Abstract
Abstract: :
Purpose:To calculate a 'true' intraocular pressure measurement (IOPT), after accounting for the effects of central corneal thickness (CCT) & corneal curvature (CC) on the highest recorded IOP & examining how many subjects with open angle glaucoma (OAG) or ocular hypertension (OH) could be reclassified into a different diagnostic group. Methods:Orssengo & Pye’s theoretical mathematical model of the cornea (Bull Math Biol 1999:61;551-72), which takes into account CCT & CC, was used to calculate IOPT. CC & CCT were measured in one randomly selected eye of 127 subjects (81 in Norwich, 46 in Chennai) none of whom had undergone intraocular surgery. 41.7% had OH, 37.8% had primary open angle glaucoma (POAG) & 18.1% had normal tension glaucoma (NTG). CCT was measured using ultrasonic pachymetry. The highest recorded IOP measured using Goldmann tonometry, was obtained from the patient charts. The model was then applied to produce IOPT. Results:There was good correlation between the two pachymeters used (Pearson r=0.994 p<0.001). Linear regression was used to combine the data of the pachymeters. Based on the IOPT, overall, 11.0% of our subjects were reclassified. 8.9% of the OH group were reclassified as normal i.e. IOPT < 21mmHg. 8.3% of the POAG group were reclassified as NTG i.e. IOPT 21mmHg. The mean percentage error between the highest recorded IOP & IOPT was 10.3% (SD 7.9) with the maximum difference being 42.8%. Paired t-tests showed statistically significant differences between IOPT & the highest recorded IOP in the OH, POAG & NTG groups (p-values 0.0001, 0.012 & 0.020 respectively). Conclusion:Applying the Orssengo & Pye model to the classification of OAG & OH results in 11.0% of subjects being reclassified. Using this model, the number of subjects with NTG being reclassified as POAG is on a par with other studies whereas the number of subjects with OH being reclassified as normal is rather less than has been suggested in other studies.
Keywords: 444 intraocular pressure • 369 cornea: clinical science