Surgical procedures

(–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

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

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 underor 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

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

40Lian 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.

41Alio 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.

42Siganos 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.

43Asbell PA and Ucakhan OO (2001). Long term follow up of Intacs from a single center. J Cataract Refract Surg. 27, 1456–1468.

44Charpentier DY, Nguyenkhoa JL, Duplessix M, Colin J and Denis P (1995). Intrastromal thermokeratoplasty for correction of spherical hyperopia – one year

prospective-study. J Fr Ophtalmol. 18, 200–206.

45Hoyos 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.

46Baikoff 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.

47Rosen E and Gore C (1998). Staar Collamer posterior chamber phakic intraocular lens to correct myopia and hyperopia. J Cataract Refract Surg. 24, 596–606.

48Landesz 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.

49Brauweiler PH, Wehler T and Busin M (1999). High incidence of cataract formation after implantation of a silicone posterior chamber lens in

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.

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-epithe- lialization, 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-epithelializa- tion 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-opera- tively. 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

Figure 5.2

Flap-edge defect post-LASIK. (Courtesy of Michelle Hanratty)

decide whether the location and extent of the ingrowth warrant intervention.

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-sur- gery.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-opera- tive 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 quot-

ed 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

increase in myopia and an associated reduction in vision have been reported at high altitude after LASIK.31

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)

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 5.7

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

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 syn-

drome, cataracts and myopic maculopa- thy.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

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

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-sur- gery, 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 post- PRK.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

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 spa-

tial frequencies is reduced for the first 3 months,107–109 although some spatial fre-

quencies 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.

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 highand 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

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 highand 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 highand 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-sen- sitivity 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.

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 underor 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

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-opera- tive keratometry readings, since post-oper-

ative keratometry readings misrepresent the power of the cornea,138 with the pos-

sibility 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 exhib-

it 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

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 contrast-

sensitivity 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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