The effect of excimer laser photorefractive keratectomy for myopia on nerve fiber layer thickness measurements as determined by scanning laser polarimetry

Scanning laser polarimetry measures the retardation of polarized laser light as it passes through the birefringent retinal nerve fiber layer (RNFL). Because retardation is directly proportional to the number of retinal nerve fibers, indirect measurements of the RNFL thickness are obtained. A fixed compensatory mechanism is used to correct for retardation attributed to birefringence in the cornea and anterior segment. Excimer laser photorefractive keratectomy (PRK) corrects myopia by ablating tissue from the cornea. This study was conducted to determine the effect of this ablation on RNFL measurements. Prospective comparative case series. Eighteen patients. Patients undergoing PRK had RNFL measurements performed with the GDx Nerve Fiber Analyzer (Laser Diagnostic Technologies, San Diego, CA), a scanning laser polarimeter, in both eyes before and 90 days after PRK in the first eye. Fourteen different GDx parameters were analyzed for statistically significant differences preoperatively and 3 months postoperatively in the treated and untreated eye using Hotelling’s T-squared generalized means test. Eighteen patients underwent PRK in the nondominant eye first. Preoperative refractions averaged −3.8 ± 1.6 diopters (D), with an average correction of 3.4 ± 1.9 D, corresponding to 49.7 ± 20.5 microns of ablation. There were no statistically significant preoperative differences between the fellow eyes in refractive error ( P = 0.65). Postoperatively, there were no statistically significant changes in any GDx parameter in the treated eye compared with the untreated control eye ( P = 0.21). Excimer laser PRK for moderate myopia has no significant effect on RNFL thickness measurements as determined by scanning laser polarimetry.