Correlation Between Goldmann Applanation Tonometry and Rebound Tonometry in Relation to Central Corneal Thickness

Abstract

Purpose: : The Goldmann applanation tonometry (GAT) is still today the gold-standard of IOP-Measurement, although its disadvantages have been known since the method has been introduced. Within the last years new ways of measuring the IOP have been developed. One of the recently developed instruments is the ICare Rebound Tonometer (Tiolat Oy, Helsinki, Finland). The purpose of this study is to investigate the influence of central corneal thickness (CCT) on the RBT measurements and to verify the correlation of RBT and GAT.

Methods: : The current study contains data from 230 eyes from a mixed population of the outpatient departments of the University Eye Clinic Salzburg. The mean-age was 67.19 years (SD +/- 11.87). The study was explained to the patients and their informed consent was obtained. The IOP was measured with both the GAT and the ICare tonometer; in addition the CCT was determined with standard pachymetry.

Results: : We were able to show a good linear correlation between GAT and RBT readings (r=0.9). Over the whole range of IOP we could observe a good correlation of both tonometers with the tendency of a slight overestimation of the IOP in RBT readings. According to our data CCT has a stronger impact on the results of RBT than on the results of GAT. In thin corneas RBT tends to underestimate the GAT readings, whereas in thick corneas RBT overestimates the GAT readings. Although the correlation factor between GAT and RBT is high, significant differences can be seen in the Bland-Altman analysis. Only in 15% of all measurements no differences in results between the two methods were observed. In 26% the difference was 1 mmHg, in 23% 2 mmHg, in 19% 3 mmHg and in 16% 4 or more mmHg. The analysis also revealed a correlation of r=-0.4 between CCT and the difference in results of the two methods.

Conclusions: : In cases where an exact measurement of the IOP is important RBT would not be the first choice. Also formulas to correct for the dependency of RBT on CCT would improve the results only marginally.

Keywords: intraocular pressure