Purpose: To characterize in vivo response of Muller's muscle to phenylephrine about time and diurnal variation. Methods: A nonrandomized trial of adults without (n = 20, 40 eyes) and with ptosis (n = 5, 5 eyes) measured for response of marginal reflex distance-1 (MRD1) to topical phenylephrine (2.5%) every 15 seconds in the morning (6-10 am) and evening (6-10 pm). Mean MRD1 was plotted over time, and morning and evening MRD1 were compared with the Spearman correlation coefficient (r) and paired t test. Results: Regardless of the time of day, subjects with and without ptosis had maximal response to phenylephrine within 2 minutes, with persistence of response at 30 minutes. There was good correlation (r = 0.92), with no statistically significant difference in rate or amount of change of MRD1 between morning and evening time points. Conclusions: Maximal response to phenylephrine occurs within 2 minutes of instillation with persistence of response at 30 minutes; there appears to be no diurnal variation in phenylephrine testing.
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