From a therapeutic perspective, an understanding of anatomy is particularly important for managing anal fistulae, preventing nerve injury during surgical dissection, and understanding the consequences of rectal resection. Left-sided colectomy may result in postoperative colonic transit delays in the unresected segment; this likely represents parasympathetic den-ervation, as ascending intramural fibers travel in a retrograde manner from the pelvis to the ascending colon. The sigmoid colon and rectum are also supplied by descending fibers that run along the inferior mesenteric artery. These nerves may be disrupted during a low anterior resection, leaving a denervated segment that may be short or long depending on whether the dissection line includes the origin of the inferior mesenteric artery . A long denervated segment is more likely to be associated with non-propagated colonic pressure waves and delayed colonic transit than is a short denervated segment. In addition to colonic denervation, a low anterior resection may damage the anal sphincter and reduce rectal compliance ; in contrast to anal sphincter injury, rectal compliance may recover with time . Defecation may also be affected after surgical section of pelvic nerves in humans [70, 71].
Denonvilliers' fascia is intimately adherent to the anterior mesorectal fat but only loosely adherent to the seminal vesicles. During anterior rectal dissection, the deep parasympathetic nerves situated in the narrow space between the rectum and the prostate and seminal vesicles may be damaged, leading to impotence . For benign disease, most surgeons will tend to stay posterior to Denonvilliers' fascia in an attempt to protect the pelvic nerves. For malignant disease, the choice is less straightforward, because dissection behind rather than in front of the fascia may, in theory, be associated with incomplete resection and/or local recurrence.
Because vaginal delivery can damage the anal sphincters and the pudendal nerve, up to 10% of women develop fecal incontinence after a vaginal delivery . The incidence of post partum fecal incontinence is considerably higher (i.e., 15-59%) in women who sustain a third-degree (i.e., anal sphincter disruption) or a fourth-degree tear (i.e., a third-degree tear with anal epithelial disruption) [74, 75]. The only prospective study that imaged the anal sphincters before and after vaginal delivery demonstrated that anal sphincter defects and pudendal nerve injury after vaginal delivery were often clinically occult and that forceps delivery was the only independent factor associated with anal sphincter damage during vaginal delivery . A Cochrane Review concluded that restrictive episiotomy policies were beneficial (i.e., less posterior perineal trauma, less suturing, and fewer complications) compared with routine episiotomy policies . However, there is an increased risk of anterior perineal trauma with restrictive episiotomy. Both the external and internal anal sphincters may be damaged during a severe per-ineal laceration. When possible, lacerations that require complex repair should be carried out in the operating room, under regional or general anesthesia, with appropriate instruments, adequate light, and an assistant . A randomized controlled study demonstrated that compared with end-to-end repair, primary overlapping repair of external anal sphincter defects was associated with a significantly lower incidence of fecal incontinence, fecal urgency, and perineal pain at 12 months . Though some experts have suggested that both the internal and external anal sphincters be repaired, there are no trials comparing concurrent repair of the internal and external anal sphincters to repair of the external sphincter alone after obstetric injury [80, 81].
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