Refractive changes after phototherapeutic keratectomy

Refractive changes after phototherapeutic keratectomy MaritaAmm, MD, Gernot I.W. Duncker, MD

ABSTRACT Purpose: To evaluate refractive error changes after phototherapeutic keratectomy (PTK). Setting: University Eye Hospital, Kiel, and University Eye Hospital, Hulle, Germany. Methods: The MEL 60 excimer laser (Aesculap Meditec) was used in all cases. To even out the peaks and valleys of irregular surfaces, modulating agents were applied. The study included 45 patients with various preoperative corneal diseases: central scars, recurrent erosions, corneal dystrophies, and surface irregularities. Subjective and objective refraction, keratometry, slitlamp photography, and corneal topography were performed preoperatively and postoperatively. The follow-up was up to 24 months. Results: Twenty-six patients had stable postoperative refractions. Thirteen patients developed a hyperopic shift; the highest observed amount was +4.0 diopters. In seven patients, the astigmatic error increased, although no significant change in axis was measured. Three patients had a myopic shift. Conclusion: After PTK, all types of refractive change can occur. The greatest risk is that of a hyperopic shift. We saw a correlation between the degree of hyperopia and the ablation depth. Methods for preventing such changes include (1) a large treatment zone, (2) use of a polishing program involving a low viscosity fluid at the . end of the laser procedure, (3) a two-step treatment in selected cases to avoid ablations that are too deep. J Cataract Refract Surg 1997; 23:839-844

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he physical qualities of the 193 nm excimer laser make it an excellent instrument for lamellar corneal surgery. This wavelength has a short depth of absorption (from 3.7 to 3.9 /Lm) and a high energy per single photon (6.4 my), which result in a tissue ablation with sub micron precision. 1 The 10 to 20 nanosecond pulse minimizes the damage to adjacent cell structures; the zone of adjacent tissue damage has been measured by transmission electron microscopy as 0.08 to 0.30 /Lm. 2 ,3 These characteristics create a smooth wound profile immediately postoperatively. Generally approved indications for phototherapeutic keratectomy (PTK) are recurrent corneal erosion, Reprint requests to Marita Amm, MD, University Eye Hospital, Hegewischstrasse 2, D-24105 Kiel, Germany.

corneal scars (postoperative and post-traumatic), superficial corneal dystrophies, and surface irregularities. Laser ablation and induced healing mechanisms can change the corneal curvature dramatically. Case reports of a successful reduction of opacities and irregularities in various pre-existing corneal diseases note a hyperopic shift of up to +8.0 diopters (D),4-6 but no large, controlled investigations of the refractive changes after PTK have been published. In this study, we evaluated the refractive error changes after anatomically successful PTK.

Patients and Methods A 193 nm MEL 60 excimer laser (Aesculap Meditec) was used for all treatments. This laser has two

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modes of application: spot and scan. In spot mode, the laser beam is focused to a point 1.5 mm in diameter and can be guided manually over the pathological tissue. At a repetition rate of 3 to 5 Hz, the laser reaches a fluence of 500 to 1000 mJ/cm2 with an ablation rate of 1 to 2 /-Lm/spot. This mode is used to treat recurrent corneal erosions. From 50 to 500 laser shots have been recommended, depending on the erosion size; these are applied at the border between intact epithelium and the epithelial defect in an overlapping technique? In this study, the maximum number of single laser spots was only 270 pulses. In scan mode, a 1.0 X 7.0 mm laser beam achieves a fluence of 250 mJ/cm2 at a repetition rate of 20 Hz. The total ablation depth is 0.9 to 1.0 /-Lm/scan. This mode is best used for the continuous removal of broad areas of pathologic tissue. If the corneal surface is irregular, decreasing concentrations of modulating agents should be used. Otherwise the peaks and valleys of the preoperative profile will be repeated in deeper corneal layers after the uniform laser ablation. During the "sight and sound" technique, the photoablation is directed by the fluorescence behavior and the acoustic reflection of the blocking fluid. 8 The efficacy of different blocking agents has been investigated. 9 We applied polyacrylic acid (molecular weight 40 million) as a smoothing substance. In previous clinical studies, 10 this agent proved to be sufficiently viscous to cover deep corneal valleys but liquid enough to expose all surface protuberances. Forty-five patients with various preoperative corneal diseases (Table 1) were included in the study. The preoperative and postoperative examinations included Table 1.

Preoperative corneal diseases in the 45 patients.

