Excimer Laser Procedures
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Excimer Laser Procedures

      Photorefractive keratectomy (PRK), laser assisted in-situ keratomileusis (LASIK) and laser epithelial keratomileusis (LASEK) currently use an ultraviolet Argon Fluoride (193nm) gas discharge laser to remove minute layers of superficial corneal tissue under computer control. The computer controlling the laser is programmed to generate a specific algorithm relating to the diameter and depth required for a given refractive change, based on the average amount of tissue removed per pulse (Munnerlyn et al., 1988). To treat myopia, more tissue must be removed from the centre of the cornea than the periphery to produce a flatter refracting surface. The amount of tissue removed for low myopia is in the region of approximately 50m m - less than 10% of the overall corneal thickness. The treatment of astigmatism involves greater ablation of one principle meridian than the other. Hypermetropic treatments attempt to steepen the optically significant central cornea by producing a smoothly transitioned annular ablation in the stroma at the periphery of the optic zone. The central cornea remains either untouched or just lightly ablated. The ring-shaped ablation zone used for hyperopic correction has a relatively narrow width and the wound healing mechanisms post-surgery tend to partially fill in this region causing regression of the refractive correction.

       

      1.  
        2. Photorefractive Keratectomy (PRK)

        The first PRK procedure was performed on a human cornea in November 1989 by Mr. David Gartry at St Thomas’ hospital. Anaesthetic eye drops are instilled into the eye and the front layer of the cornea (epithelium) is scraped away over an area approximately 1mm larger in diameter than the intended treatment zone (Campos et al., 1992a) to expose Bowman’s membrane (figure 9).

        Figure 9: Remodelled corneal surface following PRK treatment. The zone over which the corneal epithelium has been removed, can also be visualised. (Courtesy of Mr David Gartry)

        The excimer laser is then employed to remodel the anterior corneal surface by the ablation of stromal tissue for a period between 5 and 90 seconds (figure 11B) (Trokel et al., 1983). The post-operative pain used to be severe for the first 24 hours but is now well controlled by medication such as topical non-steroidals and oral analgesia, and the use of bandage contact lenses. Some pain persists and the vision remains poor until the epithelium has healed over the treated area after approximately 3-6 days (McDonald et al., 1991). Eighty-three percent of low myopes (< -6D) and 61.4% of high myopes (>-6D) achieve the driving standard unaided (6/12 vision) by the end of the first week post-PRK (Reich et al., 1996). Unlike radial keratotomy, PRK involves treating the centre of the cornea therefore changes in corneal clarity are of concern. A sub-epithelial opacification referred to as haze develops over the first 2-4 weeks (figure 10), peaking in intensity around 2-3 months and gradually subsiding by approximately 12 months (Gartry et al., 1992; Gartry et al., 1993; Lohmann et al., 1991; Seiler et al., 1990; McDonald et al., 1989; McDonald et al., 1990a; McDonald et al., 1990b; McDonald et al., 1990c).

        Surgical outcomes

        As with radial keratotomy, PRK is more successful for lower degrees of myopia. 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 (Tuunanen and Tervo, 1998; McDonald et al., 1999; Pallikaris et al., 1999; Nagy et al., 2002). For higher degrees of myopia (>-6.00D), 68-74% achieve 6/12 or better (uncorrected vision), with only around 26% obtaining 6/6 at 12 months. The refractive error tends to be slightly hyperopic initially, drifting towards emmetropia, or mild myopia as the cornea heals. By 12 months, 87-99% of low and medium myopes (<–6.00D), and 79-84% of high myopes 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. PRK is now restricted to the treatment of myopia up to about –4.00D because it is able to produce highly predictable results for this group with rapid stabilisation of the refraction in the vast majority of cases.

