We performed a randomly assigned, double-masked, placebo-controlled study in 78 patients with varied iris pigmentation to evaluate the influence of iris pigmentation on the ability of 0.1% thymoxamine to reverse mydriasis produced by 2.5% phenylephrine. Patients were chosen so that a 1.6:1 ratio of dark to light irides was obtained. Within one-half hour after medication, thymoxamine-treated nonbrown irides constricted significantly compared to their fellow placebo-treated irides (P < .001). Thymoxamine-treated pupils of nonbrown irides were 1.0 to 3.1 mm smaller than placebo-treated fellow eyes. Thymoxamine- treated light brown irides constricted less (0.6 to 2.0 mm) and more slowly compared to fellow placebo-treated irides. Thymoxamine did not reverse the mydriasis in eyes of patients with dark brown irides. Thymoxamine appears similar to other adrenergic agents that bind to melanin, delaying onset and strength of action. Its efficacy as presently formulated may be limited, in part, by iris color.
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