Benign diseases of the anorectum range from relatively simple disorders such as hemorrhoids and fissures to extremely complex problems associated with pelvic floor abnormalities.
The rectum normally lies attached to its mesorectum within the curve of the sacrum, with limited mobility. The junction of the rectosigmoid is usually fixed by the inferior mesenteric artery and peritoneal attachments. The rectum and mesorectum (fat and vessels) follow the curve of the sacrum to the pelvic floor. The rectum exits the pelvis behind the prostate or vagina through a slit in the pelvic floor. The horseshoe-shaped puborectalis muscle circles from its origin on the pubis around behind the rectum and reinserts on the pubis anteriorly. Contraction of the muscle pulls the rectum forward, creating a more acute angle at the palpable anal-rectal ring. The anal canal is a 3- to 4-cm long funnel-shaped extension of the pelvic floor voluntary musculature called the external sphincter. The pressure generated by contracting this circular muscle prevents egress of rectal contents. The internal sphincter muscle is a thickened continuation of the circular muscle of the rectal wall. As such, it is an autonomic muscle and has no voluntary control. It is innervated by a local plexus of nerves that connects the stretch receptors of the rectal wall to the internal anal sphincter as a sampling reflex (anal inhibitory reflex) which produces relaxation as the rectum fills.
The anorectum receives both sympathetic and parasympathetic nerves. The sympathetic nerves originate from thoracolumbar segments and unite below the inferior mesenteric artery to form the inferior mesenteric plexus. Injury to these nerves results in retrograde ejaculation and infertility in men. These fibers then descend to the superior hypogastric plexus located on the sacral promontory just inferior to the aortic bifurcation. These purely sympathetic fibers bifurcate and descend as the hypogastric nerves. Parasympathetic fibers from S2, S3, and S4 (the nervi erigentes) join the hypogastric nerves in the side wall of the low pelvis, anterolateral to the rectum, to form the inferior hypogastric plexuses. Mixed fibers from the plexuses innervate the prostate, rectum, bladder, penis, and internal anal sphincter. These autonomic plexuses of the pelvic nerves run around the lateral aspect of the pelvic rim to enter the prostate and seminal vesicles anteriorly. The sympathetic innervation of the internal sphincter is motor, while the parasympathetic innervation is inhibitory. Injury to the pelvic autonomic nerves during pelvic surgery may result in urinary retention or erectile dysfunction.
The innervation of the voluntary muscles of the pelvic floor is via direct fibers from S2, S3, and S4 in the pelvis from the sacrum (Fig. 52-1). The motor and sensory nerves of the external sphincter are derived from S2, S3, and S4 nerve roots from the sacral plexus and they arrive at the external sphincter via the pudendal nerve around the ischial spine at Alcock’s canal. The vagina is closely ...