Disease

Number of Patients

Recurrent corneal erosion Central corneal scars Postoperative Post-traumatic Post infectious Corneal dystrophy Reis-BOckler's Macular Granular Thiel-Behnke Surface irregularities

13 18 4 9 5

840

7 1

3 2 7

subjective uncorrected and best corrected refractions with Snellen optotypes, objective refraction (Topcon refractometer), intraocular pressure, slitlamp microscopy, and fundoscopy. In addition to these routine diagnostic methods, keratometry (Zeiss instrument), slitlamp photography, and corneal topography (TMS system) were performed preoperatively and postoperatively. Haze was assessed with Fantes' scale. 11 Because of the corneal disease, it was often difficult or impossible to obtain exact and reliable data preoperatively by objective refraction and keratometry. Before the laser treatment, all patients received topical antibiotic and anesthetic eyedrops (erythromycin and tetracaine hydrochloride). Before starting the scan mode, we removed the epithelium over an 8.0 to 9.0 mm zone with a hockey knife. Then we performed an 8.0 mm central ablation without prior peripheral keratectomy or a transition zone. Postoperative care consisted of antibiotic and soothing ointments (erythromycin and dexpanthenol three times each day) until complete re-epithelialization had occurred, usually by day 2 to 4. In patients who had deeper ablations, we then instilled local corticosteroids (fluorometholone 0.1 %) four times per day at first, then tapering over 3 months according to the degree of corneal haze. Follow-up was up to 24 months.

Results During the follow-up, none of the patients showed visual loss. In 9 patients (20%), best corrected visual acuity (BCVA) did not change. In 36 (80%), it improved, sometimes up to seven logarithmic lines. Refraction did not change in the 13 patients treated with PTK in spot mode. After scan mode, which was always performed with modulating agents, various spherical and cylindrical shifts occurred in the 32 patients: (1) In 13, the refraction remained stable. (2) Thirteen developed hyperopia (highest amount 4.0 D; mean 1.7 D; range 0.5 to 4.0 D). (3) In 7, the preoperative astigmatic error increased (maximum 2.75 cyl D), but we did not observe an irregular shape in eyes in which the shape was previously regular. When subjective refraction was evaluated, no significant axis change was measured. The corneal topography revealed postoperative shifts of up to 30 degrees. Patients who did not have astigmatism preoperatively did not develop it as a result

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Figure 1A. (Amm) Slitlamp photograph of the cornea of a 42-year-old man with a recurrence of Thiel-Behnke dystrophy in a graft. Best corrected visual acuity: + 1.75 -0.25 x 45; 20/50.

Figure 1B. (Amm) Slitiamp photograph of the same patient 24 months after PTK. Best corrected visual acuity: +3.0 -3.0 x 40; 20/25.

ofPTK. (4) Three patients had a myopic shift (highest amount 1.5 D). After 6 months, the refraction remained stable in most patients, except those treated for corneal scars from inflammation. In these eyes, PTK was not successful or the results were only temporary. Refractive fluctuation occurred during the entire follow-up. The maximum amount of haze was 1.5. Figure lA is a preoperative slitlamp view of the right eye of a 42-year-old man with a recurrence of ThielBehnke dystrophy in a corneal graft 5 years after penetrating keratoplasty (PKP). With + 1.75 -0.25 X 45, his visual acuity was 20/50. Twenty-four months after PTK in scan mode, the increased corneal transparency

was obvious, with the honeycomb-like surface irregularities smoothed (Figure 1B). Corresponding to the anatomical success, both uncorrected visual acuity (UCVA) and BCVA improved. The UCVA improved from 20/200 to 20160; with +3.0 -3.0 X 40, the patient read 20/25. Figure 2 shows preoperative and 24 months postoperative corneascope photographs. Whereas the central mires formed irregular lines preoperatively (Figure 2A), after PTK the Placido rings were concentric at symmetrical distances (Figure 2B). Figure 3A shows the cornea of a 27-year-old man with macular dystrophy. The spiral opacities could be removed by PTK in scan mode with a high degree of

Figure 2A. (Amm) Preoperative photokeratoscope of the patient with Thiel-Behnke dystrophy shows irregular central mires.

Figure 2B. (Amm) Photo keratoscope of the same patient 24 months postoperatively demonstrates concentrically arranged Placido rings.

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(Amm) Slitiamp view of the cornea of a 27-year-old man with macular dystrophy. Uncorrected visual acuity: 20/200 (no improvement with spectacles, objective refraction impossible).

