Novel anterior-chamber angle measurements by high-definition optical coherence tomography using the Schwalbe line as the landmark

Objective To propose the Schwalbe line (SL) as a new anatomical landmark, independent of the scleral spur (SS) location, for assessing anterior chamber angle (ACA) width quantitatively with high-definition optical coherence tomography (HD-OCT). Methods Study subjects underwent dark-room gonioscopy and HD-OCT in one randomly selected eye. The authors developed a computer-aided program to define two new quantitative parameters for assessing ACA width: Schwalbe line-angle opening distance (SL-AOD) measured at the SL, and Schwalbe line-trabecular-iris space area (SL-TISA) measured 500 mm from the SL. The associations between SL parameters, SS parameters and gonioscopic grading were evaluated. Results Seventy-three (47 females, 26 males) subjects were recruited, the majority of whom were Chinese (89%). The authors excluded 29 images (19.9%) owing to poor image quality, leaving 117 HD-OCT images (65 nasal, 52 temporal) for analysis. SL and SS could be identified in 95% and 85% of quadrants respectively (p=0.035). SL-AOD and SL-TISA were significantly correlated with SS parameters (all r >= 0.85) and gonioscopic grading (all r >= 0.69). In eyes with closed angles (n=36), SL parameters showed strong correlations with gonioscopic grading (r ranged from 0.43 to 0.44). Conclusions Novel angle parameters, based on SL as a landmark, may be useful to quantify ACA width and to assess for risk of angle closure.