs are independent contractors of the NHS, who are able to mix private practice with NHS contracted work. Some GPs work under the general medical services (GMS) contract and others are employed under the personal medical services (PMS) scheme, which enable the GPs to have contracts negotiated locally with commissioning health bodies, such as primary care trusts (PCTs). Patients generally are referred to secondary care under the NHS, whereby the GP refers a patient for further specialized investigation in terms of the management of a particular illness. The GP delegates the responsibility for that particular condition to the consultant at the hospital. However, if the condition is resolved and the patient discharged, the GP then continues to manage or monitor his or her patient as normal under the NHS, or privately. Thus, the responsibility for the patient in terms of that particular
Co-management issues
condition is delegated to a consultant and team at the hospital, until discharged. At present under the NHS, the GP would normally only refer a patient to an optometrist if he or she believes the problem is refractive, although optometrists possess a range of sophisticated equipment (e.g., a slitlamp or visual field testing equipment) that is not available at most GPs and enables optometrists to detect eye disorders and diseases. However, once a patient is referred to an optometrist for visual correction, only the responsibility of undertaking a sight test is delegated to the optometrists; the responsibility for medical care, such as treating blepharitis and conjunctivitis, still lies with the GP. The ophthalmologist’s role The ophthalmologist is both a physician and a surgeon for conditions that occur in and around the eye and the visual pathways. Most ophthalmologists tend to work in the secondary-care environment, for example in the eye department of hospitals, and they may also hold out-patient departments at peripheral clinics. The surgical work of the general ophthalmologist may include cataract extraction, squint and glaucoma surgery, and retinal, oculoplastic and nasolacrimal surgery. Many consultant ophthalmologists also have an area of particular interest and expertise, such as glaucoma, paediatrics, retinal disorders, etc., for which they may hold special clinics. For cases in which an ophthalmologist holds a specific sub-speciality, it is not uncommon for ophthalmologist colleagues to make tertiary referrals. Consultant ophthalmologists are responsible for all the patients in their care, and for supervising and training junior doctors. However, when a referral is sent to the ophthalmologist, the consultant or registrar decides upon the urgency of appointment, and consequently the responsibility for the patient is only taken if the ophthalmologist decides to monitor or treat the condition. It is, then, possible that the patient may be sent back to the GP, in which case the GP is responsible for monitoring the patient until secondary care is available. In contrast, the patient is not normally referred back to the optometrist for monitoring, unless the optometrist is involved in a co-management and/or a shared care scheme with the hospital. The optometrist’s role Optometrists are graduates who have undertaken a 3- or 4-year university-based undergraduate degree course at an accredited optometry school followed by a period of at least 1 year supervised practice before
taking professional qualification examinations. Successful completion of these examinations, set by the College of Optometrists (a public benefit body for the improvement and conservation of human vision), leads to registration with the GOC. The GOC licences optometrists to practice in the UK under the provisions of the Opticians Act 1989 and regulates the practice of optometrists in the UK who work privately or under the NHS. Once qualified, optometrists are able to perform a sight test, which includes the detection of injury, disease or abnormality in the eye, and a refraction, which enables them to dispense spectacles, prescribe and fit contact lenses and prescribe low-vision aids. Optometrists work mainly in the primary care sector and are independent contractors. They provide NHS examinations by initially registering with the PCT responsible for the location in which they intend to offer the service. This seals a contract with that PCT, under which the optometrists agrees to abide by the ‘terms of service’ for the provision of NHS services (GOS) under the 1986 regulation Statutory Instrument (SI) 1986/975. Free NHS eye examinations are available to certain groups of qualifying patients, such as minors (under 16 years of age), senior citizens (over 60 years of age), students in full time education, patients with low income, diabetic patients, glaucomatous patients and those above the age of 40 years with direct relatives who suffer from glaucoma. If during a routine eye examination an optometrist detects an injury or disease, he or she is obliged to manage the patient under certain referral criteria set by the NHS. The GOS Amendment (No. 2) Regulations 1989 SI 1989/1175 requires that a patient who is diagnosed with diabetes or glaucoma be referred to the patient’s GP. This must also be done if a satisfactory standard of vision is unlikely to be achieved with corrective lenses. In England and Wales this procedure is carried out via the GOS 18 Referral Form, which is the standard form for an NHS referral. Under the Opticians Act 1989, the GOC is given the power under sections 31(5) and 5(A) to make rules on referral that apply to all practising optometrists, be it under the NHS or private. Thus, the ‘Rules relating to Injury or Disease of the Eye, 1999’ are the present regulation, and are set by the GOC. Therefore, under these regulations a patient who presents to an optometrist with an injury or disease must be referred to the patient’s GP, unless that patient is acting on the advice or instructions of his or her GP or if the patient is suffering from a condition
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that requires immediate attention, in which case the optometrist should refer the patient directly to the local HES, although the GP would be informed. If the professional judgement of the optometrist deems that no referral to the GP is necessary, he or she may (at his or her own discretion) decide to monitor the patient and not refer on that occasion. It is considered that, under SI 1999/3267, the optometrist transfers the authority for dealing with the patient to the GP upon referral. This includes the dispensing of optical appliances until the patient is released back to the care of the optometrist or the optometrist receives instruction from the GP or ophthalmologist to dispense the optical aid.
General optometry comanagement schemes Recognition of the basic skills and training received by optometrists has led to the establishment of various co-management schemes. These schemes deal with pathological abnormalities of the eye, such as cataract, diabetes, glaucoma and low-vision aids. These are managed differently to refractive surgery co-management, as they are set up to improve the quality of referrals to secondary care. They are undertaken in accordance with a protocol agreed with hospital ophthalmologists and GPs, in accordance with the 1996 general guidelines on co-management schemes from the College of Ophthalmologists. This agreed protocol usually involves a number of hours per year training for the optometrist with the ophthalmologist in a lecture and clinical training format. Failure to comply with this required training can lead to the removal of the optometrist from the scheme. Furthermore, the optometrist undertakes responsibility for the part of the service that is provided by him or her. Additionally, the co-management schemes provide for patients for whom a confirmatory diagnosis has been made in the secondary care sector, and the schemes are outside the GOS. Payments to practitioners are made from hospital and community health services funds, although where refraction is required as part of the agreed protocol, a NHS sight test fee is claimed by eligible patients. This is very different to the situation that exists in the co-management of refractive surgery patients. As mentioned above, the co-management schemes that exist in refractive surgery do not have guidelines in terms of training required, payment methods or apportioning of responsibility. Optometrists involved in these schemes see the responsibility for the patients as remaining with
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the treating surgeon, which is what the clinics advocate. This may lead to certain problems if the co-managing optometrist is appointed by the clinic and does not have any dealings with the treating surgeon. If a problem arises, the treating surgeon may argue that they did not approve the optometrist involved in the scheme. Thus, surgeons will need to feel comfortable with the recruitment process that the clinic uses to recruit co-management partners.
Insurance and legal issues of responsibility Surgeons involved in refractive surgery will have their own medical defence indemnity insurance cover. Some defence organizations are reviewing whom they offer cover to because of the proliferation of litigation cases among refractive surgery patients. The Royal College of Ophthalmologists recently updated its advice to patients on refractive surgery, and is currently also considering its guidelines to surgeons. It is felt that the Royal College of Ophthalmologists will suggest that only surgeons who have been providing refractive surgery for a certain number of years or those who are accredited ophthalmologists should provide refractive surgery. Optometrists in routine practice usually have indemnity insurance to protect them against any potential litigation from patients. It is a requirement of the College of Optometrists that its practising members or fellows have indemnity cover to at least a pre-set level. Many optometrists in the UK are members of the Association of Optometrists (AOP), which provides its members with some advice when entering co-management schemes, published in their fortnightly journal Optometry Today.9 The AOP’s advice to its members says that they should obtain a written contract that sets out the terms of engagement. The AOP professional indemnity insurance will cover them for
References 1 2 3
Saunders H (1981). Age-dependence of human refractive errors. Ophthalmic Physiol Opt. 1, 159–174. Ewbank A (2001). The current status of laser refractive surgery in the UK. Optician 222, 24–27. Ewbank A (2002). Trends in laser refractive surgery in the UK. Optician 224, 20–24.
work that they may normally undertake as optometrists, but not for work that they do not normally perform (such as prescribing medication or performing surgical techniques). The AOP advises good record keeping and suggests that members do not advise patients on surgery, but rather inform them of what the surgery can offer, highlighting any pros and cons. Importantly, the AOP also suggests that if a member is involved in a comanagement scheme then the patient must be given details of the agreement that exists with a refractive surgery clinic. The AOP suggests that taking this course of action will mean that professional indemnity for the patient remains with the treating surgeon and that, if required, the clinic will be able to produce satisfactory documentation to demonstrate this professional indemnity to the optometrist.
Sources of further information There are various sources for information on refractive surgery to which clinicians may wish to refer. Many of the professional bodies have their statements on their websites. Below is a list of some useful organizations and their respective web links.
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College of Optometrists – produces the refereed journal Ophthalmic and Physiological Optics. www.college-optometrists.org
Professional societies • United Kingdom and Ireland Society of Cataract and Refractive Surgery (UKISCRS) www.ukiscrs.org.uk • British Society for Refractive Surgery (BSRS) www.bsrs.co.uk • European Society of Cataract and Refractive Surgeons (ESCRS) – jointly produce the refereed Journal of Cataract and Refractive Surgery www.escrs.org • American Society of Cataract and Refractive Surgery (ASCRS) – jointly produce the refereed Journal of Cataract and Refractive Surgery www.ascrs.org • International Society of Refractive Surgery (ISRS) – produce the refereed Journal of Refractive Surgery and have just become part of the American Academy of Ophthalmology (AAO), who produce the refereed journal Ophthalmology www.isrs.org
Professional bodies • Royal College of Ophthalmologists – produces the refereed journal British Journal of Ophthalmology. www.rcophth.ac.uk
Other publications • Cataract and Refractive Surgery Today www.crstoday.com • Refractive Eye News – the supplement of Eye News
[email protected] • Ocular Surgery News www.osnsupersite.com • Eurotimes www.escrs.org/eurotimes • EyeWorld www.eyeworld.org • Optometry Today – journal of the AOP www.optometry.co.uk • Optician www.optometryonline.net
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Governing bodies • General Medical Council (GMC) – all practising medical practitioners in the UK must be registered with the GMC. www.gmc-uk.org • General Optical Council (GOC) – all practising optometrists in the UK must be registered with the GOC. www.optical.org
5 6 7
Hanratty M (2003). Optometric comanagement of Lasik: Part 1 – Preoperative assessment. Optician 225, 26–29. Hunter I (2003). Frictionless fees. Optom Today 43, 3. Optometry Today (2003). GOC seeks ban on referral fees. Optom Today 43, 4. Doshi S (2001). Co-management schemes. Optician 222, 34–35.
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Doshi S (2002). Co-management in refractive surgery: a honey trap? Optician 224, 28–29. Association of Optometrists (2001). Optometric services in refractive surgery: Advice for AOP members. Optom Today 41, 19.
8 Surface laser treatments: an alternative to LASIK? Stephen J Doyle, Sunil Shah and Balasubraminiam Ilango
In this chapter laser in situ keratomileusis (LASIK) surgery is detailed further and the arguments as to its supremacy over photorefractive keratectomy (PRK) and laser epithelial keratomileusis (LASEK) are considered. There may be some slight overlap with previous chapters, but the authors feel that this is appropriate here. LASIK is often perceived by patients as being the most up-to-date technique, partly because some clinics and surgeons only offer this type of laser surgery. However, an increasing number of surgeons advocate a glorified version of PRK (i.e., LASEK) as a more suitable option for some prescription ranges. Approximately one-quarter of the world’s population need visual aids to correct their refractive errors. In the UK, about 6.6% of the population are myopic.1 The discovery that argon fluoride (ArF) excimer laser is able to alter the corneal curvature has transformed some of these people’s lives. The key refractive elements of the eye are the cornea, lens and axial length. Of these three factors the cornea is the most easily accessible for modification. In laser refractive surgery the aim is to correct the patient’s ametropia by altering the corneal profile and changing the overall power of the eye. To correct myopia the excimer laser is used to remove corneal tissue in the central area to produce an overall flattening of the cornea shape. In hyperopic patients, corneal tissue is removed in the mid-peripheral area of the cornea to produce a net steepening of the corneal shape. One essential difference between LASIK and PRK (or LASEK) is where in the cornea the laser is applied. In PRK and LASEK the excimer laser is applied under the corneal epithelium, at the Bowman’s membrane level, whereas
in LASIK the excimer laser is applied underneath Bowman’s membrane, in the stroma. PRK and LASEK are often termed as being surface laser treatments. Taboada et al.2 reported the use of excimer laser on rabbit corneas in 1981. Trokel and Srinivasan,3 in 1983, showed the world that the 193nm excimer laser could be used to remove corneal tissues very precisely. Marshall et al.,4 during 1984, looked at the structural changes in ablated rabbit and monkey corneas, using scanning and transmission electron microscopy. In 1985, Theo Seiler performed the first phototherapeutic keratectomy procedure in Germany5. McDonald et al.6 performed the first PRK procedure in the USA on a myopic person in 1988. During the same year, Gartry and KerrMuir performed their first PRK on a sighted person’s cornea at the St Thomas’ Hospital, London.
The development of LASIK The concept of keratomileusis was introduced by Pureskin in 1966.7 In the 1970s, Jose Barraquer3 improved this idea when he removed a thin corneal wafer, reshaped it using a cryolathe and reinserted it onto the cornea. Subsequently, in 1982, an automated device called a microkeratome was used to cut a thin cap of the corneal tissue. A second pass was then made to remove tissue to flatten the cornea by a predetermined amount, calculated using Barraquer’s ‘Law of Thickness’ equation. The cap was then replaced and allowed to heal, held in place by the endothelial pump mechanism. This technique was called an automated lamellar keratectomy and was used to treat myopic refractive errors up to 20D.7
The first of the present-day microkeratomes was developed by Chiron and used by Slade. The combination of a lamellar incision and excimer laser ablation was first named laser in situ keratomileusis by Pallikaris in 1990.8 It employed the microkeratome to cut a plano-lamellar corneal flap followed by excimer laser to the stromal bed and flap replacement.
