Aqueous outflow in the beagle: unconventional outflow, using different-sized microspheres.

Normal drainage of aqueous humor from the anterior chamber of the canine eye occurred through the angular aqueous plexus and uveoscleral route. In order to understand more clearly, uveoscleral outflow with regard to larger sized tracer particles and the possible influence that varying intraocular pressure (IOP) may have on them, microspheres of 3 sizes, 0.5, 1.0, and 3.0 micron, were perfused at 3 different IOP (20, 50, and 75 mm of Hg) for 3 different times (30, 60, and 90 minutes). Presence of spheres was detected by light microscopy and scanning electron microscopy, and verified by transmission electron microscopy. By 30 minutes of perfusion, all 3 sphere sizes thoroughly infiltrated the anterior uveal trabecular meshwork. At normotensive pressure (20 mm of Hg), spheres of each size penetrated the outer anterior extension of the ciliary musculature and entered the supraciliary space. Only 0.5- and 1.0-micron spheres moved posteriorly into the suprachoridal space. At hypertensive pressures, movement of the tracer particles increased markedly toward the posterior uveal trabecular meshwork. However, 3.0- and 1.0-microns spheres did not enter the supraciliary space and suprachoroidal space, respectively. Spheres also entered the iris, mostly at its root. They remained comparably sparse at different perfusion times and IOP. The present study supported evidence for uveoscleral outflow (vs uveovortex) in the dog and indicated that passage of materials through this pathway may be influenced by IOP when these substances are within a critical size range.