In Vivo Dimensions of Schlemm’s Canal in Normal and Experimental Glaucomatous Monkey Eyes

Abstract

Purpose: To characterize in vivo structure of the Schlemm’s canal (SC) in normal and experimental glaucomatous monkey eyes

Methods: Chronic ocular hypertension (OHT) of various durations (2.5-11 y) was induced in the right eye (OD) of 17 cynomolgus monkeys by argon laser trabecular photocoagulation. The left eye (OS) was untreated with normal intraocular pressure (IOP). An additional 15 naïve animals served as controls. A serial of 49 horizontal enhanced depth imaging (EDI) optical coherence tomography (OCT) B-scans in a 15x3 degree rectangle area were obtained in gas anesthetized monkeys by Spectralis SD-OCT (Heidelberg Engineering, Germany) in the temporal limbal area each eye. A dark non-reflective area in the cross-sectional image of each EDI OCT B-scan is considered as the SC and was measured by three independent masked observers using AMIRA V. 5.5 (FEI Visualization Sciences Group, Burlington, MA). The mean SC area of the 49 scans was then calculated.

Results: In the 15 naive male monkeys, the mean cross-sectional SC area in OD and OS was 3,322±367 μm2 and 3,541±451 μm2, respectively and without significant difference. In three age-matched male experimental monkeys (1 year post laser treatment), the mean SC area in the lasered eyes (1035±323 μm2) was significantly smaller than that in the non-lasered fellow eyes (4196±387 μm2). On the contrary, the mean SC area in the normal eyes of the 9 age-matched female monkeys was only 1784±448 μm2 (about half of that in male monkeys) and the SC area in the lasered eyes became slight narrower 1563±590 μm2 but without significant difference from the fellow eyes. In the 5 older monkeys who have longer duration of OHT (11 years), the mean SC area in the experimental eyes (550±80 μm2) was markedly smaller than that in the fellow normal eyes 1663±674 μm2.

Conclusions: Schlemm’s canal can be noninvasively assessed in the monkey eye. These measurements will be useful in physiological studies of aqueous outflow and may have important implications for the development of treatments that target the trabecular outflow pathway.

Keywords: 568 intraocular pressure • 552 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • 735 trabecular meshwork