Effect of anterior resection on anal sphincter function

Minor difficulties with continence may occur after low anterior resection. Intraoperative injury to the internal anal sphincter or its nerve supply may contribute to this. To study the effect of low anterior resection on the anal sphincter mechanism, anal manometry was performed on 20 patients before and 10 days after resection. Fifteen patients were studied again 6 months after operation. Resting, maximum squeeze and squeeze increment pressures were recorded. Intraoperative manometry (n = 11) and presacral nerve stimulation (n = 6) were performed to determine whether peroperative injury to the internal anal sphincter had occurred. Resting and maximum squeeze anal canal pressures were reduced by low anterior resection, and did not recover. The squeeze pressure increment did not change. Division of the inferior mesenteric artery, full mobilization of the rectum and mesorectum, and rectal transection did not affect resting anal pressure, which was reduced after EEA anastomosis (mean (s.e.m.) before, 40(5) mmHg; after, 27(4) mmHg; P less than 0.05, n = 5). Presacral nerve stimulation produced relaxation of the internal sphincter. Anal sphincter pressures are reduced after low anterior resection. The external anal sphincter and the nerve supply to the internal anal sphincter appear intact. A direct injury to the internal sphincter is postulated.