The cutaneo-anal reflex: a useful index of neuropathy?

Recent studies have suggested that the latency of the cutaneo-anal reflex is prolonged in idiopathic faecal incontinence, suggesting neuropathic damage to the external anal sphincter. Electrical responses were recorded from the external anal sphincter following stimulation of the ipsilateral and contralateral perianal skin in normal subjects and patients with idiopathic faecal incontinence. Ipsilateral stimulation gave rise to shorter latencies of the earliest responses to stimulation compared with contralateral stimulation (P less than 0.001). When care was taken to compare results obtained by stimulating the skin on corresponding sides of the sphincter, the latency of both the early and late components of the cutaneo-anal reflex in incontinent patients was not significantly different from that in control subjects. The durations of the motor unit potentials, obtained by conventional electromyography, were prolonged in incontinent patients, indicating that they have external sphincter neuropathy. The data support the suggestion that external sphincter neuropathy in idiopathic faecal incontinence is due to a stretch injury to the pudendal nerve, and indicate that measurement of the latency of the cutaneo-anal reflex may be an inadequate means of demonstrating nerve damage in patients with idiopathic faecal incontinence.