Dependence of tonometry parameters on the curvature and thickness of the corneal periphery

Relevance Tonometry occupies a critical place in modern ophthalmological practice, being a fundamental method for diagnosing and monitoring glaucoma. The only modifiable risk factor for the development of glaucomatous optic neuropathy is intraocular pressure (IOP) [1]. For the correct interpretation of tonometry results, it is important to take into account the individual characteristics of the fibrous membrane of the eye. Objective To study the effect of curvature and thickness of the peripheral cornea on tonometric measurements. Material and methods The study included 49 patients (49 eyes) with primary open-angle glaucoma of varying degrees of IOP decompensation (a, b, c according to Nesterov - Bunin), without concomitant pathology that could affect the results [2]. Intraocular pressure was measured using the Ocular Response Analyzer (ORA) based on the principle of bidirectional applanation. The values ​​of IOPcc, IOPg (corneal and Goldman compensated IOP, respectively), corneal hysteresis (CH), and corneal resistance factor (CRF) were obtained. The difference between IOPcc and IOPg (ΔIOP) was analyzed. The corneal thickness in the central (CTR) and peripheral (PTR) zones at a distance of 1.5; 2; 3 and 4 mm from the center, keratometric parameters (anterior and posterior curvature, as well as average corneal thickness) were measured with a Pentacam Scheimpflug camera. Data processing was performed in IBM SPSS using the Shapiro-Wilk test to check the normality of distribution and the Spearman test. Statistical significance was taken to be p ≤ 0.05. Results In most cases, the data follow a normal distribution, with the exception of CRF, keratorefractometry values, and mean corneal thickness at a distance of 4 mm from the center. The ΔIOP indicator has a significant inverse correlation with the biomechanical parameters of the cornea: r = -0.652 for CRF and r = -0.873 for CH (p < 0.001). ΔIOP also shows a similar correlation with the central and peripheral corneal thickness at different distances. The correlations of ΔIOP with keratometric data obtained on the Scheimpflug camera are positive and are comparable in strength to the correlations with PTR. As a result of stepwise selection for the multiple linear regression model, the following equation was selected: ΔIOP = (–0.619 • CH) + (–0.536 • CRF) + (0.003 • PTR 1.5 mm) + 10.702, with a high level of reliability (R² = 0.974). Conclusion The accuracy of intraocular pressure measurement (tonometry) depends not so much on the thickness or curvature of the cornea, but on its biomechanical properties. Although thickness and curvature affect these properties, the CRF and CH indicators provide more complete information than just linear dimensions. Keywords: tonometry; glaucoma; intraocular pressure; peripheral corneal thickness; corneal curvature