If brimonidine is an adrenergic agonist, how and why does it reduce glaucoma?

Brimonidine acts at postsynaptic alpha-2 adrenoreceptors on blood vessels to cause vasoconstriction, reducing aqueous humour production. Long-term, there are also effects on uveoscleral drainage, perhaps secondary to reduced blood flow to the ciliary muscle.

Brimonidine alone is not as potent at reducing intraocular pressure (IOP) as beta-blockers or prostaglandin F2alpha analogues (e.g., latanoprost). The primary reason that brimonidine has come back into use is that it also has a neuroprotective action, reducing the death of retinal ganglion cells through mechanisms that remain poorly understood.