Role of rectosigmoid junction in fecal continence: an experimental study.

To investigate the reason why, during a mass contraction, the stool moving from the colon to the sigmoid colon stops short of the rectosigmoid junction (RSJ) instead of passing directly to the rectum, and whether the sigmoid colon and RSJ share in the anorectal continent mechanism, 12 mongrel dogs were studied. Under anesthesia, the anorectum was excised, sigmoid colon and RSJ were mobilized and the caudal end was anastomosed to the perianal skin within the external anal sphincter. The pressures in the sigmoid colon, RSJ, rectum and rectal neck were measured before anorectal excision. After excision, the pressure was registered in the neorectum (sigmoid colon) and the RSJ. The external anal sphincter EMG was recorded and the balloon expulsion test performed before and after anorectal excision and after anesthetization of the neorectum. Balloon distension of the sigmoid colon to a mean volume of 46.6+/-7. 6 ml raised the pressure in the sigmoid colon (p<0.001) and decreased it in the RSJ (p<0.05) and the balloon was dispelled to the rectum; no change occurred in the external anal sphincter EMG activity. Neorectal balloon distension to a mean volume of 62.3 +/- 8.2 ml effected a pressure rise in the neorectum (p<0.001) and a momentary increase in the external anal sphincter EMG activity, followed by a decrease of the RSJ pressure (p<0.05); the balloon was dispelled to the exterior. Balloon distension of the anesthetized neorectum effected no significant changes in neorectum and RSJ pressures or external anal sphincter EMG activity. In conclusion, it is assumed that the stools arriving from the colon are halted at the RSJ by the existing high-pressure-zone and the presence of a potential sphincter at the RSJ. We suggest that fecal continence occurs at 2 levels: an involuntary one at the RSJ and a voluntary one at the rectal neck.