          Figure 10: Corneal haze post-PRK (courtesy of Mr Sunil Shah)

           

          The technique for treating astigmatism using an excimer laser has become known as Photoastigmatic Refractive Keratectomy (PARK). The results are less accurate than those for myopic photorefractive keratectomy alone (see appendix) since the axis alignment is critical (Stevens and Steele, 1993) and there are variations in meridional wound healing with greater regression occurring in the meridian of greater tissue loss (Shieh et al., 1992). Shah et al. (Shah et al., 1997) reported that patients with high cylinders were unlikely to achieve 6/6 unaided and only a limited number achieve 6/12. The risk of a loss of best-corrected visual acuity is also slightly greater (Higa et al., 1997).

          As with myopia treatment, hyperopic PRK is more predictable for lower refractive errors (<+3.50D) (Daya et al., 1997; Knorz et al., 1998), however, stabilisation of the refractive error can take up to 12 months (Corones et al., 1999) and the risk of losing two or more lines of best–corrected visual acuity is greater than for myopic treatments (Nagy et al., 2001).

           

        1. Ocular integrity
          2.

          Studies have indicated that ocular integrity is not significantly compromised following PRK (Galler et al., 1995). Studies of porcine eyes examining the force required to rupture the globe after various forms of refractive surgery, indicate that PRK causes a slight weakening of the globe compared to untreated eyes but the difference is not statistically significant (Peacock et al., 1997). Rupture occurs at the sclera or near the limbus, as in untreated eyes (Campos et al., 1992b).

           
        2. Visual performance
          3.

      Forward light scatter is known to increase during the first 2 weeks post-PRK, peaking around three months and returning to normal levels comparable to spectacle wearers and soft contact lens wearers by 12 months (Miller and Schoessler, 1995; Veraart et al., 1995). There is evidence to suggest that the distribution of light scatter around the retinal image is permanently modified by PRK, with an increase in the spread of straylight leading to a reduction in retinal image contrast (Chisholm, 2002). The contrast of the retinal image can also be impaired by surgically induced aberrations (Martinez et al., 1996; Oliver et al., 1997), the most significant of which is spherical aberration (Seiler et al., 2000). The degree to which the aberrations of the eye increase varies considerably between individuals. Since both forward scatter and aberrations cause a reduction in the retinal image contrast, low contrast acuity and contrast sensitivity are affected for the first three months for myopia below –6.00D, and six months for treatments greater than -6.00D (Esente et al., 1993; Ambrosio et al., 1994; Pallikaris et al., 1996; Montes-Mico and Charman, 2001). A permanent reduction in visual performance occurs in a minority, associated with large induced aberrations or persistent scatter (Chisholm et al., 2000). Studies indicate that visual performance under low illumination when the pupil is dilated (and aberrations are greatest), may be compromised for a year or more, particularly for low contrast tasks (Strolenberg et al., 1996; Montes-Mico and Charman, 2002). High contrast acuity (Snellen letter chart) is only affected in severe cases and rarely in those treated for < –6.00D. The increase in forward light scatter and aberrations also leads to a reduction in visual performance in the presence of a glare source (glare sensitivity) during the first month post-PRK, with normal levels returning by about three months under daylight conditions, but not until 12 months in the presence of a dilated pupil (low illumination) (Seiler and Wollensak, 1991). A significant reduction in visual performance can occur if there is a mismatch between size of the ablation zone and the pupil in its dilated state.

      Unlike RK, diurnal variations in refractive error post-PRK are not clinically significant (Goldberg et al., 1997; Goldberg and Pepose, 1996), and neither hyper nor hypobaric conditions cause corneal deformation or a refractive shift (Hjortdal et al., 1996).

