The role of alpha-agonists in glaucoma therapy.

alpha-Agonists are a relatively old class of medications, the topical use of which lowers eye pressure. Clonidine was introduced for this use in 1966, brimonidine in 1974, and apraclonidine in 1978. Initial short-term attempts to use clonidine were complicated by problems with systemic hypotension. Apraclonidine is more polar and less lipophilic than clonidine. This probably allows less penetration into both the posterior segment of the eye and systemic circulation, allowing for an excellent therapeutic index. The prophylactic use of apraclonidine (1% and 0.5%) has dramatically changed the safety profile for many anterior segment laser procedures, cataract surgery, and vitrectomy. The role of alpha-agonists in the chronic treatment of glaucoma is still uncertain. Potential benefits of additional lowering of intraocular pressure must be weighed against the following potential disadvantages: tachyphalaxsis, posterior segment vasoconstriction, psychologic depression and fatigue, syncope and systemic hypotension, and a topical allergy-like syndrome.