PAC suspect (PACS) or PAC patients evaluated by one glaucoma specialist (KRS) at the glaucoma clinic of Asan Medical Center, Seoul, Korea, from March 2009 to December 2014, and who met the inclusion criteria described below, were consecutively included in this study based on medical record review. The study was approved by the institutional review board of Asan Medical Center and followed the tenets of the Declaration of Helsinki.

All participants underwent a complete ophthalmic examination, including a medical history review, measurement of best-corrected visual acuity, slit-lamp biomicroscopy, Goldmann applanation tonometry, gonioscopy, funduscopic examination using a 90- or 78-diopter lens, stereoscopic optic disc photography, retinal nerve fiber layer photography, a visual field test (Humphrey field analyzer, Swedish Interactive Threshold Algorithm 24-2; Carl Zeiss Meditec, Dublin, CA, USA), and AS OCT (Visante OCT ver. 2.0, Carl Zeiss Meditec).

PACS and PAC were diagnosed by gonioscopic examination. Eyes with appositional contact between the peripheral iris and the posterior trabecular meshwork of greater than 270° were included in the PACS group [14]. Eyes with an occludable angle (appositional contact between the peripheral iris and the posterior trabecular meshwork of more than 270°) and exhibiting features indicating trabecular obstruction by the peripheral iris were considered to have PAC [14]. Such features included an elevated intraocular pressure (IOP), iris whorling (distortion of radially orientated iris fibers), "glaukomflecken" lens opacity, or excessive pigment deposition on the trabecular surface, but without the development of a glaucomatous optic disc or any visual field change [14]. We combined patients with both PACS and PAC eyes, and defined them as having "angle closure" for our current analysis. Only reliable visual field test results (false-positive errors <15%, false-negative errors <15%, and fixation loss <20%) were included in the analysis. Eyes with peripheral anterior synechiae in the angle were excluded.

We excluded patients with a history or current use of topical or systemic medications that could affect the angle or the pupillary reflex; those with a history of previous intraocular surgery, including cataract surgery, laser trabeculoplasty, laser iridoplasty, and laser iridotomy; and those unable to fixate prior to AS OCT examination. Patients with a history of acute PAC, defined by the presence of ocular or periocular pain, nausea or vomiting, and a history of intermittent blurring of vision with haloes; an IOP of more than 30 mmHg; and the presence of at least three of the following conditions: conjunctival injection, corneal epithelial edema, mid-dilated unreactive pupil, and shallow angle closure, were also excluded [15]. Eyes diagnosed with secondary angle closure, such as those with neovascular or uveitic glaucoma, were also excluded. All eyes were newly diagnosed cases, and AS OCT imaging was performed before starting any glaucoma medication or laser treatment or intraocular surgery.

Prior to AS OCT imaging, all patients underwent a slit-lamp examination and gonioscopy, conducted by an independent observer (KRS) who had extensive experience in the performance of such examinations. All eyes were examined using a Sussman lens in a darkened room (0.5 cd/m²). Both static and dynamic gonioscopy were performed using a Sussman lens, with the eye in the primary gaze position. Indentation gonioscopy was performed to determine whether angle closure was attributable to apposition or to peripheral anterior synechiae. Care was taken to ensure that light did not fall on the pupil during the examinations.

All participants were imaged in terms of the nasal and temporal angles (0° to 180°) using AS OCT (Visante OCT, ver. 2.0, Carl Zeiss Meditec) operating in the enhanced AS single mode (scan length 16 mm, 256 A-scans). To confirm the consistency of iris root insertion according to pupillary reaction, four sessions using four different standardized lighting conditions (3.25, 100.8, 426, and 1,420 cd/m²), from dark to light, were performed by a single well-trained operator. The room in which the AS OCT imaging was performed had four-graded lighting conditions controlled by four-leveled switches. Thus, the same four-leveled lighting conditions were provided to all participants. Participants were asked to sit back after imaging and wait for 30 seconds, during which time the lighting conditions were changed. After 30 seconds of adaptation to the new lighting conditions, imaging was resumed. Four images, obtained under four different lighting conditions, were obtained for each participant. Of the four images obtained at each session, images obtained at 3.25 cd/m² were used for analysis [16]. The AS parameters in each image were evaluated by an independent examiner (JWH) who was blinded to all other test results and the clinical information of the participants.

