Iris volume decreases during pupillary dilation

Abstract

Objective: To evaluate changes in iris cross-sectional area and volume under dark and lighted conditions using ultrasound biomicroscopy.

Patients and methods: Patients underwent ultrasound biomicroscopy imaging using standardized lighting conditions. Imaging was repeated after at least 2 minutes in standardized dark room conditions (physiological dilation) or 30 minutes after instillation of tropicamide 1% (pharmacological dilation). Images were exported to an IBM-compatible computer. Iris volume and cross- sectional area were then measured using an automated software program of our own design capable of calculating iris volume with axial alignment using keratometry and corneal diameter information.

Results: Twenty two eyes of 22 healthy people were enrolled. In the pharmacologically dilated eyes, mean iris volume was 41.21 ± 2.64 mm3  before and 35.88 ± 3.92 mm3 after dilation (12.5 ± 11.4% reduction; p = 0.009, paired t-test). For eyes undergoing physiologic dilation, mean iris volume was 39.43 ± 7.83 mm3 using lighted conditions and decreased to 36.80 ± 6.70 mm3 in the dark (6.3 ± 4.8% reduction; p = 0.004). The change in pupillary diameter correlated significantly with iris volume and iris cross-sectional area change after physiological dilation (r2 = 0.41 and 0.35, p = 0.03 and 0.04, respectively, Pearson correlation), but not after pharmacological dilation (p > 0.2).

Conclusion: Cross-sectional iris area decreases with pupillary dilation. The iris volume decrease during physiological change was proportionately less than with pharmacological dilation.