Anterior segment OCT imaging (Visante; Carl Zeiss Meditec, Jena, Germany) was performed on all participants under standardized dark conditions (20 lux). We used the Anterior Segment Single-Scan protocol to obtain cross-sectional images of the anterior segment along the horizontal axis (nasal-temporal angles at 0°–180°) and vertical axis (superior-inferior angles at 90°–270°). Images were processed using the customized software, the Zhongshan Angle Assessment Program (Guangzhou, China).
16 Following the user input on the location of the two scleral spurs, the algorithm automatically measures the following angle parameters: angle opening distance (AOD250, AOD500, and AOD750), angle recess area (ARA750), and trabecular-iris space area (TISA500 and TISA750). Angle opening distance 250, AOD500, and AOD750 were the lengths of a line from the anterior iris to the corneal endothelium perpendicular to a line drawn along the trabecular meshwork at 250, 500, and 750 μm from the scleral spur, respectively.
17 Angle recess area 750 was defined as the area bordered by the anterior iris surface, corneal endothelium, and a line perpendicular to the corneal endothelium drawn to the iris surface from a point at 750 μm anterior to the scleral spur.
18 Trabecular-iris space area 500 and TISA750 were the areas modified by not including the area posterior to a line drawn from the scleral spur to the anterior iris perpendicular to the plane of the inner scleral wall. Among the three AOD and two TISA measures, AOD750 and TISA750 have been shown to have the highest diagnostic performance for angle closure, respectively.
19 In addition, the software was also used to measure iris thickness, iris curvature (I-Curv), and pupil size. Iris thickness was measured at 2000 μm from the scleral spur. I-Curv was measured by drawing a perpendicular line from the iris pigment epithelium at the point of greatest iris convexity, to the line extending from the most peripheral to the most central points of iris pigment epithelium.
8 The average of measured values of the four quadrants (nasal, temporal, superior, and inferior) was used for the analysis. Anterior segment OCT image analysis was done without the knowledge of the iris grading results.