DCT better than GAT in assessing glaucomatous damage

Dynamic contour tonometry (DCT) provides a more accurate measure of glaucomatous visual field loss and intraocular pressure (IOP) when compared with Goldmann applanation tonometry (GAT), according to a study published in the December 2007 issue of the Journal of Glaucoma.

Michael Sullivan-Mee and team of the VA Medical Center, Albuquerque, USA investigated and compared the relationships between glaucomatous visual field loss and IOP as measured by both Pascal DCT and GAT in 67 subjects (42 primary open-angle glaucoma, 25 normal tension glaucoma).

The study criteria required primary open-angle glaucoma (POAG) and normal tension glaucoma patients with at least two sets of good-quality, bilateral DCT and GAT measurements and 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.

Mean DCT-IOP was significantly higher in the higher AGIS-score eyes compared with the lower AGIS-score eyes (16.3 vs. 15.5 mmHg, p=0.004), whereas GAT-IOP was not significantly different in these same eyes (14.5 vs. 14.4 mmHg, p=0.56). Mean IOP difference between the two methods was significantly larger in higher versus lower AGIS-score eyes (p<0.001); 72% of the subjects demonstrated larger inter-method IOP differences in their higher AGIS-score eye compared with their lower AGIS-score eye (p<0.001). 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 central corneal thickness (CCT) differences between eyes (p=0.04). CCT, however, was not associated with inter-method IOP differences within or between eyes.

From these results, the authors concluded 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: this is most likely because GAT-IOP systematically underestimates IOP compared with DCT-IOP. The authors also surmized that the results support the hypothesis that corneal biomechanical factors other than CCT are major confounders of applanation tonometry measurements.