Effect of Corneal Hydration on Goldmann Applanation Tonometry and Corneal Topography

Abstract

Measured ocular tension is reported to be positively related to corneal thickness. It has been speculated that an edematous cornea will cause low measurements. We studied the effects of corneal hydration on applanation tonometry and biomechanical behavior of the cornea. This study examines the effects of corneal hydration on applanation tonometry on four human cadaver eyes with the Goldmann Schmidt tonometer and on one cadaver eye with the Tono-Pen 2. Hydration was controlled osmotically and assessed by ultrasonic pachometry. True intraocular pressure (IOP) was controlled and was measured by a piezoelectric pressure transducer. The effect of hydration and IOP on anterior curvature was also assessed with computer topography. The results of this study show an inverse relationship between corneal hydration and ocular tension measured with applanation tonometry. The Goldmann readings decreased by 9.2 +/- 0.7 mm Hg as the cornea is taken from minimum to maximum hydration. Anterior curvature was affected very little by hydration or increased IOP. This study has shown, in vitro, that the tonometer readings are inversely related to its thickness when the cornea is edematous. The clinical implication is that when performing applanation tonometry, the hydration status of the cornea must be carefully examined to avoid possible diagnostic error.