Excimer laser technology Excimer laser is a term applied to a group of lasers in which a molecule of inert gas, such as krypton or argon, is forced to associate with a molecule of halogen gas. The term ‘excimer’ comes from ‘EXCited dIMER’, which means a mixture of two inert gases that bind together to produce an unstable diatomic gas halide. In 1975 Velazco and Setser described properties of inert gas halides that suggested they could be used fore lasers in the ultraviolet range.9 The high-energy photons produced by these lasers were found to be able to destabilize valency bonds in the macromolecules and thus cause tissue destruction. This technique is known as photoablation. The first working excimer laser was produced in 1975.9 Krypton fluoride (KrF) lasers use an ultraviolet wavelength of 248nm and ArF lasers use an ultraviolet wavelength of 193nm. Ultraviolet light is strongly absorbed by most biomaterials. At 193nm the laser-head photon energy is around 6.4eV, sufficient to break the corneal intramolecular bonds of about 3.6eV, but not to cause any thermal effects. The remaining energy is used to expel particles from the surface at supersonic speeds, but does not cause any significant heating of
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the adjacent tissues. At wavelengths greater than 200nm the thermal effects become more marked locally. However, even at 248nm the photons still cannot penetrate more than a few microns.10–13 Investigations of a range of excimer lasers have shown the ArF laser to produce the smoothest ablations of the corneal tissue, with minimal collateral damage from thermal diffusion. As an excimer laser, ArF has the advantage that it provides an exact, computer-controlled tissue removal, with a linear relationship between energy density and ablation depth. Dyer and Al-Dhair first noted this property in 1990.3 The major disadvantage of the excimer lasers for refractive surgery is their expense. This arises from the need for a sophisticated containment system for toxic gases, the daily need to replace the gas fills and the requirement for high-quality optical components to prevent irregular tissue removal that results from inconsistent beam energy. For these reasons, solid-state lasers are emerging in the market. However, the present-day excimer lasers are still 193nm ArF lasers.
Laser equipment A wide variety of laser hardware and software is available. In broad terms, the types of lasers can be divided into broad-beam, scanning slit and flying spot lasers. Both second- and third-generation lasers (scanning slit and spot lasers) have the advantage of a smaller beam, which minimizes the effects of beam irregularities and results in a smoother ablation profile. Flying spot lasers have been in use in the UK since the late 1990s. Their small beam size makes them suitable for use in topographic feedback systems. Some systems utilize a combination of technologies. Each type of laser has its own advantages and disadvantages. For example, broad-beam lasers are limited as to the maximum ablation diameter and have a high tendency to produce ‘central islands’ (which have a refractive power different to the rest of the ablation), but broad-beam lasers are able to achieve the required ablation profile quicker than are scanning lasers. Software modifications have been made frequently to adjust the algorithms to provide optimum results. Recent developments have allowed ablation assisted by corneal topography to help customize the laser treatment for individual patients. The most recent developments are the wavefront analyzer (works on a variety of optical principles), and it is now possible to analyze higher order optical aberrations with a degree of accuracy. In theory, wave-
front analysis linked into an excimer laser would allow the correction of whole eye aberrations with the possibility of ‘supervision’. However, as for much of refractive surgery recently, media hype has far preceded clinical outcome data. In addition, it is possible that wavefront-guided treatments would need to be used either in ‘real time’ or with PRK rather than LASIK (because of the aberrations that LASIK causes by cutting the flap itself).
LASIK versus PRK It is clear that each of these techniques has some advantages over the other. In the UK, PRK has been by far the more popular technique as it was developed earlier. LASIK was developed to overcome some of the problems encountered with PRK. These include retaining Bowman’s layer to give a low level of post-operative haze. An intact corneal epithelium helps to reduce the pain and the chances of surface infection. LASIK patients also enjoy a fast visual recovery. With healing after LASIK, the apoptosis is limited to the lamellar interface, and so less stromal wound healing, and usually the haze is limited to the flap margin only.14 PRK is an inherently safer operative procedure is needed as no cutting of the corneal stroma is involved, and hence the instrumentation is much simpler. The complication rate for PRK rises in direct proportion to the degree of treatment. The complication rate for LASIK is relatively constant, as the major complications are related to cutting the flap. For each individual surgeon, there is probably a point at which the risks of PRK outweigh the risks of LASIK. This point varies from surgeon to surgeon and also from patient to patient, depending on other factors. In the USA, consumer pressure has meant that the vast majority of patients receive LASIK. The LASIK procedure requires sufficient corneal thickness to ensure the ablation does not approach within 250μm of the endothelium. The risk of significant endothelial damage is small – most studies quote a change in endothelial cell density similar to the physiological change with age,15 even for high myopic corrections. The risk of inducing a corneal ectasia is small if a minimum of 30% of the corneal thickness is left intact, and a thinner flap is cut for thin corneas. A central corneal thickness of <410μm usually contraindicates LASIK.16 A number of studies have shown that the long-term results for low-to-moderate
myopia are identical for PRK and LASIK. The difference is the short-term visual recovery and the anisometropia (bilateral PRK does not tend to be performed simultaneously). LASIK is able to treat –1.00D to –12.00D and +1.00D to +4.00D, depending on the corneal thickness. The range of correction of refractive errors is from –1.00D to –7.00D (maximum) and +1.00D to +2.00D with the PRK technique. The crossover point between PRK and LASIK for myopia is about –3D for many surgeons. The visual recovery is different between the two techniques. For PRK the epithelium takes up to 4 days to heal and then the vision improves as the epithelium stabilizes and the post-operative hyperopic shift settles. The modified version of PRK, known as the epithelial flap,16 is slowly gaining popularity. In the epithelial flap procedure, the epithelium is treated with 18% alcohol for about 45 seconds and then an epithelial flap is created (rather than the stromal flap of LASIK). After laser ablation, the flap is replaced. This results in less pain and quicker visual recovery, and so attracts patients to opt for the PRK treatment because of the increased safety.
LASIK surgical procedure The LASIK surgical procedure is usually performed in a clean room, preferably an operating theatre. Qualified nursing assistance during LASIK surgery is mandatory and is a prerequisite for local health authority registration. The patient is made to lie on a couch with the excimer laser delivery system just above the position of the head. The patient’s cornea is anaesthetized with local anaesthetic eye drops. Very anxious patients can be given a mild sedative about 30 minutes pre-operatively. An eye speculum is inserted to expose the cornea and to prevent the patient from blinking during the treatment. The patient fixates on the He–Ne laser beam and the cornea is marked with gentian violet to assist in realignment of the flap. Various microkeratomes (manual and automated) are available and the surgical techniques differ between them. The Hansatome has two pieces and employs the following technique. A suction ring is applied to the eye and the intraocular pressure (IOP) is increased to >65mmHg to ensure a regular cut. This can be confirmed using the Barraquer applanation tonometer. The patient may experience a transient loss of vision secondary to an
Surface laser treatments: an alternative to LASIK?
increase in the IOP in excess of the retinal arterial perfusion pressure. The microkeratome is placed onto the track and activated to pass across the cornea and back, thus cutting the flap. Then the vacuum is released and the corneal flap is reflected back, to reveal the stromal bed. Some microkeratomes (Nidek) are available as one piece and do not need to be connected together. The hinge of the corneal flap can be made nasally, although cuts made with the Hansatome are hinged at 12 o’clock. The depth of the cut is usually no more than one-third of the total corneal thickness. The microkeratomes have 130 and 160μm blades. Most microkeratomes do not always produce a flap of the exact intended thickness and have a standard deviation (SD) of about 30μm.18 However, for many newer microkeratomes, such as the Nidek MK2000, the SDs are significantly less. Ideally, pachymetry is repeated to ensure that an adequate depth of corneal tissue remains, and the excimer laser ablation is carried out on the stromal bed. The patients self-fixate throughout the laser ablation, and the corneal centration should be maintained either manually or by the built-in eye tracker mechanism. Patients must be warned that they might experience a pungent smell during the laser ablation, which takes under 90 seconds in total. The flap is washed thoroughly with balanced salt solution to remove any debris. The corneal flap is repositioned, centration checked and the edges smoothed down. The endothelial pump mechanism keeps it in place. Adherence is verified and the speculum removed. Some surgeons prefer to apply a bandage contact lens over the flap, and remove this after 24 hours. The patients are sent home with an eye shield, which prevents them from accidentally rubbing their eyes and from direct trauma to the eye. Flap position is rechecked at the
slit lamp after a few minutes. Post-operative care for the LASIK patient is discussed in Chapter 5. Complications of LASIK Complications after LASIK can be classified into three broad groups, summarized in Table 8.1: • Flap-related; • Refractive; • Miscellaneous. The flap-related complications occur mostly because of mechanical problems, but these are very rare with modern microkeratomes. Epithelial ingrowth results from the entrapment of corneal epithelial cells under the margin of the flap. Flap striae are caused by misalignment, slippage or tenting of the flap during the early postoperative period. Accidental eye rubbing could also lead to this complication. Flap striae can be reversed readily, if corrected during the early post-operative period. Total flap-related complications are quoted as 2–5%, although serious flap complications are about 0.1%. Diffuse interstitial keratitis is uncommon and needs early aggressive therapy. It is treated with topical corticosteroids and possibly by lifting the flap and cleaning the surface. Retinal haemorrhage is thought to result from pre-existing pathology.19 Epithelial ingrowth, if sight threatening, may need the flap to be lifted and the surfaces treated with absolute ethanol. Results Predictability Of the patients with a pre-operative myopia of –2.00 to –6.00D, 94.8% achieve >6/12 unaided vision at 5.2 months post-LASIK. This percentage is 62.3% and 36.8% in the –6.00 to –12.00D range and the –12.00 to –20.00D range, respectively.16 Among the –1 to –3D group, up to 36% of the patients
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achieved 6/6 unaided vision in one study.14 The predictability of achieving 6/12 or better is about 70–87% by 12 months, depending upon the initial level of myopia.17 The results of PRK and LASIK are very similar at 12 months postoperatively. Accuracy The percentage of eyes that achieve a residual refractive error within ±1.00D of emmetropia at 12 months is between 68 and 86% for <–12.00D of treatment and 40–50% for >–12.00D of treatment.8 Stability The refraction has been shown to stabilize within 3–6 months after LASIK.20 Fiander and Tayfour showed a refractive change of ±0.50D between 1 and 6 months in 90.4% of high myopes and 81% of low myopes.21 This quick recovery has been attributed to the limited stromal healing required after LASIK. Loss of best-corrected visual acuity Most studies that involve low myopes (<–6.00D) show no eyes losing more than two lines of Snellen acuity. In the high myopia group, 0–9.5% of patients lost two or more lines of Snellen acuity after LASIK. This remarkable result with LASIK is because of the lack of haze and the limited stromal healing that is required.8 Contrast sensitivity Spatial contrast sensitivity is reduced at high and middle spatial frequencies at both 1 and 3 months post-operatively.22 In one study, contrast sensitivity was studied in 14 eyes (–6.00 to –19.50D) after LASIK. A reduction in contrast sensitivity was detected at 1 month, after which a rapid recovery was noticed, so that by the third month no statistically significant reduction in sensitivity was seen at any spatial
Table 8.1 Complications of LASIK Period
Flap related
Refractive
Miscellaneous
Early
Incomplete flap Decentred flap Irregular flap Completely cut flap Lost flap Flap striae Sands of Sahara (diffuse interstitial keratitis)
Induced irregular astigmatism Primary undercorrection Primary overcorrection Decentred ablation
Glare Haloes Ptosis Infectious keratitis Retinal haemorrhage19 Central retinal artery occlusion Dry eye Corneal anaesthesia
Late
Epithelial ingrowth
Regression Undercorrection Overcorrection
Retinal detachment19 Iatrogenic keratectasia
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Refractive surgery: a guide to assessment and management
frequency (Catherine Chisholm, personal communication). High myopia The increase in IOP during surgery may cause problems in large myopic eyes with weak retinas. Arevalo et al. demonstrated that 20 of 29,916 eyes exhibited vitreoretinal diseases after LASIK (14 retinal detachment, four retinal tears and 2 miscellaneous conditions),19 although this may have been pre-existing. LASIK for high myopia is possible, but full correction is not always feasible since about 250μm of the corneal thickness has to be maintained to prevent corneal ectasia. The greatest ablation that can be performed depends on the laser and the nomogram used. Hyperopia LASIK has been shown to produce slightly better results in correcting low hyperopes than PRK. Condon showed that 80% of the eyes achieved 6/12 or better. The stabilization period was about four times longer than that of myopia. About 7.3% of the patients lost two or more lines of best-corrected visual acuity (BCVA) in the >+4.00D group.7
LASEK Anyone who investigates refractive surgery for themselves by reading advertisements, looking on the internet, etc., would be forgiven for thinking that LASIK is the latest thing and that PRK is ‘old hat’. Some providers in the UK do not offer any surface laser, even in its latest form of LASEK. They put forward LASIK as quicker, better and very safe, but is this really true? PRK was superseded by LASIK, particularly in the USA, for complex reasons. The Food and Dug Administration (FDA) made every manufacturer of excimer lasers go through an initial accreditation process for PRK, which meant that the first American lasers on the market had a head start. These were VISX and Summit, both broad-beam lasers. One of the authors used an early Summit laser and the quality of the ablations were not nearly as good as those of today’s machines. The optical zones were only 5mm, there was no blend zone around the optic zone and the surface was rough with visible rings from the expanding diaphragm. Other laser manufacturers, such as Nidek and Technolas, rapidly brought out their own machines with flying spots, scanning slits, etc., which provided much smoother surfaces on the cornea. For surface lasers in par-
ticular, ‘smoother is better’, as there is less chance of haze developing. The standard way to remove the epithelium at the time was to scrape it off with a surgical blade. This leaves a rougher surface than taking it off with alcohol or a rotating brush and, again, tends to lead to more haze. Hence, many American surgeons were put off PRK by the development of bad haze in more patients than was comfortable. There was also the issue of post-operative pain, which can be quite severe and last for several days. When LASIK was introduced it offered a pain-free procedure, quick visual recovery and no risk of haze. This is because the corneal stroma is a relatively quiescent tissue in comparison with the very metabolically active epithelium. It is the interaction between the healing epithelium and the lasered stroma that causes the deposition of glycoaminoglycans (GAGs) and collagen IV, which causes the haze. Putting the laser ablation in the middle of the stroma meant that this was one complication, at least, that LASIK did not have, although a raft of new ones transpired rapidly, as mentioned above. The public began to realize that there was an operation that was almost completely painless, and that you could see the next day, which gave what has been called the ‘WOW factor’. For a high myope to have clear vision the next day is a powerful selling point to encourage your friends to have the same surgery. Even if the complication rate was 5%, the noise from 95 very happy patients tends to drown out the unhappy five. The American psyche is also one that likes operations that are over quickly, easily and let one ‘get on with your life’. LASIK offered and still offers this. It is the nearest thing to what one might call ‘stealth surgery’ – an operation that even at one day post-operative is hard to detect, even using a slit lamp. In Europe, the surgeons could use other lasers that were not yet available in the USA. Also, perhaps being of a more conservative nature, they tended to stick with PRK as well as to learn LASIK and begin to offer it to patients. Reports of the complications of LASIK began to filter across from America. This is not to say that LASIK was not taken up in Europe. LASIK is and remains a very successful operation with a very low complication rate. Quite a number of studies have compared LASEK and PRK to LASIK, sometimes with LASIK in one eye and PRK or LASEK in the other.23–25 All these articles show that the final visual results are the same for both techniques, which is not surprising given that both operations use the same laser, albeit in a slightly differ-
ent part of the cornea. One different factor with the introduction of wavefront technology is the levels of higher order aberrations after surgery. For these, surface laser surgery seems to have the edge. LASIK, having a mechanical element as well as the laser, leads to an increase in the ‘root mean square (RMS)’ after surgery. This is a random process caused by the making and replacement of the flap in LASIK.26 The ‘street cred’ of lasers is very high, and the public perception is that any operation using a laser must be almost foolproof. However, in refractive surgery we are operating on healthy patients and not diseased ones. One might argue that a –10 myope is ‘sick’, but one cannot say that a –2 myope is anything more than a physiological variation or the body’s adaptation to its environment. One of the most important diktats of any doctor is ‘do no harm’. If we operate on 100 cancer patients and five die, this is perfectly acceptable as we are trying to cure a sickness. Equally, in doing cataract surgery, a patient who develops macula oedema will have had blurred vision before the surgery and may feel at least no worse off than before having the surgery. However, it is a disaster of a different order for a low myope with 6/5 spectacle vision to be reduced to 6/18 bestcorrected vision by an elective refractive operation. For an operation on such low myopes, 5% with significant complications is much too high. These are often young healthy people and we do not want to leave anyone with lifelong visual problems. In the same way that refractive surgery is sold as ‘something that you will appreciate every waking moment for the rest of your life’, problems such as monocular diplopia, loss of contrast sensitivity, fuzziness of objects, dry eyes, etc., can make patients regret the surgery for the rest of their lives. The popularity of the website www.surgicaleyes.com is testament of this. At this website refractive patients with problems gather to compare notes and to look for solutions. Hence, refractive surgery has a different paradigm for the surgeon and he or she should approach the refractive patient in a different way to the elderly cataract patient.23,27–29 PRK has improved much over the past few years. As well as improved lasers, epithelial management as in LASEK (epithelial PRK, also called ‘PRK epiflap’ or ‘advanced surface ablation’) has meant that haze is now much less of an issue. There is a little confusion between different authors as to the actual meaning of the acronym LASEK, as some state it as being ‘laser epithelial keratomileusis’ and
Surface laser treatments: an alternative to LASIK?
