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Anorectal disorders range from simple to complex, may be varied and multiple, and, at times, can manifest signs and symptoms of underlying serious local or systemic disorders that may be life-threatening. It may be difficult to determine a precise cause for anorectal disorders. Pertinent history and a careful examination can narrow the differential diagnosis and aid timely and appropriate management.

The rectum is an anatomic structure that begins at the S3 vertebral body and descends for about 13 to 15 cm. This entodermal intestine unites with and opens into an orifice of ectodermal origin, the anal canal, which is about 4 cm long. (Figure 88-1).

Figure 88-1.

Midsagittal section of the anorectum.

The junction of these two embryonic structures is the dentate line, which marks the anatomic beginning of the anal canal and is in continuity more distally with the perianal skin at the anal verge. The mucosa of the anal canal consists of stratified squamous epithelium but contains no hair follicles or sweat glands. At the anal verge, the anoderm thickens and includes in its structure 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 separates the internal from the external sphincters and is a continuation of the usual layers of the wall of the colon and rectum. The innermost lining, the mucosa, 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 hemorrhoidal arteries supply the anorectum, whereas the venous network in this area is the internal hemorrhoidal plexus. The superior (internal) hemorrhoidal veins drain into the portal system, whereas the inferior (external) hemorrhoidal veins drain 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 88-2).

The external sphincters, voluntary skeletal muscles, are actually a caudal ...

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