Aqueous humor flow in normal human eyes treated with brimonidine and timolol, alone and in combination

Abstract

Objective: To compare the effect on aqueous humor flow and intraocular pressure (IOP) of topically applied 0.2% brimonidine tartrate with topically administered 0.5% timolol maleate, alone and in combination. Design: A randomized, double-masked, placebo-controlled study of 20 human subjects was carried out. The topical drugs were instilled twice daily the day before and again on the morning of the day of the measurements. Aqueous humor flow was measured by clearance of topically applied fluorescein with a fluorophotometer, and IOP was measured with an applanation tonometer. Results: Brimonidine reduced the aqueous humor flow by 33.1%; timolol, by 49.9%; and the combination of brimonidine and timolol, by 58.9%. Brimonidine reduced the IOP by 20.3%; timolol, by 22.9%; and the combination of brimonidine and timolol, by 34.7%. Conclusions: Brimonidine suppressed aqueous humor flow, but not as effectively as timolol. However, the effects on the IOP of both drugs separately were comparable. The short-term effect of brimonidine was partly additive to timolol, and the combination treatment caused a further reduction in aqueous humor flow and IOP. The IOP reduction by timolol could be explained solely by aqueous humor flow reduction. Much of the IOP reduction caused by brimonidine, but not all, could be explained by suppression of the aqueous humor flow, suggesting an additional mechanism for the ocular hypotensive effect of brimonidine.