Topical phenylephrine increases anal sphincter resting pressure

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Abstract
Background: Phenylephrine is an α1-adrenergic agonist which causes contraction of human internal anal sphincter muscle in vitro. Its intra-arterial administration in animals has been shown to increase resting sphincter pressure in vivo. In this study the effect of topical application of phenylephrine on resting anal pressure in healthy human volunteers was investigated. Methods: Twelve healthy volunteers had measurements of maximum resting sphincter pressure (MRP) and anodermal blood flow taken before and after topical application of increasing concentrations of phenylephrine gel to the anus. To determine the duration of effect of the agent, readings were taken throughout the day after a single application. Results: There was a dose-dependent rise in the resting anal sphincter pressure, with a small 8 per cent rise after 5 per cent phenylephrine (P = 0·012) and a larger 33 per cent rise with 10 per cent phenylephrine (mean(s.d.) MRP 85(12) cmH2O before versus 127(12) cmH2O after treatment, P < 0·0001). Thereafter no additional response was noted with higher concentrations of phenylephrine. The median duration of action of a single application of 10 per cent phenylephrine was 7 (range from 6 to more than 8) h. Conclusion: Topical application of 10 per cent phenylephrine gel to the anus produces a significant rise in the resting anal sphincter pressure in healthy human volunteers. This represents a potential novel therapeutic approach to the treatment of passive faecal incontinence associated with a low resting anal sphincter pressure.