       

    1.  
      2. Laser Assisted in-situ Keratomileusis (LASIK)

      LASIK was developed in 1990 (Pallikaris et al., 1991) and involves the use of a microkeratome to cut a thin flap of tissue followed by ablation of the underlying corneal tissue (figure 11C). The flap is then repositioned and is held in place by strong osmotic forces until the epithelium heals over to cover the wound margins. The aggressive wound healing that occurs following PRK is not seen post-LASIK because of limited disruption of the corneal epithelium. This allows much higher refractive errors to be treated without inducing post-operative haze (Helmy et al., 1996), minimises pain (Buratto et al., 1993) and means that useful vision returns almost immediately. Post-operatively, a C-shaped ring is visible corresponding to the edge of the flap, which fades with time. Retreatment following LASIK involves lifting the initial flap and reablating the stromal bed. The time course of corneal healing post-surgery means that the flap can be lifted for about 12 months after the initial procedure. LASIK is now the most popular refractive surgery technique in both Europe and America.

      Figure 11: Diagrammatic representation of PRK (B) versus LASIK (C). The corneal epithelium seen shaded must be removed prior to PRK but remains intact during LASIK, (courtesy of Mr David Gartry)

       

        1. Surgical Outcomes
          2.

          The percentage of eyes achieving 6/12 vision or better has been quoted as 86-100% at 6 months post-LASIK for corrections of -8.00D or less (Montes et al., 1999; Yang et al., 2001), with 88-100% of eyes achieving a residual refractive error within ± 1.00D of emmetropia. The refraction tends to stabilise within 1-3 months (Balazsi et al., 2001; Magallanes et al., 2001). Between zero and 1.2% of eyes treated for myopia <-6.00D lose two or more lines of best corrected visual acuity (Montes et al., 1999). For the treatment of myopia up to –6.00D, LASIK is more accurate and predictable than PRK initially, but this difference evens out by one year post-surgery (Pop and Payette, 2000; Stojanovic and Nitter, 2001)

          Figure 12: The flap of tissue is reflected back to reveal the surface to be reshaped, (courtesy of Mr David Gartry)

           

          Hyperopic LASIK has proved slightly more successful than hyperopic PRK for the correction of low hypermetropia. In a study by Condon (Condon, 1997), 80% of eyes achieved 6/12 or better. The rate of regression was similar to hyperopic PRK but the stabilisation rate was approximately four times longer than that following myopic treatment (Corones et al., 1999). An unacceptably high percentage of patients treated for hypermetropia > +4.00D lose two or more lines of best corrected acuity (7.3%) (Ditzen et al., 1998; Goker et al., 1998).

          LASIK has been used to treated a wide range of refractive errors from +8.00D to –20.00D but the majority of surgeons now restrict LASIK treatment to those between +4.00D and –10.00D due to the reduction in accuracy, increased risk of complications and the small optic zones associated with the correction of high refractive errors (Puk et al., 1996).

           

        2. Ocular integrity
          3.

          There as been concern about the integrity of the globe post-LASIK, since it is uncertain whether or not the collagen fibres regrow between the corneal flap and the ablated stromal bed after surgery. In other words, the flap may only be attached around its margins by the corneal epithelium, in which case it would not contribute significantly to the strength of the cornea. However, a study examining the integrity of the globe following 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 significantly different (Peacock et al., 1997). LASIK eyes ruptured either at the flap edge or the limbus. A couple of other studies have also concluded that ocular integrity is not compromised by LASIK (Cowden et al., 1997; Galler et al., 1995). The risk of the flap being dislodged is also very low with one study of rabbit eyes showing that even the maximum amount of force that could be applied with an airgun (before the whole eye ball was destroyed), did not lift the flap at one week post-LASIK (Laurent et al., 2001). This can be attributed to the multiple layers of corneal epithelium that cover the flap margin. There have been reports of flap damage as result of focal trauma from the corner of a paper shopping bag at 3 weeks (Schwartz et al., 2002), and a tree branch 6 months (Geggel and Coday, 2001) post-LASIK. Following repositioning of the flap, a good visual outcome was achieved in both cases.

