Anatomic Changes in Schlemm's Canal and Collector Channels in Normal and Primary Open-Angle Glaucoma Eyes Using Low and High Perfusion Pressures

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Abstract
To examine the anatomy of Schlemm's canal (SC) and collector channels (CCs) in normal human and primary open-angle glaucoma (POAG) eyes under low and high perfusion pressure. In normal (n = 3) and POAG (n = 3) eye pairs, one eye was perfused at 10 mm Hg while the fellow eye was perfused at 20 mm Hg for 2 hours. Eyes were perfusion fixed at like pressures, dissected into quadrants, embedded in Epon Araldite, and scanned by three-dimensional micro-computed tomography (3D micro-CT). Schlemm's canal volume, CC orifice area, diameter, and number were measured using ANALYZE software. Normal eyes showed a larger SC volume (3.3-fold) and CC orifice area (9962.8 vs. 8825.2 μm(2)) and a similar CC diameter (34.3 ± 17.8 vs. 32.7 ± 13.0 μm) at 10 mm Hg compared to 20 mm Hg. In POAG eyes, SC volume (2.0-fold), CC orifice area (8049.2 μm(2)-6468.4 μm(2)), and CC diameter (36.2 ± 19.1 vs. 29.0 ± 13.8 μm) were increased in 10 mm Hg compared to 20 mm Hg perfusion pressures. Partial and total CC occlusions were present in normal and POAG eyes, with a 3.7-fold increase in total occlusions in POAG eyes compared to normal eyes at 20 mm Hg. Visualization of CCs increased by 24% in normal and by 21% in POAG eyes at 20 mm Hg compared to 10 mm Hg. Schlemm's canal volume, CC area, and CC diameter were decreased in POAG eyes compared to normal eyes at like pressures. Compensatory mechanisms for transient and short periods of increased pressure appear to be diminished in POAG eyes. Variable response to pressure change in SC and CCs may be a contributing factor to outflow facility change in POAG eyes.

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