Imaging Post-Trabecular Outflow Pathways in Spontaneous Canine Glaucoma

Abstract

Purpose : Goniodysgenesis-related primary angle closure glaucoma (PACG) is highly prevalent in dogs and is poorly responsive to therapy. We hypothesized that pathology affecting the post-trabecular outflow pathways contributes to IOP elevation in canine PACG. The goal of this study was to determine the potential for aqueous angiography (AA) and optical coherence tomography (OCT) to identify abnormalities in post-trabecular aqueous outflow pathways in canine PACG.

Methods : AA and anterior segment OCT (Spectralis HRA+OCT) were performed ex vivo in 30 enucleated canine eyes (23 normal eyes and 7 irreversibly blind eyes from clinical patients enucleated for management of refractory PACG). In phase 1 of the study, eyes were cannulated and maintained at normal physiologic intraocular pressure (IOP) prior to intracameral infusion of 2.5% fluorescein and 0.4% indocyanine green. OCT scleral line scans were acquired in regions of high and low limbal AA signal. Eyes were perfusion fixed at physiologic IOP, and cryosections were immunolabeled for vascular markers. In phase 2, normal eyes were cannulated and imaged first at physiologic IOP, then after 30 mins at 50-70mmHg. Scleral lumen cross-sectional areas were measured using Image J and compared between well-oriented OCT scans acquired in the same regions pre- and post-IOP elevation.

Results : Normal eyes showed segmental, circumferential limbal AA signal (Fig. 1A), whereas PACG eyes showed very little to no AA signal circumferentially (Fig. 1B). AA signal correlated with lumens seen on OCT in normal dogs , but were generally absent or flattened in PACG eyes (Fig.1C). Collapsed vascular profiles were identified by immunolabeling in PACG. Compared to physiologic IOP, short term extreme IOP elevation did not consistently or significantly reduce OCT luminal cross-sectional area of scleral vessels in normal canine eyes (Fig. 2).

Conclusions : In this first report of AA in either canine eyes or eyes with PACG, PACG eyes showed little evidence of distal outflow on AA despite normalization of their IOP, and vascular profiles were absent to collapsed on OCT and histopathology. In contrast, short term IOP elevation did not lead to loss of AA signal or scleral lumens on OCT in normal canine eyes.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.

 

AA signal in a normal dog (A) and absent (B) in a canine PACG eye, in which scleral lumens were absent or flattened (C).

 

AA and OCT at physiologic (A) and high IOP (B) in a normal dog eye.