           
        3. Potential complications
          4.

          Complications can arise from either the flap or less commonly the laser ablation. Flap complications include those that occur at the time of surgery, such as an incomplete or decentred flap, and complications that present after surgery, such as flap striae and epithelial ingrowth, (nests of trapped epithelial cells beneath the flap, leading to corneal irregularity and glare or rarely corneal melt). The vast majority of complications manifest within the first 6-8 weeks. Surgeon experience is a key factor in the initial outcome.

          Because the ablation takes place within the cornea, LASIK requires sufficient corneal thickness to prevent the ablation encroaching within 250µm of the endothelium, increasing the risk of inducing keratectasia. This is a rare condition in which induced corneal thinning leads to protrusion of the corneal tissue, severe irregularity and consequently a reduction in visual performance. Some cases require a corneal graft to achieve functional vision. Keratectasia can generally been attributed to miscalculation of the remaining stromal bed after flap creation, due to the limited accuracy of microkeratomes, (standard deviation of 30m m). This is a severe complication that may not present for a year or so post-surgery (mean of 1 year ± 0.3 (Argento et al., 2001)). It is almost always associated with treatment for high myopia since more tissue needs to be removed (Seiler et al., 1998; Joo and Kim, 2000). The risk of retinal detachment increases with increasing myopia above approximately –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 (Mitchell et al., 1999; Ivanisevic and Bojic, 1998; Ivanisevic et al., 2000; Kanski, 1999). Consequently, some organisations inadvertently exclude the very individuals who are at greatest risk of keratectasia should they undergo LASIK, because of potential complications related to their refractive error.

           
        4. Visual performance
          5.

        As with RK and PRK, there is still the possibility that "successful" cases with good visual acuity will suffer from reduced visual performance due to increased forward light scatter and aberrations. However, forward light scatter does not appear to increase significantly following LASIK unless the patient suffers from complications such as diffuse lamellar keratitis, which is treatable. Higher-order aberrations are known to increase following LASIK (Oshika et al., 1999b; Marcos, 2001). Limited study has been made of the effects of LASIK on visual performance, but initial work suggests that problems are less common and less severe than those resulting from PRK. Spatial contrast sensitivity recovers by 3 months in 78% of patients (Alanis et al., 1996; Perez-Santonja et al., 1998) although one studied noted a reduction in low contrast acuity under low illumination at 6 months post-LASIK (Holladay et al., 1999).

         

      1.  
        2. LASEK

      LASEK is a relatively new technique used for low myopia and hyperopia, falling part way between PRK and LASIK. It involves the production of an epithelial flap using a solution of 20% alcohol. The underlying anterior stroma is ablated, as in PRK, but the epithelial flap is then replaced, acting as a bandage lens to minimise post-surgical inflammation. It is currently being used to treat low myopia, where it produces less haze than PRK and therefore better best-corrected visual acuity (Shah et al., 2001) and avoids the potential flap complications of LASIK. There is a rapid recovery of vision following LASEK: in one recent study of 222 eyes ranging from –1D to –11D, 98% of eyes achieved 6/12 unaided vision within two weeks of LASEK and 63% achieved 6/6 unaided vision at one year (Claringbold, 2002). The procedure appears to be safe since no eyes showed a reduction in best-corrected visual acuity despite the wide range of pre-operative myopia. Another study comparing LASEK with conventional PRK for the treatment of –3D to –6.5D reported significantly less corneal haze following LASEK although there was no significant difference in uncorrected vision or refractive outcome at 3 months post-surgery (Lee et al., 2002). Although some surgeons believe that LASEK is simply a glorified version of PRK with few real benefits, others believe that PRK will be replaced by LASEK in the near future.

       

       

    2. Complications common to all excimer laser treatments
      3.

      Individuals with 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 giving the appearance of halos around lights at night (O'Brart et al., 1994a; O'Brart et al., 1994b). Halos 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 virtually eliminated halo problems in the majority of patients, although Roberts and Koester (Roberts and Koester, 1993) 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 where glare is of serious concern.

