Repeatability, Agreement and Corneal–Thickness Dependence of Goldmann Applanation Tonometry and Dynamic Contour Tonometry

Abstract

Purpose: : A potential dependency of Goldmann Applanation Tonometry (GAT) on corneal biomechanical properties is increasingly discussed. The newly developed Pascal Dynamic Contour Tonometry (DCT) is said to be unaffected by corneal biomechanics. The aims of this study are 1. to compare the repeatability and reproducibility of GAT and DCT over short–time (one day) and over long–time measurements (one week), 2. to assess agreement between GAT and DCT and 3. to evaluate the effect of central corneal thickness (CCT) on GAT and DCT.

Methods: : GAT and DCT measurements were performed in 50 eyes of 50 healthy volunteers. For the short–time examination, IOP was measured 4 times on day 1 with each technique in a random order. On day 2, 5, 8 IOP readings were repeated in the same eye with GAT and DCT (random order). The repeatability of GAT and DCT was determined from calculations of the within–subject standard deviation. For the short–time repeatability the standard deviation of the 4 measurements of day 1 were used. For the long–time reproducibility the standard deviation of the measurements of day 1, 2, 5 and 8 were taken. Agreement between IOP measurements was determined using Bland–Altman plots. The effect of CCT on GAT– and DCT–readings was assessed by linear regression analysis.

Results: : The short–time measurements` standard deviation was 1.11 (GAT) versus 1.2 (DCT), the long–time standard deviation was 1.24 (GAT) versus 1.52 (DCT). The mean IOPs were 14.81±1.77 mmHg (GAT) and 15.21±1.87 mmHg (DCT). The mean difference between GAT and DCT for the 4 measurements on day 1 was found to be –0.8 mmHg (95% confidence interval of [–3.6;2]). The mean difference for the 4 measurements between day 1 and day 8 was found to be –0.81 mmHg ([–4.35;2.73]). The linear regression analysis revealed an IOP increase of 0.9 mmHg per 100 µm for GAT and an IOP increase of 0.2 mmHg/100µm for DCT.

Conclusions: : Short–time and long–time reproducibility of DCT is comparable to that of GAT. Bland–Altman revealed a good agreement of the two methods. However, mean DCT values were in average 0.81±1.09 mmHg higher than the GAT values. GAT is more affected by CCT than DCT, measuring higher IOPs with thick corneas. GAT and DCT measurements were similarly well tolerated by the study participants.

Keywords: intraocular pressure • clinical research methodology • clinical (human) or epidemiologic studies: systems/equipment/techniques