Primary degeneration of the internal anal sphincter as a cause of passive faecal incontinence

Background Faecal incontinence is usually attributed to pelvic-floor denervation of striated muscle or direct sphincter trauma. We have identified a cause of passive faecal incontinence related to degeneration of the internal anal sphincter smooth muscle, in the absence of denervation, structural damage, external-sphincter weakness, or sensory abnormalities. Methods Patients were included on the basis of: passive faecal incontinence, no urge faecal incontinence, low anal pressure whilst at rest, normal anal-squeeze pressure, endosonographically confirmed circumferentially intact internal and external anal sphincters, and normal pudendal nerve terminal nerve latencies. In a second analysis done to assess the proportion of patients with this disorder, we recorded the cause of incontinence in consecutive patients seen during a 6-month period. Findings 45 patients (35 women, median age 63 years, range 23-80 years) fulfilled the diagnostic criteria. Median duration of symptoms was 2 years (3 months to 20 years). Nine of the 35 women were nulliparous. The median resting anal pressure was 40 cm water (16-56 cm water, normal >60 cm water). Endosonography revealed an internal sphincter that was thin and hyperechogenic, and had a poorly defined edge. The normal increase in the thickness of the internal anal sphincter with age was not seen, Anal-squeeze pressure, sensitivity, and pudendal nerve latencies were normal. In the second analysis the condition was identified in eight of 230 patients, representing 4% of new referrals. Interpretation Primary degeneration of the internal anal sphincter smooth muscle is a discrete clinical condition causing passive faecal incontinence.