Applanation Tonometry and Central Corneal Thickness

Readings with the Goldmann applanation tonometer were made at various intraocular hydrostatic pressures and compared with central corneal thickness and radius in rabbit and in man. Linear correlations were established between hydrostatic pressure and applanation readings, with correlation coefficients close to 1.0. In rabbits the tonometer readings were generally too low. In human eyes with a normal corneal thickness tonometer readings and hydrostatic pressure coincided, with thick corneas the readings were too high, with thin corneas too low. The correlation between corneal thickness and the error of applanation tonometry (ΔP) was statistically highly significant (P < 0.001). No statistical correlation could be established between corneal radius and ΔP. Multiple regression, taking thickness as well as corneal radius into consideration, revealed only slightly higher correlation coefficients. It is concluded that the central corneal thickness is a parameter which should be taken into consideration when evaluating applanation tonometer readings. A Table is presented showing the correction to be added to the applanation reading at differing corneal thickness.