Accommodation-induced changes in iris curvature

Eyes were imaged using anterior segment ultrasound biomicroscopy to compare alterations in iris contour following the onset of accommodation in eyes with narrow angles, pigment dispersion syndrome, and controls. A radial perpendicular image in the horizontal temporal meridian was obtained for one eye while the subject focused on a distant target (∼6 m, unaccommodated state) with the fellow eye. The subject then focused steadily on a near target (∼0.33 m, accommodated state) for 3 min. Images were acquired at 0, 1, 2, and 3 min. Iris curvature was determined by measuring the maximum distance between the posterior iris surface and a line from the iris root to the first point of contact between the iris and lens. In control subjects (n = 22), iris curvature decreased immediately after the onset of accommodation, but not significantly (p = 0.49), from 246 ± 37 μm (mean ± SEM) to 205 ± 82 μm; curvature increased after 3 min of accommodation to 298 ± 57 μm (p = 0.10 vs. onset of accommodation). Eyes with pigment dispersion syndrome (n = 15) exhibited curvatures of 60 ± 79 μm when unaccommodated, -3 ± 83 μm immediately after accommodation (p = 0.12), and 146 ± 94 μm (p = 0.01) 3 min later. Eyes with narrow angles (n = 16) exhibited curvatures of 449 ± 45 μm when unaccommodated, 414 ± 46 μm immediately after accommodation (p = 0.37), and 523 ± 40 μm (p < 0.01) 3 min later. The results confirm the time-dependent nature of iris contour response, with significant differences observed between the initial observation after accommodation and the observation 3 min later. The largest drop in curvature immediately after accommodation and the most rapid increase in curvature during subsequent observation were seen in the PDS subjects. We suspect that the more rapid increase in curvature in the PDS subjects is due to the elevated anterior chamber pressure caused by the "reverse pupillary block" effect.