others say ‘laser sub-epithelial keratectomy’. It could be argued that the acronym LASEK has been applied deliberately to link it closer to LASIK and move it away from PRK, since PRK is seen as ‘old hat’ and LASIK as ‘new age’. LASEK is often quoted as being ‘fancy PRK’ or a ‘halfway house’ between LASIK and PRK. Probably the former of these two quotes is nearer the mark, as LASEK is a surface-based laser procedure, like PRK, but involves more delicate manipulation of the epithelium. It maintains some of the safety features of PRK combined with some of the advantages of LASIK.30 Visual recovery is also quicker and pain is somewhat less in LASEK than in PRK.31,32 However, it is still the case that LASIK is the procedure many patients prefer because of the speed and pain-free nature of the surgery. LASIK is perceived and presented as a better surgical experience for patients. It is undoubtedly true that many prospective patients are put off PRK by the pain element and that many who have had LASIK would not have had PRK. Hence one might say that the ‘refractive market’ has developed on the back of LASIK rather than on PRK, especially in the USA. Problems that come up are resolved with new technology and with more experience. For example, with the 5mm diameter optical zones in the early lasers there were some night-vision problems. These have been cured largely by using optical zones as large as 7–9mm, along with blend zones going out to 10–12mm. It has also been realized that poor night vision is associated with treating high myopes, mostly because of an increased spherical aberration. Hence, the initial enthusiasm for operating on myopes of even –20D has been replaced with a more realistic level of about –8 to –10D as the upper end of treatment for corneal laser surgery. Similarly, when hyperopic treatment became available, there was an initial enthusiasm to treat young high hyperopes. Patients flocked from the USA, where it was not yet approved by the FDA, to just over the border in Canada. Some surgeons there found that they were operating on very large numbers of hyperopes. There is nothing like an unhappy patient sitting in front of a surgeon threatening to sue him or her unless the treatment parameters are modified; the consequence meant that these surgeons rapidly became more conservative. We now realize that we should not steepen the cornea beyond 47–48D, because beyond this the quality of vision deteriorates markedly through the creation of a kind of iatrogenic keratoconus.33
However, this is the nature of progress in medicine and we learn from experience. Some patients, by their nature, are more ‘risk takers’ than others, which is fine as long as we make clear to patients what the risk factors in this surgery are in as much detail as is necessary. Doctors always have a prime duty to do the best they can for their individual patient, even if that patient is a fit –1.5D myope, and they should not be swayed by how much money can be made.
Commercial aspects Excimer lasers are not inexpensive, with an average price of about £300,000. They are gas lasers and, as well as needing refills every few weeks, they also need regular servicing to clean the mirrors, check alignment of all the mechanical parts, and so on. Some need new laser chambers every year, which cost about £20,000. If they break down complete chaos can result for a clinic, as 20 patients may be waiting for laser treatment on any given day. Hence, one has to have emergency preparations to cover for this event, as it will certainly happen. The microkeratomes for LASIK cost around £45,000 each and at least two headsets are needed, at an extra £12,000. The blades cost between £20 and £40 each and can be used for a maximum of one patient. Finally, there are the costs of marketing, staffing, billing and all the other accoutrements of a modern business. Optometrists will be aware of these costs, as they are similar in type, if not in degree, to those of any optometric practice. Hence, it is not surprising that commercial groups have set up many of the laser centres. Individual entrepreneurs, existing optometric chains, groups of businessmen and health care groups have all played their part. In the early-to-mid 1990s up to 40 different centres in the UK provided PRK. However, a sharp downturn occurred in the market and many of theses centres ceased to operate.34,35 However, doctors used to dealing with sick patients in the NHS, with perhaps a private practice on the side, are not used to the commercial world. Doctors used to be forbidden by the General Medical Council (GMC) to advertise at all, and even now there is a fine line between what is permitted and what is not. However, there is nothing to stop laser centres from advertising widely. Different centres put forward what they think is their best selling feature. This may be price, or the qualities of a particular laser system, or the backup of a large hospital, which are all legitimate selling tactics.
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On selling features, it is interesting that three of the authors’ consultant ophthalmologist peers have had LASIK, but chose their surgeon by reputation and not by the laser being used. It could be argued that modern excimer lasers are of good quality and the quality of the individual surgeon is the most important factor. Ophthalmologists or optometrists employed by commercial laser centres have to maintain their clinical freedom to provide what they think is ethically the best treatment for each particular patient. This can lead to conflict with the business side of the enterprise, the main purpose of which is to make money for individuals or shareholders. Businessmen can apply pressure to treat unsuitable patients, and this must be resisted by any optometrist and/or ophthalmologist. In surveys of patients who have had refractive surgery, always about 5% are ‘disappointed’. Sometimes the reason for this is clear to both doctor and patient, for example bad haze in PRK or flap striae in LASIK. However, in other cases it is not so clear why the patient is unhappy. The doctor or optometrist may think that a –8D myope who has a post-operative refraction of –0.75D should be happy and grateful. However, the patient’s appreciation is just that his or her uncorrected vision now is not quite as good as it was with contact lenses or glasses before surgery. Conversely, and perhaps fortunately for the surgeons, some patients are so pleased that their uncorrected vision is better that they do not notice they have lost some best-corrected spectacle vision, for whatever surgical reason. We think that they should be unhappy, but in fact they may be delighted.
Unilateral or bilateral treatment? One question that often arises in the LASEK versus LASIK debate is whether to perform the procedure unilaterally or bilaterally. It was common in PRK surgery to separate the procedures for each eye by a few months, to allow the first eye to stabilize. LASEK has a quicker recovery period than PRK, and if applied to only low prescriptions this recovery time is very short. As a result, some surgeons advocate bilateral LASEK. In the case of LASIK, bilateral procedures have been the norm for a while, so both eyes are treated while the patient is still on the operating table.36,37 There is obviously much financial sense in this for the various clinics or doctors, as it increases the throughput of eyes treated and hence the profits. However, what is
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best for the individual patient? The arguments full into three groups: safety, accuracy and subjective. Safety Clearly, in a planned bilateral operation if there is any intra-operative complication in the first eye, treatment of the second should be abandoned. In practice, the complications are usually flap problems. The authors also include any marked epithelial loss or ‘slide’ caused by the passage of the microkeratome, because the incidence of the ‘sands of the Sahara’ syndrome is ten times higher in such cases and does not present until day 1 after the surgery if it occurs. Such epithelial problems occur more often in older patients. Any infection, inflammation, flap wrinkles, etc., almost always show up on day 1 post-operatively, so waiting 2–7 days between eyes should avoid a simultaneous bilateral problem. LASIK and PRK are dependent on the technology of the excimer laser machine, which are gas lasers that are calibrated frequently. They are complicated machines, but are generally reliable and have many safety features. However, if a technical problem occurs and both eyes are treated, it could affect both eyes adversely without the surgeon being aware of it at the time of surgery. The most likely fault is a simple over- or undertreatment, which could be corrected fairly easily in most cases. Worse is when a beam irregularity occurs. Checks are carried out on all the common machines to pick this up, but there has been a reported case in Canada of a mirror problem in a machine that resulted in irregular astigmatism in a group of bilateral patients. These patients were corrected with much angst, and it was realized that the fault in the machine should have been picked up before the surgery. However, it is a salutary lesson! Accuracy As a general rule it is fair to say the more extensive the treatment, the larger the spread of results (accuracy issues are dealt with in more detail below). If a patient falls within the normal spread of dioptres, then to treat both eyes at the same sitting does not give any significant increase in accuracy than to operate on them apart, which is the normal experience of most patients. However, if an eye behaves oddly, at the extremes of the statistical spread, operating on the eyes separately allows the surgeon to alter the laser settings for the second eye. Scientific articles disagree as to whether this really makes any difference, but the latest ones seem to indicate about a 20% improvement in accuracy.
Hence, for a –7D correction with a standard deviation of 0.93D, this leads to a mean improvement in accuracy of about 0.2D, which is small. Subjective Sometimes, especially in very high myopes, full recovery of vision can take up to 2 weeks. Thus, if both eyes are lasered on the same day, the patient may struggle to cope for this period. Similarly, the patient may find that the quality of vision after LASIK in one eye is not to his or her liking. For example, there may be night-vision problems, and so the patient may not want to go ahead with the second eye. For these and similar reasons patients may prefer one eye to be operated on at a time. However, unilateral treatment does lead to a feeling of anisometropia, which for some patients may be too uncomfortable to tolerate with a spectacle correction. This can, of course, be overcome by contact lens use in the non-treated eye, unless contact lens intolerance was the reason behind the patient’s initial option for surgery. LASIK on both eyes on the same day is more convenient, but is a slightly greater risk than operating on separate days. It is up to each patient to decide what he or she wants to do, after consultation with the individual surgeon. In most cases the risk is the same whether both eyes are operated on 2 minutes apart or several hours apart on the same day, and in some cases the risks may not be reduced significantly by separating treatment to each eye by a few days. Photorefractive keratectomy With PRK the situation is a little different. The only real complication of PRK is haze, which is not an immediate post-operative complication, as most LASIK complications are. Haze is maximal at around 6 weeks post-operatively, although there are cases of haze developing late at 6 months or occasionally even later.38,39 Hence, when PRK began, the eyes were operated on separately, at least 3 months apart, to reduce the risk of bilateral bad haze. The risk of haze is greater with higher myopic prescriptions and also with hyperopia, because of the more extensive shape changes made on the cornea with such prescriptions. Also, the speed of recovery of useful vision after PRK for high myopia is slower than that for low myopia, as the epithelium ‘models’ its new shape. Most refraction occurs at the first air–fluid interface of the cornea, which is disturbed much more by PRK than by LASIK. PRK has improved incrementally over the years. Newer lasers leave a smoother surface, and blend or transition zones also
make the shape changes more physiological. The LASEK technique, in which alcohol is used to remove the epithelium, which is replaced back on the cornea after ablation, means that haze is now quite rare and speed of recovery quicker. Hence, surgeons began to do low myopes bilaterally (e.g., up to –3D). With more experience this has now increased to about –5D and the debate today is whether to offer bilateral LASEK to anyone who wishes it in the same way, as with LASIK. Also, if a patient developed bad haze, there used to be no way to treat it, apart from letting nature take its course. In fact, most haze fades by itself if left long enough, although this may take many months in some cases. Attempts to remove the haze mechanically or with the laser worked initially, but it was soon found that the problem returned, often worse than before. However, now 0.02% mitomycin on a sponge can be used, as described in an earlier chapter, which seems to prevent such haze recurrence successfully. Hence, it may be said that even if bad haze develops bilaterally, there is now a method that goes a long way to curing this problem and allows the surgeon to undertake bilateral PRK with more peace of mind. In very high prescriptions, some surgeons use mitomycin prophylactically to prevent any haze from developing. Mitomycin has been used for some years in enhanced trabeculectomies, especially in children. It is known in these cases that late problems can be caused by mitomycin, such as corneal melt. There are hence worries that similar problems may develop in the future with its use in PRK. However, some clinicians have been using it for a few years and say that they have had no such problems as yet. They attribute this to the technique of using mitomycin on a central sponge.
Relationship with the NHS The existence of the NHS means that everyone in the UK has free access to medical care, which includes iatrogenic illness as well as primary pathology. This has, over the years, led to certain tensions between the public and private medical sectors. For example, private hospitals try to manage all their own surgical complications. However, on occasions they do not have the necessary expertise or facilities, so patients have to be transferred to a NHS facility. Sometimes the private provider will pay the NHS for these facilities and other times not. In the world of ophthalmology some private cataract services operate on large numbers of patients to clear up waiting lists. Who
Surface laser treatments: an alternative to LASIK?
should manage and pay for any surgical complications, such as yttrium–aluminium–garnet (YAG) capsulotomies for posterior capsule fibrosis, when the original surgeon may no longer be in the area? Who should pay for more complicated problems such as a dropped nucleus? Private clinics often have their own set-ups to manage these things, but there are undoubtedly times when the NHS ophthalmic unit feels unfairly used as, for example, when a private patient presents unexpectedly in a hospital casualty department. Similarly, in the world of optometry, contact lens patients develop complications, sometimes quite serious ones such as corneal infections. These are almost universally managed by the NHS, despite being iatrogenic problems caused to healthy people. It has been commented in the past that contact lens practitioners should shoulder some responsibility to contact lens wearers who develop severe adverse reactions that require medical treatment. It is now accepted that these are a normal part of the NHS ophthalmic workload and there is no attempt to either charge the referring optometrist or to be angry with him or her. In the world of refractive surgery similar problems can occur. Patients with severe pain after excimer laser refractive surgery may present at an ophthalmic casualty unit. A LASIK patient may need a corneal transplant because of keratectasia. Usually, the larger providers of laser refractive surgery have private financial arrangements with individual ophthalmic surgeons to cover such complications. Surgeons who perform refractive surgery in an independ-
ent capacity often manage their own complications or may refer privately to a more experienced colleague. However, in law there is nothing to prevent a refractive patient from being referred to the NHS for a second opinion by his or her GP. Generally, such problems are well managed through mutual respect among professionals, with a sense of proportion and goodwill on both sides. In fact, with common sense, relationships between public and private sectors can be managed to everyone’s satisfaction, including that of the most important individual, the patient.
