Evaluation of tonometric correction factors

Purpose: To investigate the efficacy of currently available correction factors in correcting intraocular pressure (IOP) measurements for the errors induced by the normal variations in corneal structural characteristics. Materials and Methods: Central corneal thickness (CCT) and corneal radius of curvature were measured on 324 individuals (175 normal: group I and 149 had either open angle glaucoma or ocular hypertension: group 2). TOP was measured in all normal subjects with the Goldmann applanation torrometer and the highest recorded IOP was obtained from patient charts for subjects with either open angle glaucoma or ocular hypertension. Regression analysis was perfon-ned on IOP, CCT, and corneal radius of curvature. The corrected IOP was also calculated using the models proposed by Ehlers and Orssengo & Pye. Linear regression analysis was used to calculate the residual association between corneal parameters and corrected IOP. Results: There was a significant positive correlation between IOP measured using Goldmann applanation tonometer and the CCT in both groups. There was no significant correlation between corneal radius of curvature and IOP in either group. There was a significant negative correlation in both the groups between CCT and corrected IOP calculated using the models of Ehlers and Orssengo & Pye. This indicates that the Ehlers and Orssengo & Pye models may significantly overestimate the effect of CCT on IOP measurement. Conclusion: The effect of CCT and TOP as observed in the present study and by other studies in literature is less than predicted by both the Ehlers formula and the Orssengo & Pye model. Correcting IOP for the effect of CCT using these models could be erroneous and lead to overcorrection of IOP, thus resulting in erroneously low corrected IOP eyes with thicker cornea and erroneously high corrected IOP in eyes with thinner cornea. Conclusion: The effect of CCT and IOP as observed in the present study and by other studies in literature is less than predicted by both the Ehlers formula and the Orssengo & Pye model. Correcting IOP for the effect of CCT using these models could be erroneous and lead to overcorrection of IOP, thus resulting in erroneously low corrected IOP eyes with thicker cornea and erroneously high corrected IOP in eyes with thinner cornea.