Abstract
Abstract: :
Purpose: Dynamic Contour Tonometry (DCT) is a procedure in which systematic errors due to corneal influences are expected to be less than 1 mmHg. In Goldmann Applanation Tonometry (GAT), errors due to physiological variation in corneal thickness (CCT) are up to ± 5 mmHg. This error is further aggravated due to variability of other corneal properties (rigidity, elasticity, hydration). According to Orssengo and Pye's non–linear model, GAT values may be corrected for CCT. In this study, we have checked the hypothesis that CCT–corrected GAT values and DCT values are less influenced by CCT than original, uncorrected GAT readings. Methods: Subjects: 118 right eyes (OD) of consecutive patients, aged >40, with uncompromised corneas. Diagnostic methods: IOP: Aplanation tonometry (Goldmann–Tonometer, Haag–Streit AG, Köniz, Switzerland), and DCT (PASCAL® tonometer, SMT AG, Port, Switzerland). CCT: optical coherence pachymetry (4optics GmbH, Lübeck, Germany). GAT readings were corrected for CCT acording to Orssengo and Pye. Results: The arithmetic means of the two tonometric methods differ insignificantly: GAT: 15.6 ± 4.3 mmHg (mean ± std.dev.); DCT: 15.5 ± 3.1 mmHg. The following linear regression parameters were computed: GAT vs. DCT: DCT = 9.2 + 0.41 * GAT; r = 0.556; p < 0.001 CCT vs. GAT: GAT = 0.43 + 0.028 * GAT; r = 0.225; p = 0.01 CCT vs. DCT: DCT = 14.7 + 0.0015 * DCT; r = 0.016; p = 0.86 CCT vs. corr.GAT: GATcorr = 22.0 – 0.013 * CCT; r = –0.103; p=0.27 Conclusions: The correlation coefficient of the GAT vs DCT comparison is indicative of a low concordance between the two methods. The steeper slope of the regression line for the dependence of GAT on CCT, compared to the dependence of DCT on CCT suggests that CCT has a stronger influence on GAT readings as compared to DCT results. The negative slope of the regression line for corrected GAT values suggests minor over–correction by the Orssengo–Pye formula. Corneal thickness has no influence on the result of dynamic contour tonometry performed with the PASCAL device. The large variability of the residuals may be indicative of the influence of other corneal properties, such as rigidity, elasticity, or hydration on IOP readings obtained with applanation tonometry.
Keywords: intraocular pressure • clinical (human) or epidemiologic studies: biostatistics/epidemiology methodology • cornea: basic science