Effects of corneal thickness on intraocular pressure measured with three different tonometers

To compare intraocular pressure (IOP) measured by a Goldmann applanation tonometer (GAT), a noncontact tonometer (NCT), and a portable noncontact tonometer (PNCT) in eyes of healthy volunteers, and to determine if a significant correlation exists between the IOP and the central corneal thickness (CCT).

A total of 144 healthy participants were randomly assigned to one of two groups; in the first group, IOP was measured first with the NCT and then with the GAT. In the second group, IOP was measured first with the PNCT and then with the GAT. Subsequently, the CCT of all subjects was measured with an ultrasonic pachymeter.

The IOPs determined by the GAT and NCT and were strongly correlated, as were those determined by the GAT and PNCT, in both groups. However, a Bland-Altman plot showed that the correlations between the GAT and NCT and between the GAT and the PNCT measurements were not significant. With all three instruments, the IOP readings varied with the CCT. The mean IOPs obtained with the GAT increased by 0.23 mmHg with each 10-microm increase in CCT (0.23 mmHg/10 microm). The comparable value for the NCT was 0.29 mmHg/10 microm, and that for the PNCT was 0.31 mmHg/10 microm.

For measurements of IOP in normal eyes, the GAT is the tonometer least affected by the CCT, compared with the PNCT and NCT. A PNCT is more likely to be affected by variations in CCT than the GAT.