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 1 and 149 had either open angle glaucoma or ocular hypertension: group 2). IOP was measured in all normal subjects with the Goldmann applanation tonometer and the highest recorded IOP was obtained from patient charts for subjects with either open angle glaucoma or ocular hypertension. Regression analysis was performed on IOP, CCT, and corneal radius of curvature. The corrected IOP was also calculated using the models proposed by Ehlers and Orssengo and 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 and Pye. This indicates that the Ehlers and Orssengo and Pye models may significantly overestimate the effect of CCT on IOP measurement. 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 and 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.

Repeatability and effects of sequential measurements with POBF tonograph.

AIMS: (1) To investigate the effect of performing sequential measurements using a Pulsatile Ocular Blood Flow (POBF) Tonograph on POBF and intraocular pressure (IOP) measurements; (2) to determine any effect on IOP caused by performing POBF measurements on the fellow eye; and (3) to assess repeatability of measurements obtained using the POBF Tonograph. METHODS: Forty-six normal adult subjects (13 men, 33 women; age range, 18 to 55 years) took part in the present study. Twenty-one subjects underwent sequential measurements with a POBF Tonograph on five different occasions with five different time intervals (1, 2, 5, 10, and 15 minutes) between recordings. Twenty subjects underwent a baseline measurement on one eye with the POBF Tonograph, followed by repeat measurements in both eyes after 15 minutes. An additional 25 subjects underwent sequential measurements on the same eye with an interval of 15 minutes between readings. The intraclass correlation coefficient and Bland-Altman plots were used to assess repeatability of the instrument for all 46 subjects for the 15-minute time interval. RESULTS: In all the groups, repeated measurement resulted in a lower recorded IOP. A multivariate analysis of variance indicated that there was a significant decrease in IOP when measurements were repeated within 15 minutes (p=0.024), but there was no significant change in the POBF values (p=0.76). The recorded IOP was significantly lower when sequential measurements were performed with an interval of 1 minute (p <0.01) compared with longer time intervals. The decrease in IOP between baseline and the repeat measurement was significantly greater in the 1-minute interval group compared with the 15-minute interval group (p <0.005). There was no significant difference in POBF values between the baseline and repeat measurements in any group. There was no significant change in IOP induced by a single measurement on the fellow eye (p >0.05). The intraclass correlation coefficient value indicated good agreement between the values of the baseline and repeat measurements using the POBF Tonograph but had wide upper and lower limits of agreement. CONCLUSION: If IOP measurements have to be repeated using the POBF Tonograph, they are best done after an interval of at least 2 minutes and preferably after 15 minutes. Use of the POBF Tonograph had no significant immediate effect on the IOP or POBF values obtained from a fellow eye.