Clinical comparison of pascal dynamic contour tonometry and goldmann applanation tonometry in asymmetric open-angle glaucoma

Abstract

Purpose: To investigate and compare the relationships between glaucomatous visual field loss and intraocular pressure (IOP) as measured by both Pascal dynamic contour tonometry (DCT) and Goldmann applanation tonometry (GAT).

Patients and methods: All primary open-angle glaucoma and normal tension glaucoma patients seen between July 2005 and June 2006 with at least 2 sets of good-quality, bilateral DCT and GAT measurements were retrospectively identified. Additional inclusion criteria required that all subjects had repeatable, asymmetric glaucomatous visual field loss that corresponded with asymmetric glaucomatous optic neuropathy. After mean IOP values were computed and visual fields were scored using Advanced Glaucoma Intervention Study (AGIS) criteria, paired-eye comparisons were conducted using right versus left eyes and higher versus lower AGIS-score eyes.

Results: Sixty-seven (42 primary open-angle glaucoma, 25 normal tension glaucoma) subjects met all criteria for study inclusion. Per paired t test, mean DCT-IOP was significantly higher in the higher AGIS-score eyes compared with the lower AGIS-score eyes (16.3 vs. 15.5 mm Hg, P=0.004), whereas GAT-IOP was not significantly different in these same eyes (14.5 vs. 14.4 mm Hg, P=0.56). Mean IOP difference between the 2 methods was significantly larger in higher versus lower AGIS-score eyes (P<0.001), and 72% of the subjects demonstrated larger intermethod IOP differences in their higher AGIS-score eye compared with their lower AGIS-score eye (P<0.001; 95% confidence interval: 0.59-0.82). Multivariate linear regression analysis revealed that AGIS-score differences between eyes were independently associated with both intermethod IOP differences between eyes (P=0.004) and central corneal thickness (CCT) differences between eyes (P=0.04). CCT, however, was not associated with intermethod IOP differences within or between eyes.

Conclusions: These findings suggest that DCT-IOP is correlated with glaucomatous damage, and moreover, DCT-IOP is more closely related to extent of glaucoma damage than is GAT-IOP. The most likely explanation for these results is that GAT-IOP systematically underestimates IOP compared with DCT-IOP. Our findings also support the hypothesis that corneal biomechanical factors other than CCT are major confounders of applanation tonometry measurements.