Purpose.: Measurements of human Schlemm's canal (SC) have been limited to histologic sections. The purpose of this study was to demonstrate noninvasive measurements of aqueous outflow (AO) structures in the human eye, examining regional variation in cross-sectional SC areas (on/off collector channel [CC] ostia [SC/CC] and nasal/temporal) in the eyes of living humans.

Methods.: SC was imaged by spectral-domain optical coherence tomography with a 200-nm bandwidth light source. Both eyes of 21 healthy subjects and one glaucomatous eye of three subjects were imaged nasally and temporally. Contrast and magnification were adjusted to maximize visualization. Cross-sectional SC on and off SC/CC was traced three times by two independent masked observers using ImageJ (ImageJ 1.40 g, http://rsb. info. nih. gov/ij/Wayne Rasband, developer, National Institutes of Health, Bethesda, MD). The mean SC area was recorded. A linear mixed-effects model was used to analyze eye, nasal/temporal laterality, and SC area on or off SC/CC.

Results.: SC area was significantly larger on SC/CCs than off (12,890 vs. 7,391 μm 2, P< 0.0001) and was significantly larger on the nasal side than on the temporal (10,983 vs. 8,308 μm 2, P= 0.009). SC areas were significantly smaller in glaucoma patients than in normal subjects, whether pooled (P= 0.0073) or grouped by on (P= 0.0215) or off (P= 0.0114) SC/CC.

Conclusions.: Aqueous outflow structures, including SC and CCs, can be noninvasively assessed in the human eye. These measurements will be useful in physiological studies of AO and will be clinically useful in the determination of the impact of glaucoma therapies on IOP as well as presurgical planning.