References
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Ren Q and Keates RH (1995). Laser refractive surgery: a review and current status. Opt Eng. 34, 642–658. Taboada J, Mikesell GW Jr and Reed RD (1981). Response of the corneal epithelium to KrF excimer laser pulses. Health Phys. 40, 667–683. Trokel SL and Srinivasan R (1983). Excimer laser surgery of the cornea. Am J Ophthalmol. 96, 710–715. Marshall JS, Trokel SL, Rothery S, and Kreuger RR (1986). Photoablative reprofiling of the cornea using an excimer laser: Photorefractive keratectomy. Lasers Ophthalmol. 1, 21–48. Seiler T and McDonnell PJ (1995). Excimer laser photorefractive keratectomy: Major review. Surv Ophthalmol. 40, 89–118. McDonald MB, Kaufman HE, Frantz JM, Shofner RS and Salmeron B (1989). Excimer laser ablation in a human eye: Case report. Arch Ophthalmol. 107, 641–642.
Accuracy issues At present there is no way to measure what an excimer laser is doing ‘in real time’ (i.e., while a cornea is under the laser). Hence, if one person’s cornea is more or less dense than the average, the laser will take off more or less tissue with each pulse. Surgical technique is particularly important in this respect. The longer the surgeon takes doing the surgery, the more the cornea dries out and compacts, which means that the laser tends to take off more tissue and overtreat the patient. Hence, it is very important for each surgeon to develop a standard technique. The excimer laser does not penetrate water. Some surgeons have a ‘dry’ technique, in which the corneal surface is wiped a lot, whereas others have a relatively ‘wet’ technique. Hence, each surgeon has to develop his or her own personal algorithms. Even using the same laser in the same clinic, two surgeons may use slightly different
Brown AD and Craig JP (1997). Laser insitu keratomileusis (LASIK): A contemporary overview. Eye News 4, 7–14. 8 Pallikaris IG and Siganos DS (1994). LASIK and PRK for correction of high myopia. J Refract Corneal Surg. 10, 489–510. 9 Naroo SA and Charman WN (2001). Refractive surgery: Review and current status. Optom Pract. 2, 1–17. 10 Setser DW, Piper LG and Velazco JE (1974). Quenching rate constants for the Ar(3P0), Ar(3P2) and Xe(3P2) states. Radiat Res. 59, 441–443. 11 Dagenhardt AH (1976). Light coagulation of the eye. Br J Physiol Opt. 31, 11–18. 12 Fitzsimmons TD, Fagerholm P, Härfstrand A and Schenholm M (1992). Hyaluronic acid in the rabbit cornea after excimer laser superficial keratectomy. Invest Ophthalmol Vis Sci. 33, 3011–3016.
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laser settings. Although the room in which a laser is situated is air-conditioned, the geography can make a difference. Hence, a laser in, for example, Calgary, which is at high altitude and has very dry air, will tend to overtreat more than a laser in Houston, with a high humidity in the operating room despite the best efforts of the air conditioning. Corneal healing also plays a part. The epithelium in both PRK and LASIK may hypertrophy and alter the result. In the final analysis, refractive surgery is about biological systems and not electromechanical ones, Attempts to try and measure laser ablation during surgery are ongoing. The main problem is that the smooth air–fluid interface is highly disturbed by the act or surgery. Topography machines of various sorts are thus not useful. Research is underway to try and establish real-time measurements by using interference patterns. Haag–Streit now supply a non-contact optical coherence pachymeter, which is claimed to measure corneal thickness to an accuracy of 1 or 2μm with great speed. Some laser manufacturers are testing whether to fit this to their systems to establish a feedback loop. Corneal thickness can be measured just before surgery and then as the ablation proceeds. When the required amount of tissue has been removed, the laser is turned off. We can be certain of one thing – that refractive technology will advance and improve with time. Professionals in the field should keep up with these advances, try to discern the reality from the hype, and so offer the best solution to their patients.
13 Kerr-Muir MG, Trokel S, Marshall J and Rothery S (1987). Ultrastructural comparison of conventional surgical and argon fluoride excimer laser keratectomy. Am J Ophthalmol. 103, 448–453. 14 Wang Z, Chen J and Yang B (1997). Comparison of LASIK and PRK to correct myopia from –1.25 to –6.00D. J Refract Surg. 13, 528–534. 15 Levy SG (2000). Refractive surgery, part 5 – LASIK. Optom Today 40, 33–39. 16 Guell B and Muller A (1996). LASIK for myopia from –7 to –8 dioptres. J Refract Surg. 12, 222–228. 17 Shah S, Sarhan AR, Doyle SJ, Pillai CT and Dua HS (2001). Br J Ophthalmol. 85, 393–396. 18 Yildirim R, Aras C, Ozdamar A, Bahcecioglu H and Ozcan S (2000). Reproducibility of corneal flap thickness in laser in situ keratomileusis using the Hansatome microkeratome. J Cataract Refract Surg. 26, 1729–1732.
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19 Arevalo JF, Ramirez E, Suarez E, et al. (2000). Incidence of vitreoretinal pathologic conditions within 24 months after LASIK. Ophthalmology 107, 258–262. 20 Salah T, Waring GO III, el Maghraby A, Moadel K and Grimm S (1996). Excimer laser under a corneal flap for myopia –2 to –20D. Am J Ophthalmol. 121, 143–155. 21 Fiander DC and Tayfour F (1995). LASIK treatment in 124 myopic eyes. J Refract Surg. 11, S234–S238. 22 Alanis L, Ramirez R, Suarez R et al. (1996). Spatial contrast sensitivity in pre- and post-operative LASIK for high myopic patients. Invest Ophthalmol Vis Sci. 37, S570. 23 Stephenson C (2002). Complications of PRK, LASIK and LASEK: Diagnosis and treatment. Refract Eye News 1, 6–11. 24 Scerrati E (2001). Laser in situ keratomileusis versus laser epithelial keratomileusis (LASIK vs. LASEK). J Refract Surg. 17, S219–S221. 25 Lee JB, Song GJ, Lee JH, Seo KY, Lee YG and Kim EK (2001). Comparison of laser in situ keratomileusis and photorefractive keratectomy for low to moderate myopia. J Cataract Refract Surg. 27, 565–570.
26 Oshika T, Klyce SD, Applegate RA, Howland HC and El Danasoury MA (1999). Comparison of corneal wavefront aberrations after photorefractive keratectomy and laser in situ keratomileusis. Am J Ophthalmol. 127, 1–7. 27 Ang RT, Dartt DA and Tsubota K (2001). Dry eye after refractive surgery. Curr Opin Ophthalmol. 12, 318–322. 28 Iskander NG, Peters T, Penno EA and Gimbel HV (2001). Late traumatic flap dislocation after laser in situ keratomileusis. J Cataract Refract Surg. 27, 1111–1114. 29 Pushker N, Dada T, Sony P, Ray M, Agarwal T and Vajpayee RB (2002). Microbial keratitis after laser in situ keratomileusis. J Refract Surg. 18, 280–286 30 Shah S, Sebai Serhan AR, Doyle SJ, Pillai CT and Dua HS (2001). The epithelial flap for photorefractive keratectomy. Br J Ophthalmol. 85, 393–396. 31 Kornilovsky IM (2001). Clinical results after subepithelial photorefractive keratectomy (LASEK). J Refract Surg. 17, S222–S223. 32 Azar DT, Ang RT, Lee JB, et al. (2001). Laser subepithelial keratomileusis: An electron microscopy and visual outcomes
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of flap photorefractive keratectomy. Curr Opin Ophthalmol. 12, 323–328. Pallikaris IG, Kymionis G and Astyrakakis NI (2001). Corneal ectasia induced by laser in situ keratomileusis. J Cataract Refract Surg. 27, 1796–1802. Doshi S (2001). Co-management schemes. Optician 222, 34–35. Doshi S (2002). Co-management in refractive surgery: A honey trap? Optician 224, 28–29. Gimbel HV, van Westernbrugge JA, Anderson Penno EE, Ferensowicz M, Feinerman GA and Chen R (1999). Simultaneous bilateral laser in situ keratomileusis: Safety and efficacy. Ophthalmology 106, 1461–1468. Pop M and Payette Y (2000). Results of bilateral photorefractive keratectomy. Ophthalmology 107, 472–479. Caubet E (1993). Cause of subepithelial corneal haze over 18 months after photorefractive keratectomy for myopia. Refract Corneal Surgery 9, S65–S70. Lohmann CP, Gartry D, Kerr-Muir M, Timberlake G, Fitzke F and Marshall J (1991). ‘Haze’ in photorefractive keratectomy: Its origins and consequences. Laser Light Ophthalmol. 4, 15–34.
9 Wavefront technology W Neil Charman
As made clear in previous chapters, excimer laser refractive surgery has now reached a stage in its development at which, in carefully selected patients, it offers a realistic, routine alternative to earlier methods used to correct refractive error (spectacles and contact lenses), at a comparable level of cost. Nevertheless, the search continues for ways to improve visual outcomes and further reduce the possibility of post-operative complications. In this chapter we discuss a relatively new development, wavefront technology, as applied to excimer laser surgery. It is hoped that, as well as aiding the traditional spherocylindrical correction of ametropia, this will lead to reduced post-operative optical aberrations and hence better optical outcomes. The question of how the opportunities offered by the availability of an effective refractive surgical option to correct ametropic patients can best be embraced by the existing ophthalmic professions is also discussed.
Introduction In the naturally emmetropic eye, the quality of vision achieved depends both on the quality of the optical image on the retina and on the neural properties of the retina and subsequent visual pathways. The quality of the in-focus foveal optical image depends upon the effects of diffraction, optical aberration and light scatter, at least the first two of which are a function of pupil diameter. With very small pupils (<2mm), the effects of diffraction dominate, but with increases in pupil diameter diffractive blur reduces and the degradative effects of aberration become progressively more important.
Although effects vary between individuals, the optimal optical performance is usually achieved with a pupil diameter of about 3mm, which corresponds to that of the natural pupil under bright photopic conditions.1 A similar general pattern of behaviour is found in ametropes corrected by spectacles or contact lenses, although it is modified slightly by effects such as spectacle magnification and the aberrations of the correcting lenses. The early years of refractive surgery using radial keratotomy (RK) were dominated by the goal of achieving a tolerably accurate refractive correction. However, it was soon realized that, even when this was achieved, the quality of vision was usually worse than that found in naturally emmetropic eyes or in ametropic eyes corrected with spectacles or contact lenses. Measurements showed that the loss in vision resulted from the much poorer quality of the ‘best-corrected’ retinal image, which was degraded both by light scatter at the corneal RK incisions and by the much-increased levels of aberration associated with the limited optical zone achieved and the discontinuities produced by the pattern of incisions.2 With the advent of methods based on the use of excimer laser ablation it was hoped that, since changes in surface curvature were produced smoothly over a broad area of corneal surface rather than at a limited number of incisions, as in RK, a much improved optical effect would be produced. Again, early efforts with both photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) concentrated on producing a satisfactory spherocylindrical refractive correction, but the final quality of vision was still often disappointing, with a loss in photopic best-cor-
rected visual acuity. Performance under scotopic conditions, when the pupil was dilated, was frequently particularly poor, with complaints of haloes and scatter from lights during night driving.3 While the increased scatter could be attributed to wound-healing problems, many of the optical problems were again found to be derived from the increased levels of aberration in the eye after refractive surgery eye.4 These increases in aberration were associated with a variety of factors. In early PRK procedures using broad-beam lasers, the ablated zone was often of smaller diameter than the natural dilated pupil, which gave a massively undercorrected spherical aberration, while any temporal or spatial beam inhomogeneity led to unwanted changes in the ablation pattern. Decentration of the ablation was a further problem in some cases. Other problems identified included central islands, probably caused by ablation plumes that affected local ablation rates, and irregular ablations caused by changes in corneal hydration. Wider ablation zones, blending zones and other measures have brought some improvement, but they have not eliminated these problems. Thus, one of the goals of more recent developments in refractive surgery is to find ways to reduce post-operative optical aberration. It should, of course, always be remembered that refractive surgery has a negligible effect on the longitudinal chromatic aberration of the eye, which remains essentially unchanged at about 2D across the visible spectrum. The monochromatic aberrations of any optical system depend upon the shapes of its surfaces, as well as other factors. In principle, then, aberration might be reduced by reshaping one or more surfaces. Early laser
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systems, which involved broad-beam lasers and devices such as expanding diaphragms, could only deliver ‘standard’ ablation patterns. The advent of the new generation of computer-controlled, spot-scanning laser systems, able to ablate different regions of the cornea selectively, rather than merely to produce ‘standard’ ablation patterns, has opened up the intriguing possibility of correcting the axial monochromatic aberrations of the eye by suitably reshaping the corneal surface with an ablation ‘customized’ for each individual. If monochromatic aberration correction can be achieved, then visual acuity should match, or perhaps surpass, that achieved in naturally emmetropic eyes.5 This, then, is the promise of the new generation of laser systems that combine a device to measure the aberration and refractive error of the individual eye, and a spot-scanning or other form of laser that can ablate the cornea selectively, not only to correct the refractive error, but also to minimize the eye’s monochromatic aberrations. Since the ocular aberrations, and also the refractive error, are analyzed in terms of the form of the corresponding wavefront, rather than in terms of the classic Seidel aberrations (spherical aberration, coma, oblique astigmatism, distortion and field curvature), this area is often called wavefront technology. In the next sections we discuss the concept of wavefront aberration, current methods of measuring it in the individual eye and the limits to the overall quality of vision that might be achievable under ideal circumstances.
