Purpose: The gold standard for measuring intraocular pressure (IOP) until now has been Goldmann applanation tonometry (GAT), which depends on the central corneal thickness (CCT) and curvature. In patients with keratoconus who have an abnormal corneal geometry and thickness, measurement of pressure with GAT is often difficult and not very reproducible. We compared the impact of the central corneal thickness (CCT) on the IOP measured with dynamic contour tonometry (DCT), a digital method which is adapted to the corneal geometry, and GAT in patients with keratoconus.
Methods: IOP was measured in 54 patients (38 men and 16 women, mean age of 36+/-9.9 years) with GAT and DCT in randomized order. All patients had a keratoconus which was assured by topography. In addition central corneal thickness (CCT) was measured with the Pentacam. For statistical analysis the Pearson correlation was calculated and a Bland-Altman diagram plotted.
Results: Mean corneal thickness was 486.2+/-45.5 microm. DCT measured the IOP at a mean value of 14.9+/-2.6 mmHg and GAT at 13.3+/-2.9 mmHg. With a mean difference of 1.6+/-2.4 mmHg DCT measured significantly higher than GAT (p<or=0.05) Neither IOP measurements with GAT (r=-0.03; p>0.05) nor those with DCT (r=0.08; p>0.05) showed a significant correlation to central corneal thickness.
Conclusion: The example of keratoconus confirms that IOP measured by GAT is lower than if measured by DCT. Because both methods are independent of the CCT they are equally acceptable for IOP follow-up in eyes with keratoconus, which may result in progressive corneal thinning in the long term.