Illumination–induced Changes in the Angle Configuration: An Evaluation by Anterior Segment Optical Coherence Tomography

Abstract

Abstract: : Purpose: To evaluate illumination–induced changes in the angle configuration by using anterior segment optical coherence tomography (AS–OCT). Methods: Patients attending the glaucoma service at the National University Hospital, Singapore were recruited into this study. Subjects included patients with primary angle closure glaucoma, primary open angle glaucoma, anatomically narrow angles and glaucoma suspects. Patients were imaged with the AS–OCT system (Carl Zeiss Meditec, Inc., Dublin, CA, USA) under dark conditions as well as bright illumination. Images of the nasal and temporal quadrants were acquired in all eyes. The AS–OCT images of the right eyes were analyzed for angle opening distance at 500µm (AOD₅₀₀), angle recess area at 500µm and 750µm (ARA₅₀₀ and ARA₇₅₀) and trabecular–iris space area at 500µm and 750µm (TISA₅₀₀ and TISA₇₅₀). Results: Twenty–nine patients were analyzed in this preliminary study. The mean age was 62.4 ± 8.3 years. Under dark conditions, the mean value of AOD₅₀₀ was significantly smaller than that measured under bright illumination in both the nasal (132µm vs. 211µm, p<0.0001) and temporal (138µm vs. 240µm, p<0.0001) quadrants. The mean ARA₅₀₀ was also smaller under dark conditions (nasal quadrant: 0.06mm² vs. 0.08mm², p=0.003; temporal quadrant: 0.07mm² vs. 0.11mm², p<0.0001). The other angle parameters measured showed a similar decrease under dark conditions. The temporal quadrant showed a trend towards being wider than the nasal quadrant with the difference being significant for ARA₅₀₀ & TISA₅₀₀ (p=0.01) as well as ARA₇₅₀ and TISA₇₅₀ (p=0.03) under bright illumination. Conclusions: Anterior segment optical coherence tomography is a promising imaging device that allows assessment of illumination–induced changes in the angle configuration. Using this instrument we observed a significant decrease in angle width under dark conditions. This finding suggests that the prevalence of anatomically narrow angles may be underestimated if screening protocols do not include evaluation under conditions of dim illumination.

Keywords: anterior chamber • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)