Wavefront aberration Consider the case of a point source of light placed at the first focal point of a converging, positive lens. If the propagation of the light is viewed in terms of rays, in the absence of aberration we can envisage the rays as divergent from the point to emerge from the lens as a parallel bundle. Alternatively, if we think in terms of Huygen’s wave theory, series of concentric spherical wavefronts spread out from the point source and, after refraction by the lens, emerge as plane wavefronts. On both sides of the lens surfaces the local wavefronts are always perpendicular to the local rays (Figure 9.1a). Now consider the case of a similar, but poorly designed and manufactured, lens with aberration. The rays that emerge from the lens are no longer parallel and the associated wavefronts are no longer spherical (Figure 9.1b). Evidently, a point object at the first focal point is a special case. In the more general ideal case, when the object is not at the first focal point and the image is not expected to lie at infinity, rays diverge from the object point to converge at a unique Gaussian image point. In wave terms, spherical wavefronts diverge from the object point and the lens reshapes these as a series of spherical wavefronts, all centred at the image point (Figure 9.1c). If there is an aberration, the rays no longer intersect at the unique image point and the imaging wavefronts are no longer spherical (Figure 9.1d). How can we quantify the aberration? Evidently, we can approach this either in
a
b
terms of the characteristics of the emergent rays or of the emergent wavefronts. In principle, we can always derive one description from the other, since wavefronts and rays are always locally perpendicular. For example, we might specify the ray aberrations in terms of the slope of each ray as it leaves the exit pupil as a function of the ray position within the pupil. This, in turn, would allow us to derive a spot diagram that shows the intersection of the rays with any chosen image plane. Alternatively, we could compare the imaging wavefronts with their ideal spherical counterparts. This is normally done in the exit pupil of the system. If the ideal image point is at infinity, the ideal wavefront is plane (i.e., it has an infinite radius of curvature). We call the ‘ideal’ spherical wavefront, centred at the Gaussian image point O⬘, the reference sphere and take the wavefront aberration at each point in the pupil as the optical path distance between the reference sphere and the aberrated wavefront (Figure 9.2a). In many applications, the radius of curvature of the reference sphere is chosen so that the wavefront aberration on axis at the centre of the exit pupil is zero, but this need not be the case. Clearly, what results is a ‘contour map’ that shows the variation in wavefront aberration across the exit pupil (Figure 9.2b). It is usual to specify the amount of wavefront aberration in microns (or sometimes wavelengths of light) and the position in the pupil in terms of either Cartesian (x, y) or polar (r, θ) coordinates (Figure 9.2a). Not surprisingly, the wavefront aberration varies with the position of the object point in the field
c
d
Figure 9.1 Rays and wavefronts. (a) Rays that diverge from the first focal point of an aberration-free, convergent lens emerge parallel (full lines). Alternatively, we can envisage divergent spherical wavefronts that leave the object point to emerge as flat wavefronts (dashed lines), which are everywhere perpendicular to the rays. (b) Situation with an imperfect convergent lens: the rays that emerge from the lens are not parallel and the corresponding wavefronts are not flat. (c) General case of an aberration-free convergent lens: the rays that diverge from the object point all pass through the image point. Alternatively, the spherical wavefronts that diverge from the object point emerge from the lens as spherical wavefronts all centred on the image point. (d) In the case of an imperfect lens, the emergent rays do not intersect at a point and the imaging wavefronts are not spherical
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y Wavefront Reference sphere r
x Chief ray O⬘
Exit pupil W a
b
Figure 9.2 (a) The wavefront aberration, W, is the distance between the actual wavefront in the pupil and the ideal spherical reference wavefront, centred on the Gaussian image point. The position within the pupil can be specified either in terms of Cartesian (x, y) or polar (r, θ) coordinates. A positive value of W(x, y) or W(r, θ) means that the wavefront is in advance of the reference sphere. (b) Typical contour map that shows the variation in wave aberration across the pupil (i.e., the field angle), but in the case of the eye we are almost always concerned with the aberration on the visual axis, when the image point lies on the fovea. Recalling that to form a diffraction-limited image all parts of the wavefront must arrive in phase at the image point, we can see that it is desirable for the wavefront aberration to remain as small as possible, so the ideal wave aberration map is completely free of contours. Rayleigh suggested that the wavefront aberration should nowhere exceed a quarter wavelength, otherwise the light disturbances from different parts of the exit pupil would start to interfere destructively. Maréchal expressed the same idea in terms of the root mean
square (RMS) value of the wavefront aberration across the pupil and suggested that this should not exceed one-fourteenth of a wavelength (the variance of the wavefront aberration is the square of the local wavefront aberration integrated over the pupil area, divided by the pupil area; the RMS aberration is the square root of the variance). For both these criteria it can be seen that, if aberration is to play a negligible role, the typical wavefront aberration across the pupil must remain small (about 0.1μm or less). Referring this tolerance to refractive surgery, this is only a small fraction of the typical total depth of stromal material removed (which is usually of the order of
tens of microns), which implies the need for a very accurate control of ablation depth. How can we relate these general ideas on wavefront aberration to the measurement of ocular aberration and perhaps combine measurements of refractive or defocus error with those of aberrations like spherical aberration and coma? Consider the ‘ideal’, aberration-free emmetropic eye shown in Figure 9.3a. If, somehow, a point source of light can be produced on the retina (usually with the aid of a low-power laser), all the light rays will emerge parallel from the eye, which corresponds to the ideal, plane reference wavefronts. Suppose, however, that the eye suffers from myopia, but not from aberration. The emergent rays
a
c
b
d
Figure 9.3 (a) Rays and wavefronts for a ‘perfect’ emmetropic eye: the emergent wavefronts are plane. (b) The myopic eye: the emergent wavefronts are spherical and convergent. (c) The hypermetropic eye: the emergent wavefronts are spherical and divergent. (d) An eye that suffers from undercorrected spherical aberration: the emergent wavefronts are non-spherical
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now converge to a far point in front of the eye, that is the emergent wavefronts are convergent and spherical rather than plane (Figure 9.3b). If the eye is hypermetropic the emergent rays diverge as though they came from a far point behind the eye, so that the wavefronts are spherical, but divergent (Figure 9.3c). In such ‘ideal’ cases of spherical refractive or defocus error, we can see that the wavefront aberration corresponds to the distance between an ideal plane reference and a spherical wavefront. Recalling the sag formula, this implies that the wavefront aberration shows a secondorder (r2) dependence on the distance from the centre of the pupil. Thus, if we find that the wavefront aberration varies as r2 in polar coordinates, or (x2 + y2) in Cartesian coordinates (second-order aberration), we must have an error of focus for the eye. Analogous effects occur if the eye is astigmatic, the difference being that the curvature of the emergent wavefronts varies with the meridian under consideration, although the wavefront aberration still has a second-order, r2, dependence in each meridian. The curvature takes its maximum and minimum values in the two principal meridians of the astigmatic eye. It is, of course, also possible for the rays that emerge from the eye to be parallel, but tilted with respect to the expected direction (i.e., that there is a prismatic effect). The corresponding wavefronts are evidently plane, but tilted with respect to the ‘ideal’ reference wavefronts, with the wavefront aberration varying linearly across the pupil in the tilt direction. Thus, first-order wavefront aberration terms in r or (x2 + y2)1/2 correspond to prismatic effects. What about ‘real’ aberrations, like a spherical aberration? This aberration implies that the outer zones of the pupil have a different power to the central zones. Figure 9.3d shows the case for an eye that is emmetropic at the centre of the pupil, but myopic in the periphery. We can see that, in this case, in comparison to a simple defocus the wavefront must be relatively more steeply curved in the outer parts of the pupil and the aberration must be a higher-order function of r: since it is rotationally symmetrical, it must be a function of r4, r6, etc. In fact, in wavefront terms, the classic Seidel aberrations, and the irregular aberrations that occur in biological structures such as the human eye, all need to be expressed as higher-order functions of the pupil variables. For this reason they are known as higher-order wave aberrations (third-order and above). Clearly, a conventional refractive correction with a spherocylindrical lens can only correct the prism and defocus (first- and
second-order) terms of the wavefront aberration, not the higher-order terms. Importantly, we must be careful not to confuse third- or fifth-order wavefront aberration (i.e., wavefront aberration that varies as the cube or fifth power of the radial coordinate r) with classic third- or fifthorder Seidel aberration theory,6 in which the power refers to the angle of incidence of the rays. It is unfortunate that this possible confusion of terminology exists.
Analysis of wavefront aberration Suppose that we have somehow produced a ‘contour map’ that shows the total wavefront aberration of a particular eye (Figure 9.4), how do we know how much of the aberration is caused by second-order, spherocylindrical defocus errors, how much by spherical aberration, how much by coma, and so on? Clearly, we need some way to break down the overall aberrations into the appropriate, simpler component parts, each related to a particular sort of wavefront distortion. In principle, this can be done in a variety of different ways, but it is currently usual in the field of refractive surgery to represent the wavefront aberration across the pupil as the sum of a series of Zernike polynomial terms,7–9 each of which represents a particular ‘component’ of wavefront distortion. These polynomials were devised by Fritz Zernike, who was awarded a Nobel Prize for his invention of the phase-contrast microscope. For enthusiasts, these polynomials have the mathematical advantage that the terms are orthogonal (i.e., independent of one another) over a unit pupil (in practice, this means that Zernike coefficients derived for a particular pupil diameter must be recalculated if the pupil diameter is changed10). Various notations can be used to represent the Zernike polynomials, but most workers and manufacturers now use a standard system devised by a committee of the
Optical Society of America.9 This refers the wave aberration to the entrance pupil of the eye (since the exit pupil is not readily accessible) and uses the line of sight as the reference axis. The latter corresponds to the chief ray from the fixation point, which passes through the centres of the entrance and exit pupils to reach the fovea. Since the Zernike polynomials are only orthogonal over a unit circle, the normalized radial distance in the pupil ρ = r/rmax is used as one polar coordinate, where rmax is the maximum pupil diameter for the measured wavefront aberration. The azimuthal angle θ is defined in the same conventional way as the cylinder axis in optometry, except that it can have values between 0 and 360° (2π radians). The wavefront aberration W(ρ, θ) is broken down as the sum of the Zernike polynomials, as in Equation (9.1), W(ρ,θ) = ∑nmCnmZnm(ρ, θ) = C00Z00 + C1–1Z1–1 + C11Z11 + C2–2Z2–2 + C20Z20 + C22Z22 + … etc. (9.1) where Cnm is the coefficient for each of the Zernike polynomials Znm(ρ, θ) and the coefficients vary with the aberration of the particular eye. The subscript n represents the highest order (power) of the radial parameter ρ contained in the particular polynomial, which also contains a cosine or sine term of a multiple, mθ, of the azimuthal angle θ, so that m is often termed the azimuthal frequency. Note that when, for example, a fifth-order Zernike term is mentioned, it is the value of n that is being referred to (n = 5). Each polynomial Znm(ρ, θ) is the product of three components: a normalization term, a polynomial in ρ of order n and an azimuthal component of the form sinmθ or cosmθ. Table 9.1 lists polynomials up to the fifthorder. Details of still higher-order polynomials are given in, for example, Thibos et al.9 As noted earlier, each polynomial essentially describes a particular type of deformation of the wavefront, and the magnitudes of
Figure 9.4 Typical colour-coded contour map of the wavefront aberration of an eye that shows a mixture of defocus errors caused by ametropia and higher-order errors caused by aberrations such as spherical aberration and coma
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Table 9.1 Listing of Zernike polynomials up to the fifth order (Optical Society of America format9) Index j
Order n
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
0 1 1 2 2 2 3 3 3 3 4 4 4 4 4 5 5 5 5 5 5
Frequency m 0 –1 1 –2 0 2 –3 –1 1 3 –4 –2 0 2 4 –5 –3 –1 1 3 5
Zernike polynomial Znm(ρ, θ)
Description
1 2ρsinθ 2ρcosθ 61/2ρ2sin2θ 31/2(2ρ2 – 1) 61/2ρ2cos2θ 81/2ρ3sin3θ 81/2(3ρ3 – 2ρ)sinθ 81/2(3ρ3 – 2ρ)cosθ 81/2ρ3cos3θ 101/2ρ4sin4θ 101/2(4ρ4 – 3ρ2)sin2θ 51/2(6ρ4 – 6ρ2 +1) 101/2(4ρ4 – 3ρ2)cos2θ 101/2ρ4cos4θ 121/2ρ5sin5θ 121/2(5ρ5 – 4ρ3)sin3θ 121/2(10ρ5 – 12ρ3 + 3ρ)sinθ 121/2(10ρ5 – 12ρ3 + 3ρ)cosθ 121/2(5ρ5 – 4ρ3)cos3θ 121/2ρ5cos5θ
Piston Tilt about x axis Tilt about y axis Astigmatism, axis 45°, 135° Spherical defocus Astigmatism, axis 0°, 90° Trefoil (base on x axis) Primary coma along x axis Primary coma along y axis Trefoil (base on y axis)
their coefficients Cnm give the amount of deformation of that type present in the particular overall aberration map. Rather than always using the double-indexing system Znm to describe a particular Zernike polynomial or mode, a single-indexing system, Zj, is used occasionally. The relation between the j, m and n terms is given in Table 9.1, which also gives some of the names that are often attached to the polynomials. If we examine the polynomials in more detail, it may initially seem a little odd that, for example, third-order polynomials often include first-order terms, fourth-order polynomials second-order and constant terms, and so on. The role of these terms is to reduce the RMS deviation contributed by each polynomial. Thus, in Equation (9.2), Z40 = 51/2(6ρ4 – 6ρ2 +1)
Z00
Z1–1
Z2–2
(9.2)
the wavefront aberration contributed by the 6ρ4 term is balanced by that from the –4ρ2 term, which corresponds to the ‘best focus’ in the case of spherical aberration that lies between the paraxial and marginal foci. The constant piston term 1 is added to make the mean wavefront error zero. In fact, the form of all the polynomials except the Z00 piston term is such that in each case the mean wavefront error across the pupil is 0. The normalization term (51/2 in the case of Z40) is chosen so that the coefficient of the polynomial (e.g., C40 ) represents the contribution made by the corresponding type of wavefront deformation to the overall RMS wavefront error.
Primary spherical aberration
Z3–3
Z4–4
Z11
Z20
Z3–1
Z4–2
Z22
Z31
Z40
Z33
Z42
Z44
Figure 9.5 Contour maps that show the form of the wavefront deformation associated with each of the Zernike polynomials (modes)
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less degradative effect than individual terms. This is, of course, not surprising. It is, for example, well known in optometry that the effect of a given cylindrical refractive error (i.e., a constant magnitude of second-order wavefront aberration) on visual acuity varies with the orientation of the cylinder axis. Useful information on the distributions of the values of the various Zernike coefficients in normal (unoperated) eyes is given by Porter et al.12 and Thibos et al.13 For most of the coefficients the values are symmetrically balanced about zero, as shown in Figure 9.7. This suggests a central tendency for natural eyes to be free of most higher-order aberrations, although biological variability means that any individual is equally likely to have a positive or negative aberration. The only clear exception is the C40 coefficient, which is systematically biased towards positive values (undercorrected spherical aberration). The spread of values becomes smaller as the mode order increases.
Z00
Z1–1
Z2–2
Z3–3
Z4–4
Z11
Z20
Z3–1
Z4–2
Z22
Z31
Z40
Z33
Z42
Z44
Figure 9.6 Isometric views of the wave aberrations that correspond to the first 15 orders. Note that the mean wavefront error is zero in all cases except the Z00 piston term Figures 9.5 and 9.6 show the wavefront deformations associated with each of the polynomials. It can be seen that the polynomials can be arranged in a pyramidal manner, in which higher-order Zernike modes represent increasingly complex patterns of deformation. Although there is no exact term-by-term equivalence, the terms can be related broadly to traditional concepts of refractive error and aberration according to the order of their radial components (see Table 9.1): • The zero-order (piston) term is not significant; • first-order terms represent prismatic effects; • Second-order terms represent spherical and astigmatic defocus; • Third- and fifth-order terms represent coma-like aberrations; and • Fourth and sixth-order terms represent spherical-like aberrations. Although, theoretically, the Zernike terms continue to higher and higher orders, it is rarely of interest to go further than the sixth-order for the eye, since the corresponding coefficients are very small and so these terms contribute little to the overall aberration. It is easy to see why this is so: the very high-order terms represent a wavefront with aberrations that change
rapidly with position in the pupil, whereas under most circumstances the wavefront after laser surgery (and in the natural eye) is normally relatively smooth. As noted earlier, one of the advantages of using normalized Zernike polynomials is that the absolute value of the coefficient Cnm for each polynomial mode represents the mode’s contribution to the overall RMS wavefront deviation. Thus, for example, if we want to know the combined contribution of the four third-order coma-like terms to the overall wavefront variance σ2 (i.e., the square of the RMS deviation) for the pupil diameter for which the Zernike terms are valid, we can write Equation (9.3), σ2 = (C3–3)2 + (C3–1)2 + (C31)2 + (C33)2
(9.3)
However, it is unfortunately not true that equal coefficients for different modes imply equal visual effects. Applegate et al.11 have shown that, for equal coefficients, spherical defocus (Z20) has a relatively greater degradative effect on visual acuity than the astigmatism modes Z22 and Z2–2, and that the coma terms Z31 and Z3–1 decrease acuity more than the trefoil terms Z33 and Z3–3. Combinations of terms may have a
Wavefront aberration and refractive correction At first sight, it might appear that if we are to use wavefront measurements as the basis on which to choose a spherocylindrical refractive correction, we need to consider only the values of the second-order defocus coefficients. However, if this approach is used, the derived prescription is usually found to vary with pupil size,14 even though in practice subjective refraction seems to change little under photopic conditions as the pupil size changes.15,16 The reason for the failure of the second-order coefficients C2–2, C20 and C22 to provide a good predictor of optimal refractive correction under all circumstances is because of the presence of second-order terms in several of the higherorder Zernike polynomials. As discussed previously, these second-order terms help to balance the degradative effects of spherical and other aberrations. Obviously, the quality of the retinal image is improved by their inclusion and hence they should be incorporated in the correction. Atchison et al.17 give appropriate equations that include all the relevant Zernike coefficients in the calculation of the corresponding objective spherocylindrical prescription. They suggest that an objective refraction should be based on either just the second-order Zernike coefficients for a small pupil (for which the effects of higher-order aberrations are usually small) or on both the second- and fourthorder Zernike aberrations deduced with a larger pupil (say 6mm).
