Imaging the Iris with Swept-Source Optical Coherence Tomography: Relationship between Iris Volume and Primary Angle Closure

Abstract

To measure iris volume and anterior segment parameters using a swept-source anterior segment optical coherence tomography (OCT) and investigate factors associated with iris volume and iris volume change after pupil dilation in eyes with open angles and angle closure. Cross-sectional study. A total of 86 eyes, including 31 eyes from 21 patients with primary angle closure (PAC) or PAC suspect, 31 eyes from 20 patients with primary open-angle glaucoma (POAG), and 24 eyes from 15 normal subjects, were included. The anterior segment parameters and iris were imaged and measured by the Casia SS-1000 OCT (Tomey, Nagoya, Japan) in room light, dark, and after pharmacologic dilation. Linear mixed models were used to examine the association between iris volume and change in iris volume after dilation and each of the following: age, sex, anterior chamber volume (ACV), axial length, pupil diameter, and angle width. Iris volume. The mean iris volume significantly decreased from light to dark and after pharmacologic dilation in angle closure (40.0 ± 5.2, 38.8 ± 5.4, and 32.5 ± 4.5 mm(3), respectively), POAG (40.2 ± 5.3, 39.4 ± 5.4, and 33.6 ± 4.2 mm(3), respectively), and normal eyes (40.1 ± 4.2, 39.1 ± 3.9, and 33.0 ± 4.4 mm(3), respectively). From room light to dark, the iris volume of 16.7% normal, 19.4% POAG, and 19.4% angle closure eyes increased iris volume (P = 0.960). After pharmacologic dilation, iris volume decreased in all eyes. Iris volume was negatively associated with ACV and positively associated with axial length (P<0.001). The change in iris volume per millimeter change in pupil diameter was 2.11, 2.01, and 1.80 mm(3)/mm in the angle closure, POAG, and normal groups, respectively (P≥0.414). A smaller ACV (P = 0.049) and older age (P = 0.036) were associated with a smaller change in iris volume per millimeter change in pupil diameter. A larger iris volume, smaller ACV, and greater pupil diameter were significant determinants of a smaller angle width (all P≤0.003). The mean iris volume decreased after pupil dilation in open-angle and angle closure eyes, and the degree of reduction was less in eyes with a smaller ACV. Both iris volume and ACV were important determinants of the anterior chamber angle.