Characterisation of Schlemm's canal cross-sectional area

PURPOSE To compare three methods of Schlemm's canal (SC) cross-sectional area (CSA) measurement. METHODS Ten eyes (10 healthy volunteers) were imaged three times using spectral-domain optical coherence tomography (Cirrus HD-OCT, Zeiss, Dublin, California, USA). Aqueous outflow vascular structures and SC collector channel ostia were used as landmarks to identify a reference location within the limbus. SC CSA was assessed within a 1 mm segment (±15 frames of the reference, 31 frames in all) by three techniques. (1) Using a random number table, SC CSA in five random frames from the set of 31 surrounding the reference were measured and averaged. (2) The most easily visualised SC location (subjective) was measured, and (3) SC CSA was measured in all 31 consecutive B-scans, and averaged. (comprehensive average, gold standard). Subjective and random CSAs were compared with the comprehensive by general estimating equation modelling, and structural equation modelling quantified agreement. RESULTS The average from five random locations (4175±1045 µm(2)) was not significantly different than that obtained from the gold standard comprehensive assessment (4064±1308 µm(2), p=0.6537). Subjectively located SC CSA (7614±2162 µm(2)) was significantly larger than the comprehensive gold standard SC CSA (p<0.0001). The average of five random frames produced significantly less bias than did subjective location, yielding a calibration line crossing the 'no-bias' line. DISCUSSION Subjectively located SC CSA measurements produce high estimates of SC CSA. SC assessed by measuring five random locations estimate CSA was similar to the gold standard estimate.