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Figure 9.8 Essentials of the H-S aberrometer. (a) Typical basic design of aberrometer: LB, input laser beam, which produces a small spot light on the retina; BS, beam splitter; HS, H-S lenslet array; CCD, CCD camera. (b) Spot images formed on a plane wavefront: the array of spots in the focal plane of the H-S lenslet array is regular. (c) Spot images formed with an aberrated wavefront yield an irregular array of HS image spots. In practice, many more lenslets are used
BS LB
HS a CCD
b
c
a
b
scatter of intersections, or ‘spots’, is obtained (Figure 9.9a), and the greater the concentration of points in the ‘spot diagram’, the more closely the eye corresponds to the ideal case. Whereas the spot diagram relies purely on geometrical optics, the point-spread function (PSF) or image of a point includes the effects of diffraction (Figure 9.9b). When the amount of aberration is large, there is little difference between the spot diagram and the PSF, but for eyes with very little aberration the PSF is more realistic. The PSF is calculated by first using the wave aberration to determine the variations in the phase of the light disturbance across the pupil. The local phase is simply the wave aberration in microns, divided by the wavelength of the light in use and multiplied by 360°. The amplitude across the pupil is either treated as being constant or can be weighted to allow for the Stiles–Crawford effect. From the amplitude and phase distribution across the pupil (the pupil function), the PSF can be calculated.7 A third descriptor of image quality that is often of interest is the modulation transfer function (MTF), which shows how the contrast of sinusoidal gratings is degraded in the image as a function of spatial frequency. When the wavefront aberration, and the associated spot diagram and PSF, lack rotational symmetry, the MTF varies with the orientation of the gratings (Figure 9.9c). The ocular MTF can also be calculated from the pupil function for the eye, as the real part of its Fourier transform. Lastly, software often generates a representation of the retinal image of a chosen object (e.g., a Snellen E) in the presence of a known ocular wavefront aberration. This may be obtained by convolution of the object luminance distribution with the appropriate PSF (i.e., by blurring each point in the object so that it appears as a PSF of appropriately weighted illuminance and then summing the combined PSFs to give the overall image). Alternatively, the same calculation may be carried out by Fourier methods. Care must be exercised when interpreting these calculated images, since neural and other factors may mean that they do not correspond very closely to what the patient actually sees.
Using the wavefront information in refractive surgery c
d
Figure 9.9 Examples of (a) a spot diagram, (b) a PSF, (c) a two-dimensional MTF and (d) a retinal image, calculated from wavefront data by current commercial aberrometer software
Wavefront-guided surgery involves applying an ablation that attempts to neutralize the measured wavefront aberration of the original eye, that is both the ametropia
Wavefront technology
and the higher-order aberrations. Success in this balancing procedure is not easy to achieve. Not only must the original wavefront measurements be valid and reliable, but also many laser beam, tissue ablation and healing characteristics will affect the final result. It is, for example, obvious that laser-spot diameters of less than 1mm are necessary if higher-order aberrations are to be corrected. Yet this, in turn, means that more spot pulses are required to cover the full ablation area, so that at any given pulse frequency the ablation takes longer than with a larger spot and the problems of maintaining patient alignment and corneal hydration tend to increase. In LASIK, the variables introduced by the need to replace the flap offer further difficulties. This may favour the development of alternative methodologies, such as LASEK. Calculation and control of the number and position of the laser spots required must be extremely accurate. Fast eye tracking is likely to be necessary to avoid problems caused by eye movement. A further constraint on the degree to which aberration can be corrected is set by the need to preserve an adequate thickness of undisturbed cornea (at least 250μm): this is a particular problem for higher refractive corrections, for which the diameter of the ablation zone may have to be limited, even though this leaves an aberrated eye for larger pupil diameters. At the present time, most manufacturers of laser systems have begun to offer wavefront measurement integrated with their ablation systems, and experience in their combined use is beginning to build up. Early results show promise, but indicate that, although it can be demonstrated that eyes with less post-operative aberration tend to show higher levels of visual performance, low levels of ocular aberration cannot be achieved routinely at present. The main immediate value of the technique is undoubtedly that, since preand post-operative wavefront maps can be compared, the exact effects of the particular ablation pattern can be assessed in relation to the other parameters of the individual eye. Such comparisons should lead to a fuller understanding of the factors involved and to the development of improved ablation algorithms able to produce more consistent refractive outcomes. In the longer term, however, it is difficult to see how the various factors involved in a single excimer laser ablation procedure can be controlled routinely to the degree of precision required to correct accurately not only spherical and astigmatic errors (second-order wavefront errors), but also the higher-order
aberrations. It seems reasonable to hope, however, that higher-order aberrations may be kept at or even below the levels found in naturally emmetropic eyes, rather than being substantially higher, as is the case without wavefront technology. A major routine application of wavefront technology may be to help identify why some post-operative eyes have poor acuity and to help plan enhancement procedures to reduce unusual levels of aberration.
Super vision Let us suppose, perhaps optimistically in the light of the previous section, that understanding of the factors involved in aberration correction improves to the extent that ablation-corrected eyes can give diffraction-limited performance in monochromatic light for distance vision (or, if required, for any other distance). With such excellent optics, what improvements in visual performance over those achieved by natural emmetropes could be expected? As normal eyes suffer from aberration, which blurs the retinal image, will the ideal aberration correction yield ‘super vision’ with levels of acuity much better than the values of 6/4 usually achieved by normal, young adults.18 The key factor here is that visual performance is not just limited by optics: it also depends upon the retina and subsequent stages of neural processing. It is clear that the sampling limitations imposed by the finite size of the cone photoreceptors that form the foveal retinal mosaic would set a limit on achievable acuity, even if there were no optical degradation whatsoever, as
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illustrated in Figure 9.10. Although this sampling limit varies somewhat with the individual, the spacing of the foveal cones is such that it typically lies at about 60 cycles per degree (c/°) for grating objects, corresponding to about 6/3 Snellen equivalent. Finer gratings may detected, but will appear as some other form of coarser pattern, typically as ‘zebra stripes’. This phenomenon is known as aliasing. We can see, then, that the limits set by retinal neural factors mean that a reduction in the natural level of aberration in the eye is unlikely to produce high-contrast Snellen acuities much better than 6/3. This is, of course, much better than the levels typically achieved after current refractive surgery (in which only about 80% of patients achieve uncorrected acuities of 6/6), but not much better than that achieved by the best of natural eyes. What combined optical and neural performance levels might be achieved in practice if higher-order monochromatic aberrations were eliminated? Since in the natural and current post-surgical eye the degradative effects of aberration worsen as the pupil diameter rises, while diffractive blur reduces, the greatest improvements in optical retinal image quality are potentially obtainable when the pupil diameter is large. However, unless the pupil is artificially dilated, large pupils only occur when light levels are low and the spatial resolution achieved by the retina is degraded because of a shift towards rod vision and increased spatial integration. We must remember, too, that even if the monochromatic aberrations are corrected, the retinal image will still be blurred by longitudinal chromatic aberration.
Figure 9.10 Resolution limit set by the sampling limit of the foveal cone mosaic. To resolve the bars of a Snellen ‘E’, there must be unstimulated cones between the stimulated cones
Refractive surgery: a guide to assessment and management
To illustrate these effects, Figure 9.11 shows the MTF for the eye when the entrance pupil diameter is either 3.0 or 6.0mm. Three MTFs are shown for each pupil diameter: • A typical experimentally measured MTF for a natural eye with normal levels of aberration; • MTF for an ideal aberration-free eye working in monochromatic light of wavelength 555nm; and • The same eye working in white light with the degradative effects of chromatic aberration included. Also shown in Figure 9.11 is the photopic contrast threshold at the retinal level. If the MTF falls below this threshold level, the grating cannot be resolved, even at maximal contrast. The highest spatial frequency of high-contrast grating that can be resolved lies at the intersection of the MTF with the threshold curve. As discussed earlier, it can be seen that to eliminate optical aberration gives much more benefit at the larger pupil diameter, but only if the retinal threshold is unchanged (i.e., the pupil is artificially dilated). With smaller, normal photopic pupils, it can be seen that it is likely that the main benefit would be an overall improvement in image contrast at spatial frequencies below the cut-off, with only minor improvements in the cut-off frequency itself. Finally, even if the technical problems associated with the corrective ablation and possible regression effects can be overcome, it is likely to be impossible to correct monochromatic aberration fully at all times by surgical means.19 The natural aberrations vary as a function of accommodation (i.e., object distance) and age, while in any case lags and leads of accommodation are known to typify the accommodation system, and introduce variable second-order defocus aberrations. Further problems are caused by the fast (0.1–2.0Hz) fluctuations in accommodation that occur with amplitudes of 0.1–0.2D: these demand a dynamic correction of aberration.20 There is also evidence that aberrations may change as a result of tear film changes, prolonged near work or diurnal variation in corneal thickness and curvature.21,22
Modulation transfer or contrast threshold
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1.20 1.00 0.80 0.60 0.40 0.20 0.00 0
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Figure 9.11 The ocular MTF for (a) 3mm and (b) 6mm pupil diameters. For each pupil diameter the MTFs shown are for an eye free of aberration and working in monochromatic light, for the same eye working in white light and for an experimentally measured curve for a real eye. Also shown is the photopic retinal contrast threshold. The highest spatial frequency of high-contrast grating that can be resolved lies at the intersection of the MTF with the threshold curve
In summary, then, the possibility of producing a marked enhancement of the vision of those who already have good acuity by surgical means appears limited. However, it has been suggested that the implantation of an intraocular lens of which the aberration could be adjusted in
situ, perhaps by ablation or by changing its local index with a control beam, might present a possible future practical path to improved vision, since secondary non-invasive adjustments could be made to maintain overall ocular aberration at a low level. This lies some way in the future, however!
Wavefront technology
References 1 2
3
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6 7 8
Campbell FW and Gubisch RW (1966). Optical quality of the human eye. J Physiol (Lond.) 186, 558–578. Applegate RA, Howland HC, Sharp RP, Cottingham AJ and Lee RW (1998). Corneal aberrations and visual performance after radial keratotomy. J Refract Surg. 14, 397–407. Fan-Pau NI, Li J, Miller JS and Florakis GJ (2002). Night vision disturbances after corneal refractive surgery. Surv Ophthalmol. 47, 533–546. Oliver KM, Hemenger RP, Corbett MC, et al. (1997). Corneal optical aberrations induced by photorefractive keratectomy. J Refract Surg. 13, 246–254. Macrae SM, Krueger RR and Applegate RA (2001). Customized Corneal Ablation: The Quest for Supervision. (Thorofare: Slack Inc.). Freeman MH (1990). Optics, p. 231–236. (London: Butterworths). Born M and Wolf E (1993). Principles of Optics, Sixth Edition, p. 464–466 and p. 965–973 (Oxford: Pergamon Press). Kim C-J and Shannon RR (1987). Catalog of Zernike polynomials. In Applied Optics and Engineering, Vol. X, p. 193–221, Ed. Shannon RR and Wynant JC (London: Academic Press).
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Thibos LN, Applegate RA, Schwiegerling JT and Webb R (2002). Standards for reporting the optical aberrations of eyes. J Refract Surg. 18, S652–S660. Schwiegerling J (2002) Scaling Zernike expansion coefficients to different pupil sizes. J Opt Soc Am A 19, 1937–1945. Applegate RA, Sarver EJ and Khemsara V (2002). Are all aberrations equal? J Refract Surg. 18, S556–S562. Porter J, Guirao A, Cox IG and Williams DA (2001). The human eye’s monochromatic aberrations in a large population. J Opt Soc Am A 18, 1793–1803. Thibos LN, Bradley A and Hong X (2002). A statistical model of the aberration structure of normal, wellcorrected eyes. Ophthalmic Physiol Opt. 22, 427–433. Mrochen MC, Bueeler M and Seiler T (2002). Influence of higher-order optical aberrations on refraction. Invest Ophthalmol Vis Sci. 43, S82. Charman WN, Jennings JAM and Whitefoot H (1978). The refraction of the eye in relation to spherical aberration and pupil size. Br J Physiol Opt. 32, 78–93. Atchison DA, Smith G and Efron N (1979). The effect of pupil size on visual
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acuity in uncorrected and corrected myopia. Am J Optom Physiol Opt. 56, 315–323. Atchison DA, Scott DH and Charman WN (2003). Hartmann–Shack technique and refraction across the horizontal visual field. J Opt Soc Am A. 20, 965–973. Elliott DB, Yang KC and Whitaker D (1995). Visual acuity changes throughout adulthood in normal, healthy eyes: Seeing beyond 6/6. Optom Vis Sci. 72, 186–191. Charman WN (2003). The prospects for super acuity: Limits to visual performance after correction of monochromatic ocular aberration. Ophthalmic Physiol Opt. 23, 479–493. Hofer H, Artal P, Singer B, Aragon JL and Williams DR (2001). Dynamics of the eye’s wave aberration. J Opt Soc Am A 18, 497–506. Handa T, Mukuno K, Niida T, Uozato H, Tanaka S. and Shimizu K (2002). Diurnal variation of human corneal curvature in young adults. J Refract Surg. 18, 58–62. Harper CL, Boulton ME, Bennett D, et al. (1996). Diurnal variations in human corneal thickness. Br J Ophthalmol. 80, 1068–1072.
