Midperipheral corneal thickness affects noncontact tonometry.

PURPOSE

To determine if midperipheral corneal thickness (MCT) has an influence on intraocular pressure (IOP) measurements using noncontact tonometry (NCT).

METHODS

The corneal thickness profile over a 5-mm diameter (Orbscan IIz) and the IOP (Topcon CT-80 noncontact tonometer) was measured for 51 eyes of 51 healthy subjects aged 21.1+/-1.6 years; the relationships between corneal thickness and IOP were assessed by Pearson correlation and linear regression.

RESULTS

The central corneal thickness (CCT) was 556.1+/-32.9 mum and generally increased toward the midperiphery, though the magnitude of the midperipheral increase in corneal thickness (MICT, where MICT=MCT-CCT) was not consistently correlated to the CCT. The IOP was related to the CCT (r=0.445, P=0.001) and to the MCT at all midperipheral locations (r=0.357 to r=0.527, P<0.05), but to the MICT at only 4 corneal locations. Stepwise linear regression showed that the MICT measured 2.5-mm temporal to the central cornea, along with the CCT, was the most able to predict the measured IOP (r=0.563, P<0.001). CCT and MICT were both independently related to the measured IOP at a magnitude of 0.37 mm Hg per 10 mum increase in CCT and 1.03 mm Hg per 10 mum in MICT; at the limits of the 95% confidence interval of CCT and MICT for this study population, the measured IOP could vary by 4.8 and 3.7 mm Hg, respectively due to variations in corneal thickness at these locations.

CONCLUSIONS

In addition to the effect of CCT on IOP measurement by NCT, the MICT in the temporal corneal can exert a clinically significant and independent effect.