Impact and Permanence of LASIK-induced Structural Changes in the Cornea on Pneumotonometric Measurements: Contributions of Flap Cutting and Stromal Ablation

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Abstract

Purpose

To determine the factors that lead to changes in intraocular pressure (IOP) measurements after laser-assisted in situ keratomileusis (LASIK) and their long-term stability.

Patients and Methods

Five hundred twenty-two myopic eyes and 296 hyperopic eyes were enrolled in the study. Pneumotonometry was used to measure IOP once in the preoperative stage and twice in the postoperative stage—1 month after the operation and 1 year later. Ultrasonic pachymetry was used to determine preoperative and intraoperative corneal thicknesses and axial length of the eye, whereas optical pachymetry was used in the preoperative stage and 1 month after surgery. Corneal topography was used to determine the preoperative and postoperative mean curvature of the anterior surface of the cornea over 3 and 5-mm diameter regions. Comparative statistical analysis of the retrospective data series was performed.

Results

A highly significant reduction of IOP readings is found after LASIK for both myopic and hyperopic eyes. The reduction is stable 1 year after LASIK. In the case of myopic eyes, the reduction has a highly significant linear correlation with the amount of tissue ablated in the central region of the cornea.

Conclusions

Pneumotonometric IOP readings after LASIK are reduced, without recovering preoperative values even 1 year after surgery, because of flap cutting and tissue removal in the central region of the cornea. The contribution of flap cutting is estimated to be (1.6±0.8) mm Hg, whereas ablation contributes an additional (0.029±0.003) mm Hg/μm of removed tissue. This effect should be considered when evaluating the accuracy of IOP measurements in LASIK patients who are at risk for developing glaucoma.

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