Effects of Corneal Thickness, Curvature, Astigmatism and Direction of Gaze on Goldmann Applanation Tonometry Readings

Abstract

Background: The aim of this study was to evaluate the impact of various sources of error in Goldmann applanation tonometry (GAT). Objectives: We evaluated the effect of corneal thickness, curvature, astigmatism and direction of gaze as sources of error in GAT. Methods: Orbscan-II^® (Bausch & Lomb, Inc., Rochester, N.Y., USA) examinations were made on 30 healthy subjects and 9 keratoconus patients, and the intraocular pressure (IOP) was measured with GAT centrally, temporally and inferiorly, with the tonometer prism set horizontally and vertically. Orbscan-II images from 50 younger subjects and 49 older subjects were analysed retrospectively. Results: IOP was lower on nasal gaze (p = 0.009) but higher on upward gaze (p < 0.001) compared with forward gaze. IOP and the corneal thickness were independently correlated (R² = 0.04; p = 0.003), as were the difference in astigmatic vector in the horizontal and vertical meridians and the difference in IOP measured with a horizontal and vertical prism (R² = 0.17; p < 0.001). No correlation between IOP and corneal curvature was found. In the keratoconus patients, IOPs were generally low, with large astigmatic differences. Conclusions: Corneal thickness, astigmatism and direction of gaze are clinically important sources of error in GAT. IOP should preferably be measured with the prism both horizontally and vertically. If only one direction is chosen, a vertical prism is less sensitive to different directions of gaze. Direction of gaze should be carefully monitored, especially in an irregular cornea.