Evaluation of the intrinsic innervation of the internal anal sphincter using electrical stimulation

Stimulation of the rectal mucosa with a bipolar electrode leads to relaxation of the internal anal sphincter. Intraoperative studies in two subjects showed that transmission of the impulse was independent of extrinsic nerves and was interrupted by circular myotomy. Characteristics of the reflex were studied in 11 healthy women and 19 women with severe idiopathic constipation. One control subject and two patients did not tolerate the test. In the remainder the stimulus caused a clearly defined fall in internal sphincter pressure. The mean resting maximum anal canal pressure before stimulation was the same in both groups (90 (10) v 104 (7) cm H2O, p = 0.3, controls v patients). The threshold stimulus for relaxation (12 (2) v 14 (1) mamps, p = 0.5), the maximum percent fall in resting pressure (43 (7) v 46 (4)%, p = 0.7) and the lowest absolute resting pressure produced by stimulation (48 (13) v 49 (6) cm H2O, p = 0.9) were the same in both groups. The stimulus required to achieve maximum relaxation was significantly higher in the patient group (23 (3) v 32 (2) mamps, p = 0.012) suggesting abnormal intrinsic innervation of the sphincter in these patients. Electrical stimulation should not replace balloon distension for routine testing of the rectoanal reflex but it may be useful in quantitative studies.