AIM:To evaluate the potential posterior segment effects of topical application of brimonidine-purite 0.15％ through measurement of choroidal thickness (CT) in healthy eyes using enhanced depth imaging spectraldomain optical coherence tomography (EDI-SD-OCT).METHODS:Thirty-two eyes of 32 healthy subjects were included in this prospective,placebo controlled interventional clinical trial.They received one drop of topical preservative-free artificial tears as placebo for the first day and one drop of brimonidine-purite 0.15％ for the second day.Intraocular pressure,ocular perfusion pressure (OPP),and EDI-SD-OCT were performed at baseline,at 1,3 and 5h after the treatments.RESULTS:Compared to the measurements obtained at baseline,the CT measurements obtained after the topical application of brimonidine-purite 0.15％ significantly increased at the sub-fovea (P=0.001),at temporal 1500 μm to the fovea (P=0.003) and at nasal 1500 μm to the fovea (P=0.003).Choroidal thickness was unchanged in placebo group during the study (P ＞0.05).There was no significant reduction in the OPP in both groups (P ＞0.05).There were no adverse events during the study.CONCLUSIONS:Contrary to expectations,topical administration of brimonidine-purite 0.15％ resulted with thickening of sub-foveal,temporal and nasal CT.This might be related to altered auto-regulation mechanisms in choroidal vessels.