10 Future trends in refractive surgery Shehzad A Naroo and W Neil Charman
Introduction
The UK market place
Instrumentation and techniques in refractive surgery are improving all the time. This is evident when we look at the two video clips (see Chapter 6 and the CD Rom) of a flap being cut in a LASIK procedure. Video clip 2 shows the Hansatome microkeratome from Bausch and Lomb, which is by far the most popular microkeratome in the UK today. It is a two-piece device and cuts a hinge in the superior position. If we compare this to video clip 3 of the Amadeus microkeratome from Advanced Medical Optics (AMO), it is clear that the cutting of the flap seems to be simpler with the one-piece Amadeus. The Amadeus cuts a flap in the nasal position, which some surgeons argue is better than a superior hinged flap, while others argue the opposite viewpoint. Currently, there are a few very large manufacturers of refractive surgery equipment (excimer laser, microkeratomes, aberrometry equipment, etc.) and a handful of smaller companies. The past few years have seen many smaller companies being purchased by the larger ones to strengthen expertise in a particular area previously lacking in the larger company’s portfolio. This seems to benefit both sizes of company, as the larger players are able to support the research and development of the smaller ones and to merge their ideas with those that may exist in another area in which the larger company already has expertise. When discussing future developments of refractive surgery it is possible to speculate on many things. A few of these are outlined in this chapter but first the likely growth in the industry is considered.
The past decade has seen an exponential growth in the refractive surgery market. In the UK the number of surgeons who perform a variety of techniques (laser and nonlaser) seems to be relatively steady, but an increasing number of surgeons are becoming involved with laser refractive surgery. The number of specialist refractive surgery clinics in the UK is interesting. These clinics offer only refractive surgery, for which some employ full-time refractive surgeons, while others rely on session surgeons, who may be involved in other areas of ophthalmology in a NHS hospital the rest of the time. A few years ago, photorefractive keratectomy (PRK) was gathering momentum in the UK, but then many clinics closed. More recently, we have seen an increase in clinics that specialize in laser in situ keratomileusis (LASIK), but some of these have closed also. One high-street clinic that relied heavily on co-management schemes with nation-wide optometrists was taken over recently by a high-street optical retailer. Some of the other group optical retailers have also chosen to become involved in refractive surgery, while some have decided not be involved at all. A few large-volume refractive surgery clinics appear to meet most of the nation’s refractive surgery needs and these clinics seem to be able to adapt to incorporate each new wave of popular techniques and hence seem to be here for the long term. Some of these clinics have grouped together to form the Eye Laser Association (ELA), which is able to promote refractive surgery and increase awareness of the area for interested patients and practitioners. It is often suggested that the market place in the UK is a step behind that in the
USA. In the USA, the Food and Drugs Administration (FDA) began to approve excimer lasers for refractive surgery in 1996. Initially, the approval was granted to some excimer laser manufacturers for limited refractive errors. Gradually, further approvals were granted and the industry continued to grow. Meanwhile, in the UK excimer lasers were already being used for PRK and LASIK was beginning to emerge. The USA surgeons did not have the same early exposure to PRK as many of their global counterparts, which may partly explain some of the trends seen in the USA. For example, in the USA radial keratotomy remained popular, whereas in many other parts of the world it had a limited or short-lived popularity. Over the past few years there has been a steady growth in the refractive surgery industry in the USA (Table 10.1). However, the percentage growth year on year is declining steadily, which might suggest that growth in the industry is actually slowing down. Maybe this is to be expected, as the initial large group of patients who had waited to undergo this type of surgery have now explored this opportunity. There may now remain a slower flow of patients who reach the correct age or stability of ametropia to have refractive surgery. Perhaps when the next ‘big thing’ happens in refractive surgery, we will see another boom and then a gradual decline again.
Possible use of new types of laser Over the first two decades of laser refractive surgery, the argon fluoride (ArF) excimer laser, which emits at 193nm, reigned supreme in terms of its ability to create
Future trends in refractive surgery
Table 10.1 Growth of the refractive surgery market in the USA Year
Number of treatments
Growth (%)
1997 1998 1999 2000 2001
182,000 409,500 980,000 1,550,000 2,250,000
125 140 58 45
effective corneal ablations safely and within an acceptable time span. Nevertheless, such gas-based lasers have their disadvantages in terms of cost, safety, beam stability and maintenance. For this reason, the search continues for both alternative laser sources and different approaches to laser refractive surgery. Several solid-state lasers have been developed with outputs at wavelengths around 200nm, similar to that of the ArF excimer laser. These include a flash-lamp pumped laser that employs the fifth harmonic of neodymium–yttrium aluminium garnet (Nd:YAG) and emits at 213nm, a diode-pumped fifth-harmonic neodymium–yttrium lithium fluoride (Nd:YLF) laser at 209nm and a fourth harmonic titanium–sapphire crystal laser that works at 208nm. None of these appears to have gained widespread acceptance to date, but undoubtedly the search for other alternatives will continue. A more radical departure from current practice is offered by the use of pulses of infrared laser light of ultra-short duration to operate on the cornea.1 The Nd:YLF picosecond laser is used, which emits at 1053nm. The cornea is highly transparent at these wavelengths. When used to cut a corneal flap, the laser is focused at the required depth (e.g., 160μm) and the focused spots are moved in a spiral fashion outwards from the centre of the cornea. Since the pulses are of very short duration, each achieves a photodisruption effect mediated by plasma formation, stress waves and cavitation bubbles at the application site, rather than causing a thermal burn. High repetition-rate pulsing (1–2kHz) and the spiral movement mean that the ‘cut’ normally achieved by a microkeratome is created by a joining up of the ‘sheet’ of photodisruption sites, the flap being brought to the surface of the cornea by additional pulses anterior to the periphery of the spiral pattern. Once the flap is lifted, ablation of the underlying stroma can be undertaken with an excimer laser, as normal. A more ambitious procedure (picosecond laser keratomileusis) involves placing two overlying
spiral patterns at different and varying depths, so that an intrastromal lenticule is cut. This can be removed through a suitable aperture to correct the eye. As yet, only limited trials have been carried out, but doubtless further refinements, perhaps that employ different lasers, will follow.
Surgical restoration of accommodation In normal eyes, the amplitude of accommodation declines steadily through the early and middle years of adulthood to reach zero at around the age of 50 years. Beyond this age, there is no true change in the power of the eye when objects at different distances are observed, although ocular depth-of-focus allows objects to be seen with reasonable clarity over a limited range of distances, which gives a subjective amplitude of accommodation of around 1D. Some form of near-vision correction is therefore normally required by most people older than about 40 years. The loss in amplitude of accommodation is thought to be multifactorial in origin. As the lens ages, it increases in thickness and loses elasticity, as also does its capsule. There are changes in the geometry of the attachment of the zonule to the lens and the gap between the ciliary ring and the lens equator diminishes. The ciliary muscle is, however, thought to retain its power until much later in life. This situation has led to two approaches to try to extend the period over which accommodation is possible into later life. In the first of these, the natural, relatively rigid lens is removed from the capsule and is replaced by a more elastic synthetic material with suitable characteristics.2,3 In the second approach, it is reasoned that the reduced gap between the ciliary ring and lens equator limits the possible extension of the zonular fibres and their associated changes in tension: hence, if the gap could be increased, accommodation would be restored. It is therefore suggested that the diameter of the ciliary ring be increased via ‘scleral
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expansion’, in which a series of cuts are made in the sclera around the cornea and plugs are inserted to expand the sclera and, with it, the ciliary body. At present neither technique has demonstrated real success, perhaps because each concentrates on only a single factor related to presbyopia. Objective measurements of patients who have undergone scleral expansion surgery have so far shown no evidence for restored accommodation,4,5 although claims have been made that subjective amplitudes are increased. With the development of better materials, the lens replacement method may eventually be at least partially effective, although it obviously cannot compensate for the loss in capsular elasticity or for other changes in the lens–ciliary body complex. Accommodation is obviously also lost after cataract or clear lens extraction and intraocular lens (IOL) insertion. Bifocal, multifocal and varifocal IOLs can offer patients reasonable distance and near vision, but with the penalty of a generally reduced image contrast at all object distances. For some patients, ‘pseudo-accommodation’ – actually enhanced depth-of-focus caused by small pupil diameters, and small amounts of astigmatism and, perhaps, higher-order aberration – can also give a reasonable range of clear vision. Recently, however, several ‘accommodative’ single-vision IOLs have been produced and show considerable promise. The concept of these is that, although the IOL itself does not change power, it moves forward within the eye for near vision, so that the combined power of the eye and cornea increases. Several designs of lens are being evaluated currently. All are designed so that pressure on the lens supports (haptics) flexes them in such a way that the desired movement of the lens optic can be achieved. Different designs use changes related to near-vision in the ciliary body, capsular and vitreous pressures to produce the required movement. Optical and physiological constraints limit the achievable objective accommodation to about 1.0–1.50D, but (unlike the multifocal IOLs) no compromise in retinal image quality is involved. Although the longer-term stability and performance of these lenses have yet to be explored, the initial results are encouraging.6 Finally, it is clear that, in principle, PRK or LASIK-type ablations could be used to produce many of the types of ‘static’ corrections for presbyopia achieved by contact lenses or IOLs. Thus, for example, one eye could be corrected for distance vision and one for near, to give a monovision correction. With scanning-spot lasers and suitable controlling algorithms, bifocal,
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multifocal or varifocal correction to an eye should be possible. Attempts along these lines have so far not been particularly successful, however, possibly because the scanning-spot sizes were too large to achieve the desired corneal topography. Replacement of the flap in LASIK will obviously tend to smooth out the abrupt transitions in power across the pupil required for true bifocal or multifocal geometries.
Intraocular contact lenses The main role of IOL implants has been to replace the power of the crystalline lens after cataract removal, but manipulation of the lens parameters can offer more possibilities for refractive purposes. Calculations of the required lens parameters from ocular dimensions and powers for intraocular insertion are widely known. There are now very useful systems for accurate measurements of these ocular parameters, which enable better visual outcomes from the insertion. An example is the IOLMaster (Carl Zeiss, Jena, Germany), a recent non-invasive device that provides a complete set of ocular dimension measurements. It uses partial coherence interferometry to measure anterior chamber depth and axial length. The measurements obtained have been proved to be as accurate as those of ultrasound biometry,7 and are highly repeatable.8 It has the benefits of its noncontact character, and its corneal power measurements, obtained using image analysis, show a high correlation with those obtained with conventional keratometry7 and videokeratoscopy.8 The use of phakic IOLs for refractive correction, without crystalline lens removal, is discussed in Chapter 4, but here it is worth
Figure 10.1 Bausch and Lomb Orbscan IIz
mentioning these devices when used in conjunction with laser surgery. In cases of high ametropia, some surgeons are beginning to advocate the use of anterior or posterior chamber phakic IOLs – often referred to as implantable contact lenses9 – in conjunction with a partial correction by excimer laser surgery. This combined technique of a phakic IOL and laser epithelial keratomileusis (LASEK), PRK or LASIK is known as bioptics. It can be expected that for patients with high ametropia, future wavefront analysis systems and lasers will allow an IOL to be inserted to correct the majority of the ametropia. The cornea would be reshaped by excimer laser to yield total wavefront aberration correction, if this could not be achieved with the IOL itself. This may increase the chances of creating a visual acuity better than would be expected with other refractive surgery procedures in some patients (e.g., patients with over 10D of myopia). A single laser procedure on a patient with this degree of refractive error may require the removal of so much corneal tissue that the patient must have quite a thick cornea originally: at this level of laser correction the chances of reaching a satisfactory post-operative refractive error may be slim. The bioptics technique may give the patient a better chance of reaching emmetropia, as the laser part of the surgery would only top up the change of refractive error achieved with the phakic IOL. Of course, one immediate problem that comes to mind is that the patient would have potential risks from both procedures: that is, not only the complications of the laser refractive surgery, but also the risk of complications of cataract, chronic iritis or
Figure 10.2 The Pentacam. Courtesy of Birmingham Optical Group
endothelial cell loss with the phakic lenses commercially available at the moment. Another implantable contact lens device that is worth noting is the corneal inlay. This has actually been around for some time in various forms. In the early days, synthetic or human corneal implants were placed underneath corneal buttons created in keratophakia.10,11 More recently, with the development of different synthetic materials and more precise methods to create a hinged corneal flap with a microkeratome, this technique has appeared again. It may prove to be useful for certain groups of refractive errors.
Advances in measurement devices Equipment used in pre- and post-operative assessment in refractive surgery has improved greatly over the past few years. One example of this is the corneal topography devices. Although interest in assessing corneal topography has been around for many years, the real turning point was probably the introduction of colour-coded maps, initially by Klyce12 and later by Maguire et al.13 The revolution in corneal topography was probably aided by the growth in other areas of ophthalmic work, such as contact lenses, but probably the major driving force was the commercial nature of the growing refractive surgery market. Nowadays, nearly all manufacturers of excimer lasers have a compatible corneal topography device. We have also seen a move from the traditional Placido devices (as described in Chapter 2) to instruments like the Orbscan Corneal Analysis unit from Bausch and Lomb (Figure 10.1). Although the Orbscan has been around for a few years it has remained almost exclusively the only instrument able to calculate the anterior and posterior corneal shape, corneal pachymetry and anterior chamber. One version of the Orbscan incorporates a Placido disc to enable more accurate data to be obtained from the anterior cornea. The most recent adaptation of the Orbscan uses software that links directly into the Bausch and Lomb Zyoptix custom-ablation system, which includes an excimer laser and wavefront aberrometer. Other manufacturers have made similar systems to create custom ablations, linking their excimer lasers with either corneal topographers or aberrometers, or both. A recent instrument from Oculus, the Pentacam (Figure 10.2), uses a rotating Scheimpflug camera and is able to show the shape of the anterior and posterior cornea (Figures
Future trends in refractive surgery
10.3 and 10.4), the corneal thickness and the anterior chamber depth, as well as the shape of the crystalline lens. If the patient’s pupil is dilated, the Pentacam can map the anterior and posterior lens curvatures. This is a very useful device for looking at IOLs, as well as the results of corneal surgery.
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Figure 10.3 Image of the anterior eye cornea viewed by the Oculus Pentacam. The anterior and posterior corneal surfaces are visible, as well as the anterior crystalline lens surface. Early opacities can be seen in the crystalline lens. (Courtesy of Birmingham Optical Group)
Conclusion Although challenging problems remain to be solved completely, some millions of patients have already received substantial benefits from refractive surgery. The prospects are good for further developments and refinements that will reduce complications from their already low level and give standards of vision that will consistently equal, or even surpass, those provided by other methods of refractive correction. Eyecare professionals will, undoubtedly, want their patients to have full and informed access to the new opportunities for improved quality of life brought about by these advances in refractive surgery.
