Effect of brimonidine on optic nerve blood flow in rabbits

PURPOSE: Brimonidine is a highly selective alpha(2)-adrenoreceptor agonist that lowers intraocular pressure. The aim of the present study was to analyze in vivo the vasomotor effects and the influence of brimonidine on blood flow within the optic nerve, by means of intraluminal microvascular corrosion casting technique and intravascular injection of colored microspheres. METHODS: New Zealand white rabbits received either brimonidine tartrate 0.2% or placebo (vehicle) topical drops in one eye for 4 weeks. Intraocular pressures were measured at baseline and 4 weeks. The anterior optic nerve microvasculature of four rabbits was examined with corrosion castings for regions of focal vasoconstriction. Optic nerve blood flow was determined in 16 rabbits by means of nonradioactive colored microspheres. RESULTS: The vasoconstriction values of the short posterior ciliary arterial branches in the brimonidine eyes were 16.7% +/- 3.7%. In the fellow untreated eyes, the mean vasoconstriction was 16.6% +/- 2.4%. In the placebo-treated eyes, the average constriction was 15.9% +/- 3.2%; the fellow eyes showed a mean constriction value of 16.1% +/- 5.3%. There was no statistical difference between any of the groups (P = .2). The optic nerve blood flow in the brimonidine-treated rabbits was 0.18 +/- 0.06 ml/mg/min and 0.17 +/- 0.04 ml/mg/min in the treated and the fellow eyes, respectively. The difference between the optic nerve blood flow in the brimonidine-treated eyes and the optic nerve blood flow in all of the untreated eyes (0.19 +/- 0.06 ml/mg/min) also was not statistically different (P = .82). CONCLUSIONS: Long-term application of brimonidine 0.2% does not affect the blood flow or vasomotor activity of the anterior optic nerve. (C) 1999 by Elsevier Science Inc. All rights reserved.