EFFECTS OF BRIMONIDINE ON AQUEOUS-HUMOR DYNAMICS IN HUMAN EYES

Objective: To evaluate the mechanism by which brimonidine, a selective alpha(2)-adrenergic agonist, lowers intraocular pressure (IOP) in hum ans. Subjects: Twenty-one volunteers with ocular hypertension. Methods : Brimonidine tartrate (0.2%) was given topically twice daily for 1 we ek to one eye in a randomized, double-masked study. The fellow eye was similarly treated with brimonidine vehicle. Before (baseline) and aft er 1 week (day 8) of dosing, IOP, aqueous flow, episcleral venous pres sure, and tonographic outflow facility were directly measured. Fluorop hotometric outflow facility and uveoscleral outflow were calculated. B rimonidine-treated eyes were compared with vehicle-treated contralater al control eyes and with baseline measurements after 1 week of dosing. Results: Brimonidine significantly (P < .001, Student's two-tailed t test) reduced IOP mean +/- SE of 4.7 +/- 0.7 and 4.2 +/- 0.4 mm Hg com pared with the baseline day and with the vehicle-treated contralateral control eyes, respectively. Compared with the baseline day, aqueous f low was reduced by 20% (P = .002) and uveoscleral outflow was increase d (P = .04). A slight contralateral decrease in IOP of 1.2 +/- 0.6 mm Hg (P = .05) and in aqueous flow of 12% (P = .05) was noted. No signif icant difference was seen in the outflow facility values or episcleral venous pressure compared with the baseline day or with the contralate ral control eye. Conclusions: The brimonidine-induced reduction in IOP in humans is associated with a decrease in aqueous flow and an increa se in uveoscleral outflow. The decrease in IOP and aqueous flow in the contralateral control eye on day 8 compared with the baseline day sug gests a mild contralateral effect.