Abnormalities of innervation of internal anal sphincter in fecal incontinence

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Abstract
Physiological and histological studies have shown that the internal anal sphincter is abnormal in idiopathic fecal incontinence. We have recently demonstrated that the invitro contractile response of the internal anal sphincter to the sympathetic neurotransmitter noradrenaline is decreased in incontinence. In this study we have further defined this reduced sensitivity and provided more information about the intrinsic innervation in both the normal and the incontinent sphincter muscle. Muscle strips from 12 incontinent patients undergoing post anal repair and from 11 controls undergoing rectal excision for low rectal carcinoma were studied. Responses to noradrenaline were recorded initially alone and then in the presence of phentolamine, an α-adrenoceptor blocker. In the presence of phentolamine, noradrenaline caused relaxation: there was no significant difference in the relaxation-response curves and the EC50 was the same in the two groups. These results demonstrate that the previously documented reduced sensitivity to noradrenaline is due to an altered sensitivity of the α-adrenoceptors. Electrical field stimulation produced relaxations in all muscle strips, but only in the controls was the magnitude of the relaxation significantly increased in the presence of phentolamine. This indicates that there is an α-adrenergic excitatory component of the response to electrical field stimulation of the intramural nerves, which was present in tissues from control patients but which was absent in tissues from patients with idiopathic fecal incontinence.