Figure 38. (Amm) Slitlamp view of the same patient 8 weeks after PTK. Uncorrected visual acuity: 20/25; objective refraction:

corneal smoothing (Figure 3B). Uncorrected acuity improved from 20/200 to 20/25, but postoperative objective refraction indicated hyperopia of 3.0 D. Preoperative objective refraction or retinoscopy was not possible because of corneal scarring. The patient's ophthalmic history did not indicate a refractive error. A comparison of the preoperative and postoperative photokeratoscopes (Figures 4A and 4B) revealed good corneal regularity after PTK.

age to surrounding structures, an otherwise necessary PKP could be postponed or even prevented. . "filCant posta slgm Sorne auth ors 4-6 "12 14 mentlon operative refractive change, particularly a hyperopic shift of up to 8.0 D. One report presumed that a centrifugal contraction of the superficial corneal collagen lamellae was the main reason for the induction of hyperopia. 1s Another explanation was that debris rising from the photo ablation shielded the peripheral laser beam. IS Increased obliquity of incident radiation at the edge of the beam, due to the curvature of the cornea, could result in an irregular and lower ablation rate in the periphery of the treatment area than in the center, creating an ablation profile similar to that for correcting myopia. A multizone ablation mode, which exposes the axial cor-

Figure 3A.

Discussion Many case reports 10 ,12-14 demonstrate the efficacy of PTK in treating anterior corneal diseases. Because of the precise tissue ablation depth and the minimal dam-

Figure 4A.

(Amm) Preoperative photokeratoscope of the cornea with macular dystrophy. Because of the surface irregularities, the rings are broken and not concentric.

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+3.0 D.

Figure 48. (Amm) Postoperative photokeratoscope of the same cornea shows the regained concentric order of the Placido rings at symmetrical distances.

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nea, may also lead to the development of hyperopia. However, these latter explanations are only applicable to a wide-field delivery system, not to a scanning-slit approach. Other theories point out the relative concavity of the surface structure because of the capillary forces of the masking liquids at the treatment border. This profile may cause an irregular photoablation with preferential thinning of the central cornea. Epithelial hyperplasia at the transition zone after the healing course may also be a £: • h . h·fi 16 lactor III yperoplc SIt. Pronounced epithelial hyperplasia at the optical zone may be a reason for the myopia observed after surgery.17 In our study, the three patients who developed low-grade myopia postoperatively did not have a typical myopic corneal topography. We assume there may be another reason for the refractive change: After PTK, the cornea's optical zone is much dearer, and subjective and objective refractions can be done more reliably. The most important finding of our study seems to be the lack of refractive change after PTK in spot mode. In this mode, the laser energy is applied to the outer layer of the cornea; it does not extend beyond Bowman's layer. After scan mode, we observed postoperative refractive changes in the spherical and cylindrical components. The greate~t risk, which has been reported by others, was a hyperopic shift, which was seen in 40.6% of the eyes treated in scan mode. The refraction remained stable in another 40.6% of these eyes. We believe there was a correlation between the development of hyperopia and ablation depth; 18 the more laser pulses applied, the greater the hyperopic shift. All patients with deeper preoperative stromal pathology and a dear optical zone postoperatively had a higher incidence of hyperopia than patients with only superficial corneal disorders, who therefore had less tissue removed. We have no exact data, however, to prove there is a positive correlation between the number of laser pulses and a hyperopic shift because we used blocking agents that absorbed some of the pulses. The various corneal layers have different ablation rates; scar tissue and corneal pathologies also respond differently to the ablation procedure. 19 This, along with the use of masking substances, makes it impossible to define the exact intraoperative ablation depth. An abla-

tion depth limited to 100 J-Lm of corneal thickness is generally recommended and probably has little effect on refraction and corneal transparency. 1 We tried to identify the suitable patient by slitlamp. A better, more exact method for determining the depth of the corneal disease is by Haag-Streit optical pachymetry.20 Methods for preventing unwanted hyperopia are combining a myopic flattening with a consecutive hyperopic laser program 14 or performing laser keratectomy in the middle periphery before starting the actual 4 smoothing procedure. Another suggested technique for reducing surface irregularities and refractive shifts is a transition zone in the laser profile. The induced concave curvature at the treatment border may, however, have a detrimental effect on the refractive results. We prefer the following prophylactic strategies: (1) Enlarging the ablation zone over a broad corneal area, which minimizes the risk of cutting a new "refractive corneal lens" and producing sharp tissue edges that could stimulate proliferative mechanisms during the healing process. (2) Using a "polishing program" involving a low-viscosity fluid at the end of the laser procedure to prevent abrupt and sharp tissue "steps." We usually apply isotonic sodium chloride solution in this last modulating procedure after polyacrylic acid. (3) Considering the correlation between ablation depth and hyperopia, a two-step treatment appears advantageous in selected cases. Corneal smoothing is often sufficient to achieve visual improvement without total elimination of hazed tissue. 10 An unnecessarily deep ablation can therefore be avoided.

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