Preface

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.

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.

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

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-threat- ening infection is extremely rare after refractive surgery.7,8 Also, in photorefractive keratectomy (PRK) surgery the two

Visual acuity

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 post-

operative problems, the patient does not show a reduction in BCVA.27,28 Most cli-

nicians 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

Figure 1.3

High-contrast (90 per cent) distance Bailey–Lovie chart

Full refraction

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.

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 meas-

ured 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,

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

Intraocular pressure measurement

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.

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

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

Figure 1.7

Pelli–Robson CSF chart

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–stro- mal 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

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