      All techniques involving modification of the cornea are capable of causing an increase in forward light scatter and aberrations, and hence a reduction in visual performance. The effect of these factors on the quality of the retinal image is discussed further in section 4.

      Customised ablations for excimer laser surgery

      Technology has recently been introduced to allow the pre-operative aberrations of the eye to be measured and taken in to account during the reprofiling of the cornea by the excimer laser. The aim is to prevent or minimise the surgically induced increase in ocular aberrations and perhaps even reduce the aberrations below their pre-operative level. It remains to be seen whether this technology will make a significant difference to the average refractive surgery patient, however, it should significantly reduce the risk of poor visual performance associated with reduced optical quality. At this stage it is too early to make an informed judgement as to whether this technique will provide significant benefit over conventional laser surgery techniques. Initial results seem to suggest that aberrations are reduced but there is little benefit for the average patient (Mrochen et al., 2001; Reinstein, 2002).

  1. Potential Long-term consequences of Photoablation

    2.

Since photoablation is a relatively new technique, the long-term side effects are as yet unknown. There appear to be no adverse effects on the endothelium, which controls the hydration of the cornea (Ehlers and Hjortdal, 1992; Amano et al., 1993; Cannamo et al., 1997). Since the wavelength emitted by the ArF laser is in the Ultraviolet C range, mutagenesis and carcinogenesis are a potential worry. However, Nuss and colleagues (Nuss et al., 1987) found no difference in the unscheduled DNA synthesis, (a measure of the tissue repair mechanism), between the laser and a gemstone knife, suggesting that mutagenesis is not a problem. Extensive animal studies have not detected the development of epithelial or connective tissue neoplasms resulting from exposure to the laser. One study detected the presence of short-term changes in the aqueous humour and prolonged biochemical changes in the crystalline lens of rabbit eyes, which may be the precursor to cataractogenic changes (Costagliola et al., 1996). In another study, no significant elevation of MDA (malondialdehyde) levels, (a possible indicator of oxidative effects), was seen following PRK on rabbits, although levels were two to three times higher in the LASIK group than the PRK or control groups, implicating the microkeratome rather than the excimer laser (Wachtlin et al., 2000). No increase in the incidence of cataract following PRK or LASIK has been noted to date.

 

The change in corneal thickness following corneal refractive surgery has implications for the measurement of intraocular pressure. When using an applanation tonometer, a thinner cornea causes underestimation of the intraocular pressure and vice versa. The removal of Bowman’s membrane and the deposition of newly synthesised collagen alter the resistance of the cornea to indentation. A reduction in the apparent intraocular pressure also occurs when using a non-contact (air puff) tonometer. A mean decrease of 3.1 ± 2.6 mmHg was found, relating to the degree of myopia treated. Pressure drop (mmHg) = 1.6 - (0.4 * treatment mean spherical error in dioptres) (Chatterjee et al., 1997).

A number of refractive surgeons advocate monovision for presbyopic patients, allowing them to continue to manage without spectacles for the majority of tasks. The dominant eye is corrected to achieve a refractive error close to emmetropia and the non-dominant eye is left with a small myopic undercorrection, usually in the region of about -1.25D. The majority of patients are happy with monovision although some find the imbalance disturbing and opt to have the undercorrected eye retreated (Goldberg, 2001). Wright and colleagues (Wright et al., 1999) examined binocular function in a group of 21 patients with a monovision correction and compared them to a group of patients who had been fully corrected in both eyes. Both groups were treated with PRK. All patient’s maintained binocular fusion and some degree of stereopsis. From personal experience in Optometric practice (CMC), some monovision patients complain of poor contrast acuity at night, probably due to the reduced stimulus to the binocular cortical cells. Those who are most satisfied appear to be those with lower visual expectations. The implications of monovision for tasks such as rapid response driving have not been considered.

© British Society for Refractive Surgery and Catharine Chisholm