PURPOSE. Clonidine and brimonidine, two ␣-2 agonists, have been shown to reduce intraocular pressure (IOP) in patients with glaucoma. Little is known, however, about the exact role of ␣ receptors in the control of ocular blood flow in the posterior pole of the eye. Hence, the study was conducted to investigate the effects of topical clonidine versus topical brimonidine on choroidal blood flow and intraocular pressure during squatting. METHODS. This was a randomized, double-masked, controlled, two-way crossover study. Twelve healthy male nonsmoking volunteers, aged between 19 and 35 years were included in the study. Two drops of clonidine or brimonidine were administered in the subjects' study eyes. Continuous measurement using the compact laser Doppler flowmeter was performed during a 6-minute squatting period, to assess choroidal blood flow regulation during an increase in ocular perfusion pressure. RESULTS. Both substances induced a pronounced but comparable (P ϭ 0.8) decrease in IOP. Squatting increased mean arterial pressure (MAP) and ocular perfusion pressure (P Ͻ 0.01). This increase was comparable between the clonidine and the brimonidine study day (P ϭ 0.88). Squatting induced an increase in choroidal blood flow that was less pronounced than the increase in ocular perfusion pressure. Compared with baseline the ␣-2 agonists decreased choroidal blood flow during squatting (P ϭ 0.0026) to a comparable degree (P ϭ 0.86). Vascular resistance increased at baseline and during squatting after administration of the ␣-2 agonists (P Ͻ 0.01) in both groups to a comparable degree (P ϭ 0.56). CONCLUSIONS. Topical ␣-2 agonists may induce changes in choroidal blood flow, even after a single administration. Long-term studies are needed to study potential effects of brimonidine and clonidine in the clinical setting. (Invest Ophthalmol Vis Sci. 2007;48:4220 -4225
