Dynamic analysis of iris configuration with anterior segment optical coherence tomography

PURPOSE. To evaluate dynamic changes in iris configuration and their association with anterior chamber angle width by using anterior segment optical coherence tomography (ASOCT). METHODS. Forty-six normal subjects with open angles and 40 with narrow angles (Shaffer grade ≤2 in three or more quadrants during dark room gonioscopy) were analyzed. The dynamic ASOCT dark-light changes of iris bowing were captured with real-time video recording and nasal iris bowing, nasal anterior chamber angle, and pupil diameter were measured in serial image frames selected from the video capture. The associations between iris bowing, iris thickness, anterior chamber depth (ACD), age, anterior chamber angle, and pupillary diameter measurements were evaluated with univariate and multivariate regression analyses. RESULTS. The relationship between iris bowing and pupil diameter was largely linear, with three dynamic patterns observed: (1) convex-to-convex (iris remains convex in dark and light); (2) concave-to-convex (iris changes from concave to convex from light to dark); and (3) concave-to-concave (iris remains concave in dark and light). All the subjects with narrow angles had convex-to-convex anatomy, although 43% of the subjects with open angles also demonstrated this pattern. These individuals were older and had shorter axial length (both with P < 0.001). Older age (r = -0.352, P = 0.001), smaller ACD (r = 0.382, P < 0.001), and smaller difference in angle opening distance in light and dark (r = 0.472, P < 0.001) were associated with smaller differences in iris bowing in the light and dark. ACD and iris bowing were independently associated with anterior chamber angle width. CONCLUSIONS. Independent of ACD, iris bowing is an important biometric parameter that determines angle width. Investigation of iris dynamics may offer a new perspective in understanding the risk and mechanism of primary angle closure.