Brimonidine is an alpha-2 adrenergic agonist used to lower intraocular pressure. Which mechanism best describes its action?

• A. It increases trabecular outflow
• B. It decreases aqueous production and increases uveoscleral outflow
• C. It is a prostaglandin analog
• D. It is a beta-blocker

Answer: (B)

Brimonidine lowers intraocular pressure by activating alpha-2 adrenergic receptors in the eye. This action reduces the production of aqueous humor by inhibiting adenylate cyclase in the ciliary body, which lowers cAMP and decreases secretion. In addition, alpha-2 activation can modestly increase uveoscleral outflow, helping drainage through that pathway. So the net effect is decreased aqueous production with a secondary boost in outflow, leading to lower IOP. It’s not a prostaglandin analog (which mainly increases uveoscleral outflow) and it’s not a beta-blocker (which reduces production mainly via beta-receptor blockade).