Anorectal disorders range from simple to complex and can manifest signs and symptoms of underlying serious local or systemic disorders that may be life threatening. Precise causes may be difficult to determine; thus a focused history and careful examination can narrow the differential diagnosis and aid timely and appropriate management.
The rectum begins at the S3 vertebral body and descends for about 13 to 15 cm becoming the anus, comprised of the anal canal, anal verge, and anal margin. The rectum narrows and traverses through the muscular pelvic floor, at the level of the levator ani and coccygeal muscles, and becomes the anal canal, 4 cm in length, surrounded by the anal sphincter muscle (Figure 1).
Midsagittal section, anorectum.
The dentate line marks the junction of these two structures as the anal canal continues more distally joining the perianal skin at the anal verge (Figure 2). The anal canal mucosa consists of stratified squamous epithelium and contains no hair follicles or sweat glands. At the anal verge, the anoderm thickens and includes hair follicles and other cutaneous appendages. Proximal to the dentate line, the rectal ampulla narrows to conform to the opening of the anal canal. In doing so, its mucosa takes on a pleated appearance, forming 8 to 14 convoluted longitudinal folds: the columns of Morgagni. Each adjacent column is connected at the dentate line by a flap of mucosa that forms a small anal crypt, normally 1 to 3 mm deep. Anal sepsis, cryptitis, perianal abscesses, and fistulas result from inflammation, obstruction, and infection of the crypts and glands. The anal wall is a continuation of the usual layers of the wall of the colon and rectum, and the innermost mucosal lining continues to the anal verge. Just proximal to the dentate line, the mucosa transitions from rectal columnar to cuboidal to squamous epithelium. The submucosa, which normally contains the bulk of the bowel's blood vessels and autonomic nerves, thickens considerably proximal to the dentate line. The superior hemorrhoidal artery, from the internal mesenteric artery, supplies the proximal two thirds of the rectum, whereas the middle hemorrhoidal artery, from the internal iliac artery, supplies the distal one third of the rectum. The inferior hemorrhoidal artery supplies the anus but also supplies the rectum by a submucosal network. The venous and lymphatic system mirrors the arterial supply. The superior rectal vein drains into the portal system, whereas the middle rectal vein drains into the inferior vena cava. The inner circular muscle layer of the rectum thickens considerably as it terminates distally in the anorectum to form the involuntary internal sphincter muscles. The more attenuated longitudinal muscles of the rectum extend caudally, blending with fibers of voluntary skeletal muscles from the levator ani and external sphincter groups, to form the intersphincteric space (Figure 2).
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