All parameters were determined using the Image J software ver. 1.46 (National Institutes of Health, Bethesda, MD, USA). The analyzed parameters are described in Fig. 1. Anterior chamber depth was defined as the distance from the corneal endothelium to the anterior surface of the lens. The scleral spur was defined as the point at which a change in the curvature of the inner surface of the angle wall became apparent, often presenting as an inward protrusion of the sclera [17]. After determination of the scleral spur location, iris thicknesses at the scleral spur (IT₀) and 500, 750, and 1,500 µm from the scleral spur (IT₅₀₀, IT₇₅₀, and IT₁₅₀₀) were measured [11]. IA was defined as the cross-sectional area of the iris. The anterior chamber area was defined as the cross-sectional area bordered by the corneal endothelium and anterior surface of the lens and iris. IC was defined as the maximum perpendicular distance between the iris pigment epithelium and the line connecting the most peripheral and most central points of the epithelium [11]. Lens vault was defined as the perpendicular distance between the anterior pole of the crystalline lens and the horizontal line joining the two scleral spurs [18]. Angle opening distances (AOD₅₀₀ and AOD₇₅₀), which were defined as the linear distance between the point of the inner corneoscleral wall (500 and 750 µm anterior to the scleral spur, respectively) and the iris, were also assessed. The angle recess area (ARA₇₅₀) was defined as the triangular area formed by the AOD₇₅₀. The corners of the triangle were the angle recess (the apex), the iris surface, and the inner corneoscleral wall. The trabecular iris space area (TISA₇₅₀) was defined as the trapezoidal area with the following boundaries: anteriorly, the AOD₇₅₀; posteriorly, a line drawn from the scleral spur perpendicular to the plane of the inner scleral wall to the opposing iris; superiorly, the inner corneoscleral wall; and, inferiorly, the iris surface. The measurement variability of the parameters was checked prior to full analysis by calculating the intra-class correlation coefficients. Intra-examiner intra-class correlation coefficient values for the various AS parameters ranged from 0.933 to 0.951 [19]. The image acquisition procedure and analysis methods have been previously described [19,20,21,22].

All parameters except for anterior chamber depth, lens vault, anterior chamber area, and pupillary distance were measured at both the nasal and temporal sides, and an average of the two values was used for the analyses. The iris root insertion configuration was independently assessed by two glaucoma experts (KRS and JEL) who were blind to the other AS OCT parameters and all other test results, including the clinical information of the participants. Four images obtained with different lighting conditions were reviewed by two experts. Iris root insertion was categorized into a non-basal insertion group (NBG) or a basal insertion group (BG), according to the presence of a space between the scleral spur and iris root (Fig. 2A and 2B). Each grader classified each eye as NBG or BG. If the opinions of the two observers differed, those eyes were excluded.

Descriptive analyses of the AS OCT parameters were conducted. Descriptive statistics were calculated to determine the frequencies, means, and standard deviations of each variable. For comparisons of sex ratios and age between the two groups, the chi-square test and unpaired Student's t-test, respectively, were used. Comparisons of baseline IOP, spherical equivalent (SE), and pupillary distance between the NBG and BG were performed using linear mixed-effects regression models, which accounted for within-individual associations. Comparisons of all AS OCT parameters between the two groups were performed with the same statistical analysis procedures, which were adjusted for age, sex, eye (right or left eye), SE, and pupillary distance. Residual diagnostic plots were used to detect features of concern in the model. Exploratory analyses of the residuals suggested that the chosen models were appropriate for all of the parameters. All reported p-values were two-sided, and a value of p < 0.05 was considered statistically significant. SAS ver. 9.2 (SAS Institute Inc., Cary, NC, USA) was used for statistical analyses.