Selectivity of site of action and systemic effects of topical alpha agonists

Clonidine hydrochloride, apraclonidine hydrochloride, and brimonidine tartrate constitute the three topical alpha agonists that are used in the treatment of elevated intraocular pressure. All the alpha agonists have prejunctional (alpha 2) as well as postjunctional (alpha 1) effects. Within the past year, questions have arisen about their local and systemic effects, and their effects upon the optic nerve. We will, therefore, attempt to clarify these points, to provide a greater understanding of the role of alpha agonists in glaucoma therapy.