To determine the effect of small increases in corneal hydration on the accuracy of Goldmann applanation tonometry estimates of intraocular pressure (IOP).
Materials and Methods
Twenty-five young healthy subjects presented on 3 separate days approximately 1 week apart. On 2 visits, subjects were required to wear a hydrogel contact lens with either a center thickness of 0.3 and 0.7 mm (HEMA 38% water content, parallel surface curve) in 1 eye only under closed-eye conditions for 2 hours to induce corneal swelling. The third visit acted as a control. IOP, corneal thickness, and corneal curvature were measured in both eyes before and after contact lens wear on all visits.
There was a statistically significant increase in corneal thickness of 40.2±14.4 μm (P<0.001) and 41.9±16.4 μm (P<0.001) after wearing the 0.3 and 0.7 mm thick contact lenses, respectively (2-tailed paired t test). There was an increase in IOP of 2.8±2.2 mm Hg (P<0.001) after wearing the 0.3 mm thick contact lens, and a statistically insignificant difference of 1.3±3.0 mm Hg (P=0.058) after wearing the 0.7 mm thick contact lens (2-tailed paired t test). There was a statistically significant Pearson correlation between the change in corneal thickness and the change in IOP after lens wear (r=0.500, P<0.001, 0.3 mm lens and r=0.399, P<0.001, 0.7 mm lens). The corneal hydration-induced measurement error was 0.46 mm Hg per 10 μm change in corneal thickness (0.3 mm lens) and 0.35 mm Hg per 10 μm change in corneal thickness (0.7 mm lens).
A small increase in corneal hydration and thickness may cause a clinically significant overestimation of IOP when measured using Goldmann applanation tonometry.