Agreement Between Goldmann Applanation and Dynamic Contour Tonometry Measurements: Effects of Corneal Morphometry

Abstract

Purpose: : to determine agreement between intraocular pressure (IOP) measurements made by Goldmann applanation tonometry (GAT) and dynamic contour tonometry (DCT).

Methods: : IOPs were measured by GAT and DCT in 140 eyes of 70 patients with glaucoma and 138 eyes of 69 healthy subjects. The GEE method was used to ensure independence of eye measurements in a single subject. Agreement was determined by calculating intraclass correlation coefficients (ICC) for the sets of measurements obtained using the different instruments. Bland Altman plots were constructed to determine whether differences between measurements were conditioned by their size and to establish the existence of systematic and/or proportional biases. Multivariate regression analysis was used to examine whether the differences between the two instruments were affected by the power of the steepest and flattest axes of the cornea, their orientation, by age or by central corneal thickness (CCT).

Results: : ICCs were 0.272 (p<0.0001) (95%CI: 0.109-0.42) in the glaucomatous eyes and 0.569 (p<0.0001) (95%CI: 0.441-0.675) in the normal eyes. Mean differences were 3.013 (95%CI: 2.451-3.575) for the glaucomatous and 1.633 (95%CI: 1.092-2.173) for the normal eyes. We detected a significant systematic bias in the two groups of eyes (single sample t-test; t=10.615, p<0.0001 in the glaucomatous and t=5.972, p<0.0001 in the normal eyes). Regression lines obtained from the Bland-Altmann plots indicated no proportional bias in the glaucomatous eyes (r=0.037; p=0.67) but identified such a bias in the normal eyes (r=0.398; p<0.0001) with differences diminishing with increasing IOP (B=-0.372; p<0.0001). Multivariate regression analysis in the normal eyes revealed that the DCT readings were independent of the CCT or powers or orientations of the corneal axes while GAT measures were biased by CCT (B=0.025; p<0.028), age (B=-0.058; p<0.008) and power of the flattest corneal axis (B=1.12; p<0.012). In the glaucomatous eyes, differences between the instruments were artefacted by CCT (B=-0.18; p<0.007) and by the orientation of the flattest corneal axis (B=0.015; p<0.002).

Conclusions: : in normal eyes DCT readings were independent of the morphometric characteristics of the cornea while GAT measurements were affected by CCT, age and the power of the flattest axis of the cornea; in the glaucoma group, differences between both instruments were affected by CCT and by the orientation of the flattest corneal axis.

Keywords: intraocular pressure