Internal anal sphincter in neurogenic fecal incontinence

In neurogenic fecal incontinence there is denervation of the external anal sphincter and pelvic floor muscles but the role of the internal anal sphincter is incompletely understood. We have evaluated the internal anal sphincter in 6 patients with neurogenic incontinence undergoing postanal repair and in 7 control subjects. All the incontinent subjects, but none of the controls, had evidence of pudendal neuropathy. Surface electromyography studies of the internal anal sphincter showed absence of electrical activity in 4 of 6 incontinent subjects; in the remaining 2 subjects and in 6 of 7 controls normal slow waves were present. Internal sphincter muscle strips from control subjects showed normal in vitro responses to noradrenaline, isoprenaline, dimethyl-phenylpiperazinium, and electrical field stimulation; muscle strips from the incontinent patients showed complete insensitivity except in 2 patients in whom there was contraction to noradrenaline and relaxation to isoprenaline. Electron microscopy showed normal smooth muscle in 5 control subjects and minor changes in 1 subject; all the incontinent patients showed abnormalities in the smooth muscle cells of the internal anal sphincter. These findings indicate that in neurogenic fecal incontinence neurogenic weakness of the external anal sphincter and pelvic floor muscles is associated with damage to the internal anal sphincter.