Effect of Pilocarpine on Intraocular Pressure and Schlemm’s Canal in Experimental Glaucomatous Monkey Eyes

Abstract

Purpose: To assess the usefulness of enhanced depth imaging optical coherence tomography (EDI OCT) for evaluating in vivo microarchitecture of the trabecular-Schlemm’s canal (SC) outflow pathway.

Methods: Chronic ocular hypertension (OHT) was induced in the right eye (OD) of 9 female, 4 year old, cynomolgus monkeys by argon laser trabecular photocoagulation. The left eye (OS) was untreated with normal intraocular pressure (IOP). The effect of 300 µg Pilocarpine on IOP in eyes of conscious animals was measured with a pneumatonometer following a single topical ocular instillation. In a separate study, the effect of same dose Pilocarpine on SC was in vivo imaged in the same group animals under gas anesthesia. A serial of 49 horizontal EDI OCT B-scans in a 15x3 degree rectangle area were obtained by Spectralis SD-OCT (Heidelberg Engineering, Germany) in the temporal limbal area each eye. SC was visualized as a dark non-reflective area in the cross-sectional image of each B-scan, and was measured by three independent masked observers using AMIRA V. 5.4.5 (FEI Visualization Sciences Group, Burlington, MA). The parameters measured before and post dose were then processed for comparative analyses.

Results: In the IOP study, a single dose of 300 µg Pilocarpine induced, at 1 hour post-dose, the maximum and significant IOP reduction (7.6 mmHg or 21.5±2.9%) from baseline in the lasered OHT eyes and (3.5 mmHg or 13.9±2.7%) in the fellow normal eyes, respectively. In the imaging study, the mean SC area was significantly increased from 1,563±590 µm2 to 2,568±620 µm2 (64%) in the lasered OHT eyes and from 1,784±448 µm2 to 3,218±449 µm2 (80%) in the normal fellow eyes 1 h post dose, respectively.

Conclusions: EDI OCT is useful for evaluating the in vivo microarchitecture of the trabecular-SC outflow pathway. This technology has important implications for the development of new drugs and techniques to reduce intraocular pressure.

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