EPITHELIAL ALTERATIONS FOLLOWING PHOTOREFRACTIVE KERATECTOMY FOR MYOPIA

BACKGROUND: A retrospective observational study was conducted to test the hypothesis that there is clinically measurable epithelial hyperpla sia after photorefractive keratectomy (PRK), and to determine its effe ct on the epithelial oxygen uptake rate. METHODS: One hundred myopic e yes who had been previously treated unilaterally with PRK were examine d. Fifty eyes were treated with the Summit exicimer laser (Summit Tech nology, Waltham, Mass) 27 +/- 7 months previously with ablation zone d iameters of 4.1, 4.3, 4.5, or 5.0 mm. Fifty eyes were treated with the VISX excimer laser (VISX Inc, Sunnyvale, Calif) 5 +/- 4 months previo usly with one ablation zone diameter of 6.0 mm. The untreated eyes ser ved as controls. Epithelial thickness was measured at a standardized c entral area within the ablation zone with a modified optical pachomete r. Oxygen uptake rate was measured in a subgroup of 30 eyes (20 Summit and 10 VISX). RESULTS: The epithelium after PRK was 24% thicker than in the control eye in the Summit group (mean difference 13 +/- 10 mum; p < .01) and 7% thinner in the VISX group (mean difference - 4 +/- 10 mum; p < .01). A higher oxygen uptake rate correlated with a thicker epithelium (r = 0.42; p < .05). CONCLUSIONS: The epithelium was signif icantly thicker after PRK between 13 and 37 months after treatment wit h the Summit excimer laser using ablation zone diameters of 4.1 to 5.0 mm. The epithelium was thinner between 1 and 15 months after treatmen t with the VISX laser using an ablation zone diameter of 6.0 mm. An in crease in oxygen uptake rate reflected the metabolic rate of a greater number of cells in the hyperplastic layer.