Abstract
Purpose::
To investigate and compare the relationships between glaucomatous visual field loss and intraocular pressure as measured by both Pascal® dynamic contour tonometry (DCT) and Goldmann applanation tonometry (GAT).
Methods::
All primary open-angle glaucoma (POAG) and normal tension glaucoma (NTG) patients seen between July 2005 and June 2006 with at least two 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 vs. left eyes and higher vs. lower AGIS-score eyes.
Results::
Sixty-seven (42 POAG, 25 NTG) subjects met all criteria for study inclusion. Per paired t-test, mean DCT-IOP was significantly higher in higher AGIS-score eyes compared to lower AGIS-score eyes (16.3 vs. 15.5 mm Hg, p=0.004), while mean GAT-IOP was not significantly different in these same eyes (14.5 vs. 14.4 mm Hg, p=0.56). Mean central corneal thickness (CCT) was also not different in higher versus lower AGIS score eyes (536 vs 538um, p=0.08). Mean IOP difference between the two methods was significantly larger in higher versus lower AGIS-score eyes (p<0.001), and 72% of the subjects demonstrated larger inter-method IOP differences in their higher AGIS-score eye compared to their lower AGIS-score eye (p<0.001; 95% CI: 0.59-0.82). Multivariate linear regression analysis revealed that AGIS-score differences between eyes were independently associated with both inter-method IOP differences between eyes (p=0.004) and CCT differences between eyes (p=0.04). CCT, however, was not associated with inter-method IOP differences within or between eyes.
Conclusions::
These findings suggest that 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 to DCT-IOP. Our findings also support the hypothesis that corneal biomechanical factors other than CCT are the major confounders of applanation tonometry measurements.
Keywords: intraocular pressure • clinical (human) or epidemiologic studies: systems/equipment/techniques • clinical (human) or epidemiologic studies: risk factor assessment