Imaging Canine Post-Trabecular Aqueous Outflow Pathways: Effect of Acute Intraocular Pressure Elevation in Normal Eyes.

Purpose: To investigate the relationship between increased intraocular pressure (IOP) and the structure of the post-trabecular aqueous outflow tract of dogs.

Methods: Ex vivo aqueous angiography (AA) and anterior segment optical coherence tomography (OCT) were performed concurrently in 19 normal canine eyes, following cannulation and intracameral infusion of 2.5% fluorescein and/or indocyanine green (ICG) after maintaining eyes at physiologic IOP (10-25 mmHg) for 30 min and/or elevated IOP (55-68 mmHg) for 30 or 120 min. Scleral lumen heights (SLH) of vascular profiles in OCT scans were measured by Image J, and values analyzed in linear regression models. In a subset of 10 eyes, each quadrant was subjectively characterized as being either a high-flow or low-flow region based on angiographic signal intensity at 2 or 10 min post-tracer infusion.

Results: Segmental AA outflow signal correlated with lumens seen on OCT scleral line scans in healthy canine eyes at physiologic IOP. These lumens were also observed at elevated IOP but appeared collapsed in profile. After both 30 and 120 min of IOP elevation, SLH was significantly reduced (p < 0.0001). Large vessels tended to collapse more than small vessels after IOP elevation.

Conclusion: Acute IOP elevation resulted in a significant reduction in SLH in healthy canine eyes, and the number of AA determined high flow regions was reduced. Future studies to understand the nature and timing of this pressure-dependent change in the post-trabecular outflow pathway in glaucomatous eyes, including its potential reversibility, will be crucial in developing novel diagnostic tools and treatments for canine glaucoma.