Figure 10.4 In this Oculus Pentacam image of the anterior eye the brightness has been adjusted so that the anterior and posterior corneal surfacesare less clear, but there is a clear image of an IOL resting in the lens capsule after phacoemulsification cataract extraction and IOL insertion. (Courtesy of Birmingham Optical Group)
REFERENCES 1
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Krueger RR (1998). The picosecond laser for nonmechanical laser in situ keratomileusis. J Refract Surg. 14, 467–469. Nishi O, Nakai Y, Yamada Y and Mizumoto Y (1993). Amplitudes of accommodation of primate lenses refilled with two types of inflatable endocapsular balloons. Arch Ophthalmol. 111, 1677–1684. Haefliger E and Parel JM (1994). Accommodation of an endocapsular silicone lens (Phaco-Ersatz) in the aging rhesus monkey. J Refract Corneal Surg. 10, 550–555. Elander R (1999). Scleral expansion surgery does not restore accommodation in human presbyopia. J Refract Surg. 15, 604. Mathews S. (1999). Scleral expansion surgery does not restore accommodation
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in human presbyopia. Ophthalmology 106, 873–877. Kuchle M, Nguyen NX, Langenbucher A, Gusek-Schneider GC, Seitz B and Hanna KD (2002). Implantation of a new accommodative posterior chamber intraocular lens. J Refract Surg. 18, 208–216. Nemeth J, Fekete O and Pesztenlehrer N (2003). Optical and ultrasound measurement of axial length and anterior chamber depth for intraocular lens power calculation. J Cataract Refract Surg. 29, 85–88. Santodomingo-Rubido J, Mallen EAH, Gilmartin B and Wolffsohn JS (2002). A new non-contact optical device for ocular biometry. Br J Ophthalmol. 86, 458–462. Zaldivar R, Davidorf JM and Oscherow S (1999). Combined posterior chamber
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phakic intraocular lens and laser in situ keratomileusis: Bioptics for extreme myopia. J Refract Surg. 15, 299–308. Barraquer JI and Gomez ML (1997). Permalens hydrogel intracorneal lenses for spherical ametropia. J Refract Surg. 13, 342–348. Werblin TP, Patel AS and Barraquer J (1992). Initial human experiments with Permalens: Myopic hydrogel intracorneal lens implants. Refract Corneal Surg. 8, 23–32. Klyce SD (1984). High resolution graphic presentation and analysis of keratoscopy. Invest Ophthalmol Vis Sci. 25, 1426–1435. Maguire LJ, Singer DE and Klyce SD (1987). Graphic presentation of computer-analyzed keratoscope photographs. Arch Ophthalmol. 105, 223–230.
Index Aberrometry, 15–16 see also Wavefront aberrations, Wavefront technology Accommodation, surgical restoration of, 77–8 Accuracy, 63 LASIK, 59 unilateral versus bilateral treatment, 62 Adhesion, 23 AK see Arcuate keratotomy American Society of Cataract and Refractive Surgery, 56 Ametropia, 5, 12 intraocular lenses, 78 Anisometropia post-PRK, 36 with unilateral treatment, 62 Arcuate keratotomy, 27, 33 Argon fluoride excimer laser, 76–7 Astigmatism, 12 post-surgical, 14 Autorefractors, 41 Bailey–Lovie chart, 3, 41 Bausch and Lomb keratometer, 11 BCVA see Best-corrected visual acuity Best-corrected visual acuity, 3 LASIK, 59 phakic intraocular lenses, 31 Binocular vision problems, 43 Bioptics, 78 Bowman’s layer, 17, 19 innervation, 21 microstriae post-LASIK, 38 ultrastructure, 19 British Society for Refractive Surgery, 54, 56 Calcitonin gene-related protein, 21 Cancer risk of refractive surgery, 44 Case reports, 49–52 Cataracts, risk of, 44 Cell migration, 22 Cell proliferation, 22 CIL see Corneal inlay lenses Clear lens extraction, 27, 32 complications, 32 results, 32 surgical procedure, 32 Co-management, 53–6 general schedules for, 55–6 GP’s role, 54–5 insurance and legal issues, 56 ophthalmologist’s role, 55 optometrist’s role, 53–4, 55 professional relationships/ responsibilities, 54 training, 54
Complications of surgery, 39–40 clear lens extraction, 32 corneal infections, 40 dry eye, 39–40 intraocular pressure elevation, 40 LASEK, 29 visual complications, 41 LASIK, 30, 37–9, 59 corneal integrity, 38–9 diffuse lamellar keratitis, 38 epithelial ingrowth, 37–8 interface debris, 38 keratectasia, 39 microstriae, 38 retinal detachment, 39 visual complications, 41 overcorrection, 39 presbyopic surgery, 33 PRK anisometropia, 36 epithelium irregularity, 36 haze, 36 visual complications, 41 regression, 39 stromal infiltrates, 40 undercorrection, 39 visual, 40–1 binocular vision, 43 contact lens fitting, 43 haloes, 43 management of, 41 non-tolerance of monovision, 43–4 poor-quality night vision, 42–3 poor-quality vision, 42 starburst effects, 43 Confocal microscopy, 23–4 Contact lenses corneal warpage, 15 factors influencing decision to cease use, 2 fitting post-surgery, 43 intraocular, 78 polymegathism, 20 Contour maps, 68 Contract sensitivity function, 6 Contrast Acuity Assessment test, 42 Contrast sensitivity post-LASIK, 59–60 Corneal anatomy, 17–22 Corneal endothelium, 20–1 long-term damage, 44 microanatomy, 20 replication and regeneration, 20–1 ultrastructure, 20 wound healing, 20–1, 23 Corneal epithelium, 17–19 basement membrane, 18 ingrowth post-LASIK, 37–8 irregularity post-PRK, 36
microanatomy of, 17–18 non-native cells of, 18–19 refractive index, 17 remodelling, 50 resident cells of, 18 stem cell theory, 19 ultrastructure, 18–19 wound healing, 22–3 adhesion, 23 cell migration, 22 cell proliferation, 22 latent phase, 22 Corneal hypoaesthesia, 21 Corneal infection, 40 Corneal inlay lenses, 27, 78 Corneal innervation, 21 Corneal integrity post-LASIK, 38–9 Corneal oedema, 22 Corneal pachymetry, 4–5, 59 Corneal profile, post-operative measurement, 41–2 Corneal regeneration and healing, 20–1 Corneal sensitivity, 21–2 Corneal shape, 10–11 descriptors of, 10 Corneal stroma, 19 microanatomy, 19 ultrastructure, 19 Corneal surface regularity index, 10 Corneal topography, 4, 9–16 and aberrometry, 15–16 classification, 9–10 corneal shape, 10–11 history, 9 limitations of, 15 measurement methods, 11–14 cornea as projector system, 13–14 cornea as reflector system, 11–12 presentation of data, 12–13 videophotokeratoscopy, 12 in refractive surgery, 14–15 Corneal transparency, 22 Corneal uniformity index, 10 Corneal warpage, 15 Corneal wound healing, 20–1, 22–3 endothelium, 20–1, 23 epithelium, 22–3 adhesion, 23 cell migration, 22 cell proliferation, 22–3 latent phase, 22 stroma, 23 Cost issues, 61 Descemet’s layer, 20 Diffuse interstitial keratitis, 59 Diffuse lamellar keratitis, 38, 50–1
Index Diode thermokeratoplasty, 31 Directorate for Optometric Continued Education and Training, 54 Dry eye, 39–40 Drysdale effect, 12 Elevation, 11 Emmetropia, 67, 78 European Society of Cataract and Refractive Surgeons, 56 Excimer lasers, 28, 57–8 argon fluoride, 76–7 cost of, 61 equipment, 58 krypton fluoride, 28, 57–8 new developments, 76–7 see also LASEK; LASIK Eye Laser Association, 76 EyeSys, 4, 12 Flap infection, 50–1 Flap melt, 51 Flap transaction, 49–50 Fleischer’s rings, 4 Fuchs’ endothelial dystrophy, 2 Fundus examination, 5 General Medical Council, 54 General Optical Council, 54 General practitioners, role in comanagement, 54–5 Glaucoma, 2 Glycosaminoglycans, 19 Haloes, 43 Hartmann–Shack, 71 Haze post-PRK, 36 Hemidesmosomes, 18, 22 Holliday diagnostic summary, 11 Holmium laser thermokeratoplasty, 31 Hypermetropia, 67 Hyperopia, LASIK for, 30, 60 Iatrogenic ectasia, 39 ICRS see Intracorneal ring segment Indemnity insurance, 56 Infiltrates, 19 post-surgery, 40 International Council of Refractive Surgery, 56 Intracorneal ring segments, 27, 31 advantages and disadvantages, 31 surgical procedure, 31 Intraocular lenses, 77 contact lenses, 78 see also Phakic intraocular lenses Intraocular pressure, 2, 5 elevation post-surgery, 40, 60 IOLMaster, 78 Javal–Schiötz keratometer, 11 Keeler Tearscope, 6 Keratectasia, 39, 51–2
Keratitis, 23, 40 Keratoconus, 2, 4, 27 diagnosis of, 12 Keratocytes, 19, 23 loss of, 24 morphology, 24 Keratoglobus, 2 Keratometry, 11 data presentation, 12–13 Keratomileusis see Laser in-situ keratomileusis Keratophakia, 78 Keratoscopy, 11 KR-7000P, 12 Krypton fluoride excimer laser, 28, 57–8 Lamina densa, 18 Lamina lucida, 18 Langerhans’ cells, 18–19 LASEK see Laser subepithelial keratectomy Laser in-situ keratomileusis, 3, 27, 29–30 clinical outcome, 30 complications, 30, 37–9, 59 corneal integrity, 38–9 diffuse lamellar keratitis, 38 epithelial ingrowth, 37–8 interface debris, 38 keratectasia, 39 microstriae, 38 retinal detachment, 39 confocal microscopy, 23–4 contraindications, 29 corneal hypoaesthesia, 21 damage to microvilli in, 18 development of, 57 for hyperopia, 30 indications, 29 keratocyte morphology, 24 with LASEK in other eye, 51 late flap lift, 52 for myopia, 30 nerve fibre regeneration after, 22 post-operative care, 30 post-operative follow-up, 36–9 results, 59–60 accuracy, 59 contrast sensitivity, 59–60 high myopia, 60 hyperopia, 60 loss of best-corrected vision, 59 predictability, 59 stability, 59 surgical procedure, 29–30, 58–9 versus LASEK and PRK, 28, 30–1, 58 visual complications, 41 visual outcome, 40 Laser subepithelial keratectomy, 27, 28–9, 60–1 clinical outcome, 30 complications, 29 contraindications, 29 increased prescription after, 50 indications, 29 with LASIK in other eye, 51
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post-operative care, 29 post-operative follow-up, 39 refractive outcome, 39 surgical procedure, 29 versus LASIK and PRK, 28, 30–1, 60–1 visual complications, 41 visual outcome, 40 Lasers argon fluoride, 76–7 broad beam, 58, 65 excimer, 28, 57–8 holmium, 31 krypton fluoride, 28, 57–8 picosecond, 77 solid state, 77 LASIK see Laser in-situ keratomileusis Lipofuscin, 24 Long-term implications of surgery, 44 Microkeratomes, 58–9 cost of, 61 Microstriae post-LASIK, 38 Microvilli, 18 Mintel report, 53 Modulation transfer function, 72, 74 Monovision, non-tolerance of, 43–4 Muscle balance, 5–6 Myopia, LASIK for, 30, 60 National Health Service, 62–3 Neovascularization, 4 Nidek OPD, 15 Night vision, poor-quality, 42–3 Oculus Keratograph, 4 Oculus Pentacam, 14 Ophthalmologists, role in co-management, 55 Opticians Act (1989), 55 Optometrists co-management fees, 53–4 indemnity insurance, 56 role in co-management, 55 Orbscan, 4, 13, 14, 42, 49, 78 Overcorrection, 39 Pachymetry see Corneal pachymetry Patient selection, 1–8 decision to cease contact lens use, 2 occupational groups, 1 Pelli–Robson chart, 5, 42 Penetrating keratoplasty, 14 Pentacam, 78, 79 Phakic intraocular lenses, 27, 31–2 advantages and disadvantages, 32 anterior chamber lens implantation, 31–2 in conjunction with laser surgery, 78 posterior chamber lens implant, 31–2 Photoablation, 27 Photorefractive keratectomy, 2, 27 complications anisometropia, 36 epithelium irregularity, 36 haze, 36
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confocal microscopy, 23–4 corneal aesthesia, 21 keratocyte morphology, 24 nerve fibre regeneration after, 22 post-operative follow-up, 36 refractive outcome, 36 removal of corneal epithelium in, 18 unilateral versus bilateral treatment, 62 versus LASEK and LASIK, 28 visual complications, 41 visual outcome, 40 Phototherapeutic keratectomy, 2 cornea after, 15 Placido Antonio Placido, 9 cone systems, 12 disc systems, 4, 9, 12 Polymegathism, 20 Polymorphonuclear leukocytes, 19, 22 Poor-quality vision, 42 Post-mitotic cells, 22 Post-operative follow-up, 35–48 initial post-operative period, 35–40 complications, 39–40 LASEK, 39 LASIK, 36–9 PRK, 36 time scale for, 35 Pre-operative assessment, 1–8 contract sensitivity function, 6 corneal pachymetry, 4–5 corneal topography, 4 full refraction, 3 fundus examination, 5 intraocular pressure, 5 muscle balance, 5–6 pupil diameter, 3–4 slit-lamp examination, 4 tear secretion, 6 visual acuity, 3 Predicted corneal acuity, 10 Presbyopia, 3 Presbyopic surgery, 27, 32–3 complications of, 33 corneal, 32 intraocular, 32–3 outcome, 33 scleral, 32
PRK see Photorefractive keratectomy Professional responsibilities, 54 Projection techniques, 13–14 Proteoglycans, 19, 23 PTK see Phototherapeutic keratectomy Pupil diameter, 3–4 Purkinje image, 11, 14 Quad maps, 13 Radial keratotomy, 27, 44–5, 65 ocular integrity, 44–5 refractive stability, 44 visual performance, 45 Reflection techniques, 11–12 Refraction, 3 post-operative measurement, 41 Refractive correction, 70 Regression, 39 Retinal detachment post-LASIK, 39 RK see Radial keratotomy Rods, 24 Royal College of Ophthalmologists, 56 SimK see Simulated keratometry readings Simulated keratometry readings, 10–11 Slit-lamp examination, 4 post-operative, 42 Snellen chart, 3 Spot diagrams, 72 Staphylococcus aureus, 51 Starburst effects, 43 Stromal wound healing, 23 Super vision, 73–4 Surgical procedures, 14–15, 27–34 decision-making, 27–8 excimer laser technology, 28 history, 27 treatment plan, 28 see also LASEK; LASIK; PRK TACs see Transient amplifying cells Tear secretion, 6 Topographic Modelling System, 12 Training, 54 Transient amplifying cells, 19, 22 Treatment plan, 28
Undercorrection, 39 Unilateral versus bilateral treatment, 61–2 accuracy, 62 PRK, 62 safety, 62 subjective issues, 62 United Kingdom and Ireland Society of Cataract and Refractive Surgery, 56 United Kingdom, refractive surgery in, 76 United States, refractive surgery in, 77 Videokeratoscopy, 11 see also under Corneal topography Videophotokeratoscopy, 12 Visual acuity, 3 post-operative measurement, 41 Visual complications, 40–1 binocular vision, 43 contact lens fitting, 43 haloes, 43 LASEK, 41 LASIK, 41 management of, 41 non-tolerance of monovision, 43–4 poor-quality night vision, 42–3 poor-quality vision, 42 PRK, 41 starburst effects, 43 Visual outcome, 40–1 complications, 40–1 unaided vision, 40 Visual quality, 42 Vogt’s striae, 4 von Hippel–Lindau syndrome, 51 Wavefront aberration, 66–8 analysis of, 68–70 measurement of, 71 and refractive correction, 70 super vision, 73–4 use in refractive surgery, 72–3 Wavefront technology, 65–75 Wound healing see Corneal wound healing Zernike polynomials, 68, 69 Zonula occludentes, 18
