Ultrasound biomicroscopy of the rat eye: effects of cholinergic and anticholinergic agents

Over the past few years the rat has gained prominence as an animal model for the study of glaucoma. However, no systematic study of the angle structures and the effects of medications on angle anatomy in the rat has been reported to date. We investigated the normal rat anterior segment anatomy in vivo using ultrasound biomicroscopy (UBM) and determined the effect of both cholinergic and anticholinergic medications on angle structures.

Fourteen eyes of seven 2-month-old female Wistar rats were imaged using an ultrasound biomicroscope and a modified eyecup. Baseline measurements of the anterior chamber depth (ACD), trabecular-iris angle (TIA), iris thickness at the thickest point near the pupillary margin (IT), angle-opening distance (AOD) (distance between the posterior corneal surface and anterior iris surface measured at 200 μm from the scleral spur), corneal thickness (CT) and irido-zonular distance (IZD) were obtained. Imaging was repeated 30 min after instillation of one drop of cyclopentolate 1% and 48 h later 30 min after pilocarpine 1% instillation. The same measurements were obtained and compared to baseline values.

Baseline values for all parameters recorded were not significantly different among contralateral eyes. After instillation of either pilocarpine or cyclopentolate, ACD was the only parameter that did not change significantly from baseline. In contrast, TIA, AOD, IZD, and IT were significantly different among the three groups. Post-hoc analysis (Bonferroni test) revealed differences among all three groups of eyes for TIA and AOD. A difference was also found between the pilocarpine-treated group and the other two groups for IZD and IT. A very small difference detected between the pilocarpine-treated group and the baseline measurements for CT was caused by the zero variance of measurements in the former group. Although both pilocarpine and cyclopentolate induced angle narrowing, inspection of the ultrasonic images revealed a differential effect. Pilocarpine caused a “pupillary block-like” picture, while cyclopentolate caused crowding of the iris base in the angle.

Baseline characteristics of the normal rat anterior chamber anatomy were established. Both cyclopentolate and pilocarpine cause angle narrowing in the rat eye, by different mechanisms.