Intraocular pressure and corneal thickness - A comparison between non-contact tonometry and applanation tonometry

Objective. Corneal thickness and deformation seem to have a considerable influence on intraocular pressure measurement. Due to differences in the corneal deformation in either non-contact tonometry or applanation tonometry, both methods should be compared in the same patient group depending on central corneal thickness. Methods. In 106 eyes of 55 patients (18 males, 37 females, age 17-89 years, mean 63.3 years) with glaucoma and central corneal thickness between 409 and 644 mu m (Orbscan II pachymetry) intraocular pressure was measured in each eye with non-contact tonometry (Reichert AT550) and 30 min later with Goldman applanation tonometry. Result. Non-contact tonometry as well as applanation tonometry showed a positive correlation between measured intraocular pressure and corneal thickness. The steepness of the line of regression was 0.33 mmHg per 10 mu m of corneal thickness in non-contact tonometry and 0.17 mmHg per 10 mu m of corneal thickness in applanation tonometry. Conclusion. Independently of the large differences in individual pressure measurements between non-contact tonometry and applanation tonometry, we found higher IOP values with non-contact tonometry in thicker corneas as compared with applanation tonometry. In thinner corneas there was a better correspondence between both methods. Thus, it seems very likely that corneal rigidity increases with corneal thickness.