Study Population

This was a cross-sectional study of persons aged 50 years and older who were recruited between December 7, 2005 and July 12, 2006 from a government-run polyclinic in Singapore that provides primary healthcare services for residents living in the area around the clinic. Details of the study have been described previously. In brief, subjects were identified by systematic sampling (every fifth patient registered at the polyclinic) and asked to participate in the study after obtaining written informed consent. Subjects did not specifically have ophthalmic complaints at the time of consultation. A detailed questionnaire, including socioeconomic and education details, as well as medical and ocular history, was administered. Individuals with a history of glaucoma, previous intraocular surgery, previous laser treatment, penetrating eye injury, or corneal disorders preventing anterior chamber assessment were excluded.

Ocular Examination

All participants underwent a standardized eye examination that included visual acuity measurement using a logarithm of minimal angle of resolution chart (logMAR chart; Lighthouse Inc, Long Island, New York, USA), autorefraction (Topcon RM-8000B; Topcon Corporation, Tokyo, Japan), slit-lamp examination (Model BQ 900; Haag-Streit, Bern, Switzerland), stereoscopic optic disc examination with a 78-diopter lens (Volk Optical Inc, Mentor, Ohio, USA), and intraocular pressure (IOP) measurement using Goldmann applanation tonometry (Haag-Streit AG, Koniz, Switzerland). AXL and central ACD were also measured by IOLMaster (Carl Zeiss, Jena, Germany).

Gonioscopy was performed at a low level of ambient illumination using a Goldmann 2-mirror lens (Haag-Streit AG) at high magnification (×16) with the eye in the primary position of gaze. Indentation gonioscopy with the Sussman 4-mirror lens (Ocular Instruments Inc, Bellevue, Washington, USA) was used to establish the presence or absence of peripheral anterior synechiae (PAS). An eye was considered to have narrow angles if the posterior trabecular meshwork was not visible for at least 180 degrees on non-indentation gonioscopy with the eye in the primary position using the Goldmann lens. Gonioscopy was performed by a single ophthalmologist (R.L.) with extensive experience in performing gonioscopy in epidemiologic studies, who was standardized against another ophthalmologist with subspecialty glaucoma training (T.A.).

Anterior Segment Optical Coherence Tomography Imaging

AS-OCT (Visante, Carl Zeiss Meditec, Dublin, California, USA) imaging was performed on all subjects under standardized dark conditions (20 lux) by an experienced operator who was masked to the results of the clinical ophthalmic examination. Scans were centered on the pupil and taken along the horizontal axis (nasal-temporal angles at 0 to 180 degrees), using the standard anterior segment single-scan protocol. To obtain the best-quality image, the examiner adjusted the saturation and noise and optimized the polarization for each scan during the examination. As several scans are acquired by the AS-OCT device, the examiner chose the best image, with the least image artefacts attributable to eye motion or the eyelids. One cross-sectional scan imaging the nasal and temporal angle of the right eyes was evaluated for each subject. The entire acquisition process on both eyes of each patient, on average, took less than 5 minutes per patient.

Images were then processed using customized software, the Zhongshan Angle Assessment Program (ZAAP; Zhongshan Ophthalmic Centre, Guangzhou, China). The AS-OCT images were assessed by a single fellowship-trained glaucoma specialist (L.M.S.) masked to clinical data. The only observer input was to determine the location of the 2 scleral spurs. The ZAAP algorithm then automatically calculated the lens vault ( Figure ). To calculate lens vault, the software draws a line between the 2 scleral spurs and then a perpendicular line is extended from this line to the anterior border of the lens at the point of greatest convexity. Lens vault was defined as the perpendicular distance between the anterior pole of the crystalline lens and the horizontal line joining the 2 scleral spurs, on horizontal AS-OCT scans. The intraobserver reproducibility of the anterior chamber measurements were assessed using a random subset of 101 eyes by a single examiner (L.M.S.), masked to the initial test results. To do this, measurements were performed in 2 sessions separated by an interval of 1 week. Anterior chamber width (ACW), defined as the distance between the scleral spurs in the horizontal (nasal-temporal) axis of AS-OCT scans, was obtained using the same software as previously described.

Statistical Analysis

Data from right eyes were used for analysis. Statistical analyses were performed with SPSS (version 18; SPSS Inc, Chicago, Illinois, USA). Analysis of covariance (ANCOVA) models were used to estimate mean lens vault, adjusted for age and sex. The association between lens vault and the presence of narrow angles on gonioscopy was initially assessed using logistic regression models to determine the odds ratio (OR) and its 95% confidence interval (CI), adjusting for age and sex. Multivariate adjusted OR was obtained after adjustment for age, sex, AXL, and ACD. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of these ocular parameters for identifying narrow angles were also calculated. Receiver operator characteristic (ROC) curve was generated and the area under the ROC curve (AUC) was used to assess the performance of these ocular parameters in detecting narrow angles. A P value less than .05 was considered statistically significant.