Factors affecting epithelial hyperplasia after photorefractive keratectomy

Purpose: To determine the effect of patient age, postoperative time, ablation zone diameter and depth, attempted correction, and corneal topography on postoperative corneal epithelial thickness after photorefractive keratectomy (PRK). Setting: Private clinic and university hospital, Stockholm, Sweden. Methods: This retrospective, unmasked study comprised 136 myopic patients treated unilaterally with PRK. Seventy eyes had been treated with the Summit excimer laser 27 months +/- 7 (SD) earlier using ablation zone diameters of 4.1 to 5.0 mm. Sixty-six eyes had been treated with the VISX excimer laser 6 +/- 3 months earlier using a 6.0 mm zone diameter. The untreated fellow eyes served as controls. Epithelial thickness was measured at a standardized central corneal area with a modified optical pachymeter, and corneal topography was determined using computerized videokeratoscopy. Results: In the Summit group, the epithelial layer in the PRK eyes was 12.0 mu m (21%) thicker than in the control eyes (P<.001; 95% confidence interval [CI] 9.35 to 14.3 mu m). This thickness differential correlated significantly with increased ablation depth and attempted correction. In the VISX group, the epithelium in the treated eyes was 7.0 mu m (7%) thinner (P=.0009; 95% CI-1.9 to -6.7 mu m) and thickness did not correlate with ablation depth or attempted correction. There was no correlation between epithelial hyperplasia and patient age or postoperative followup. With the laser groups combined, epithelial hyperplasia was greater with smaller zone sizes and a greater rate of change in power at the edge of the ablation zone. Conclusion: The factors associated with an increase in epithelial thickness were small ablation zones, greater attempted corrections, and deeper ablations. Larger,smoother ablation profiles may result in less epithelial hyperplasia.