Change in Intraocular pressure measurements after LASIK - The effect of the refractive correction and the lamellar flap

Objective: To study the relationship between intraocular pressure (IOP) readings after LASIK and the amount of refractive correction. Design: Retrospective noninterventional case series. Participants: Patients receiving primary LASIK for myopia and myopic astigmatism. Methods: A database of preoperative, intraoperative, and 3-month postoperative data for 8113 consecutive eyes that underwent primary myopic and myopic astigmatic LASIK was retrospectively reviewed. Linear regression analysis of measured IOP change as a function of refractive change was then performed. Age and preoperative keratometry were also reviewed by multiple regression. Main Outcome Measures: Best-fit curve relating change in measured IOP to refractive change. Results: The mean spherical equivalent of the refractive change was -4.98 +/- 2.64 diopters (mean +/- standard deviation). The mean decrease in measured IOP was 2.0 +/- 3.3 mmHg. Linear regression analysis revealed a decrease of 0.12 mmHg of measured IOP per diopter of refractive change (95% confidence interval [Cl], 0.09-0.15, R-2 = 0.009, P<0.001). Extrapolation of the data to a theoretical correction of zero diopters revealed a decrease of 1.36 mmHg (95% Cl, 1.20-1.51, P<0.001), suggesting a component of measured IOP change that is independent of laser ablation. Conclusions: The reduction of IOP readings after corneal refractive surgery is a linear function of the amount of refractive correction, with an additional constant reduction that is probably related to the lamellar corneal flap. These data suggest that the lamellar corneal flap makes no contribution to the load-bearing characteristics of the post-LASIK cornea.