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 beginning of any evaluation of anorectal problems is the examination; therefore clinicians need to understand the anatomy. The normal anatomic relationships of the rectum and pelvis are important in understanding pelvic floor abnormalities and anorectal pathology. The rectum normally lies attached to its mesorectum within the curve of the sacrum with limited mobility. The junction of the rectosigmoid is most consistently found at the sacral promontory and descends only 2 or 3 cm during a Valsalva maneuver. The rectum exits the pelvis anteriorly surrounded by a sling of muscle from the pubis through a slit in the pelvic floor. The sling is created by the horseshoe-shaped puborectalis muscle that circles 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 anal outlet. The anal canal itself measures 3–4 cm and is a funnel-shaped extension of the pelvic floor musculature. The pressure generated by this voluntary muscle prevents egress of rectal contents. The internal sphincter muscle is a continuation of the thickened circular muscle of the rectum. As such, it is an autonomic muscle and has no voluntary control.
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. These fibers then descend to the superior hypogastric plexus located 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 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 bladder dysfunction, impotence, or both.
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. 39-1). The 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 uterus and vagina are closely approximated to the anterior surface of the rectum but not attached. There is no ligamentous suspension of the rectum or the uterus at the lower aspect of the pelvis. The slit-like defect in the pelvic floor through which the rectum passes also provides an outlet for the vagina ...