Effect of posture and artificial tears on corneal power measurements with a handheld automated keratometer

To study the effect of posture and artificial tears on handheld automated keratometry.

Department of Optometry and Radiography, Hong Kong Polytechnic University, Hong Kong, China.

Thirty-five subjects were recruited to have corneal curvature measurement in 1 eye (randomly selected) by a Medmont topographic keratometer and a Nidek handheld keratometer. In handheld keratometry, the measurements were taken with the subject in a sitting and lying posture (both with and without the use of artificial tears). The sequence of measurements was randomly assigned, but the application of artificial tears was always the last. The steepest and flattest corneal curvatures were compared between the 4 conditions. The corneal power was converted to orthogonal power vector components and rectangular Fourier form (M, J(0), J(45)) for another comparison.

There was a significant difference in the steepest and flattest meridians between the 4 conditions (P<.01). However, the mean difference between the handheld keratometer and the topographic keratometer was less than 0.50 diopter, and the intraclass correlation coefficient (ICC) was very high (0.96), indicating good clinical reliability. When analyzed in rectangular Fourier form, the difference was also significant but the ICCs were lower (0.97, 0.89, and 0.64 for M, J(0), and J(45), respectively). The greatest difference was when subjects were in the lying posture and had received artificial tears.

Handheld keratometry provided different results from topographic keratometry. The difference was greatest with the use of artificial tears. Cataract surgeons should take this into consideration, especially when using the handheld keratometry in the operating theater in patients under general anesthesia. Results show that the power vector method is best for studying corneal shape.