How important is the role of the internal anal sphincter in fecal continence? An experimental study in dogs.

It is a generalized concept that the internal anal sphincter (IAS) plays a significant role in fecal continence by generating high pressure in the anal canal at rest and relaxation during rectal distention. Agreement also exists on the importance of internal sphincter-saving anoplasty on anorectal malformations in establishing anal continence. Twelve dogs were divided into four groups. Group 1, a control group, was subjected to a perirectal dissection only. Group 2 underwent the same perirectal dissection plus a 2-cm resection of the anal canal. Group 3 underwent the perirectal dissection plus a 4-cm resection of the anal canal, and group 4 underwent perirectal dissection and transposition of the anus to the posterolateral portion of the voluntary muscle mass. Clinical continence was evaluated, and manometric results were compared with preoperative measurements. All dogs in groups 1, 2, and 3 were clinically continent without soiling except one in group 3, and also manometric results showed minimal change between preoperative and postoperative anal pressure profiles. The transposed anus of group 4 showed continuous fecal soiling. The anal resting pressure (ARP) was also decreased but still existed in this group. This experimental study showed that the IAS contributes to the anal resting tone. However, resection of the IAS did not completely interfere with fecal continence. The smooth muscle of pulled-through rectum seemed to partly take over the function of the IAS.