In vivo observations of the episcleral vasculature in the albino rabbit.

The recently described arteriovenous anastomoses of the episcleral vasculature were studied in the albino rabbit by in vivo microscopy and microendoscopy. The arteriovenous anastomoses lead directly into the episcleral venous plexus which drains aqueous humor via well-visible aqueous veins. In the arteriolar segment of the arteriovenous anastomoses the blood flow velocity, measured by intra-arterial bolus injections of Evans blue, decreases drastically toward the venular segment of the arteriovenous anastomoses. After topical application of epinephrine, the arteriolar segment contracts faster and to a greater degree than all other parts of the episcleral vasculature. In the aqueous veins this stoppage of the blood inflow from the arteriovenous anastomoses may allow an increase in the amount of aqueous humor drained from the anterior chamber. The simultaneously measured intraocular pressure decreases significantly. Experimental elevation of the intraocular pressure from 18 to 30 mm Hg causes a decrease in diameter and blood flow in the arteriovenous anastomoses. These effects are reversed by returning to normal intraocular pressure levels. It is assumed that the arteriovenous anastomoses play a role in aqueous humor drainage by the episcleral venous plexus.