Comparison of Goldmann and Pascal tonometry in relation to corneal hysteresis and central corneal thickness in nonglaucomatous eyes

Abstract

Objective: To compare measurements obtained by Goldmann applanation tonometry (GAT) and Pascal dynamic contour tonometry (DCT), and to study their relationship to corneal thickness and biomechanical properties in nonglaucomatous eyes.

Methods: This is a prospective and randomized study of 200 eyes from 200 non-glaucomatous subjects who underwent intraocular pressure (IOP) measurements by GAT and DCT. The two methods were compared and assessed for agreement by means of the Bland-Altman plot. Central corneal thickness (CCT) and corneal hysteresis (CH) were obtained by ultrasound pachymeter and Ocular Response Analyzer, respectively. The effect of CH and CCT was correlated with the DCT/GAT IOP differences.

Results: Mean age was 57.4 ± 14.7 years (range 24-82 years). Mean IOP measurements obtained were 16.7 ± 3.2 mmHg by GAT and 19.4 ± 3.3 mmHg by DCT. DCT showed a statistically significant higher mean IOP (2.7 ± 1.9 mmHg, P < 0.001) compared with GAT. Mean CCT and CH were 546.5 ± 40 μm and 10.85 ± 2.0 mmHg, respectively. The differences in IOP (DCT - GAT) were significantly correlated with CCT and CH (Pearson's correlation coefficient r = -0.517 and -0.355, P < 0.0001, respectively). The difference between the two correlation coefficients was statistically significant (P < 0.05, Z-statistic). According to the Bland-Altman plot, the results of the two methods were clinically different.

Conclusion: Significantly higher IOP readings were obtained by DCT than by GAT in nonglaucomatous subjects. The IOP differences between the two methods were associated with CCT and CH, suggesting that DCT was less dependent on corneal parameters. Each method provides clinically different IOP values, indicating that DCT and GAT should not be used interchangeably.

Keywords: Goldmann applanation tonometry; Pascal dynamic contour tonometry; central corneal thickness; corneal hysteresis; glaucoma.

Figure 1

IOP normal distribution according to GAT measurements in 200 nonglaucomatous eyes, 28 (14%) of which had an IOP > 21 mmHg. Abbreviations: GAT, Goldmann applanation tonometry; IOP, intraocular pressure.

Figure 2

IOP normal distribution according to DCT measurements in 200 nonglaucomatous eyes. Compared with the GAT IOP curve in Figure 1, a shift of the entire DCT IOP distribution curve to the right is observed. Ninety-four (47%) eyes had an IOP > 21 mmHg. Abbreviations: DCT, dynamic contour tonometry; GAT, Goldmann applanation tonometry; IOP, intraocular pressure.

Figure 3

CCT measurements plotted against the difference between DCT and GAT IOP measurements show a strongly negative correlation (Pearson’s correlation coefficient r = −0.517, P < 0.0001). IOP differences tend to increase in eyes with thinner corneas and to decrease in eyes with thicker corneas. Abbreviations: CCT, central corneal thickness; DCT, dynamic contour tonometry; GAT, Goldmann applanation tonometry; IOP, intraocular pressure.

Figure 4

CH measurements plotted against the difference between DCT and GAT IOP measurements show a significantly negative correlation (Pearson’s correlation coefficient r = −0.355, P < 0.0001). The IOP differences tend to increase in eyes with lower CH and to decrease in eyes with higher CH. Abbreviations: CH, corneal hysteresis; DCT, dynamic contour tonometry; GAT, Goldmann applanation tonometry; IOP, intraocular pressure.

Figure 5

Corneal curvature measurements plotted against the difference between DCT and GAT IOP measurements do not show a statistically significant correlation (Pearson’s correlation coefficient r = −0.028, P = 0.689). Abbreviations: DCT, dynamic contour tonometry; GAT, Goldmann applanation tonometry; IOP, intraocular pressure.

Figure 6

Bland–Altman plot testing the clinical agreement of the IOP measurements obtained by DCT and GAT. The results of the 2 methods are clinically different, since the measurements are on average >2.7 mmHg apart. Therefore, the 2 methods are not interchangeable. The tendency of the IOP differences to decrease at higher IOPs was not statistically significant (dotted line, r = −0.083, P = 0.242). Abbreviations: DCT, dynamic contour tonometry; GAT, Goldmann applanation tonometry; IOP, intraocular pressure; SD, standard deviation.