The relative effects of corneal thickness and age on Goldmann applanation tonometry and dynamic contour tonometry

Aims: To establish the effects of central corneal thickness (CCT) on intraocular pressure (IOP) measured with a prototype Pascal dynamic contour tonometer (DCT), to evaluate the effect of CCT and age on the agreement between IOP measured with the Pascal DCT and Goldmann applanation tonometer (GAT), and to compare the interobserver and intraobserver variation of the DCT with the GAT. Methods: GAT and DCT IOP measurements were made on 130 eyes of 130 patients and agreement was assessed by means of Bland-Altman plots. The effect of CCT and age on GAT/DCT IOP differences was assessed by linear regression analysis. Interobserver and intraobserver variations for GAT and DCT were assessed in 100 eyes of 100 patients. Results: The mean difference (95% limits of agreement) between GAT and DCT was -0.7 (-6.3 to 4.9) mmHg. GAT/DCT IOP differences increased with thicker CCT ( slope 0.017 mmHg/mu m, 95% CI 0.004 to 0.03, r(2) = 0.05, p = 0.01), and with greater age, slope 0.05 mmHg/ year ( 95% CI 0.012 to 0.084, r(2) = 0.05, p = 0.01). The intraobserver variability of GAT and DCT was 1.7 mmHg and 3.2 mmHg, respectively. The interobserver variability was (mean difference (95% limits of agreement)) 0.4 (-3.5 to 4.2) mmHg for GAT and 0.2 (-4.9 to 5.3) mmHg for DCT. Conclusions: GAT is significantly more affected than DCT by both CCT and subject age. The effect of age suggests an age related corneal biomechanical change that may induce measurement error additional to that of CCT. The prototype DCT has greater measurement variability than the GAT.