Anatomy

The anorectum is an anatomic structure in which the entodermal intestine unites with and opens into an orifice of ectodermal origin: the anal canal. The junction of these two embryonic structures (the anorectal line) is the dentate line, which marks the anatomic beginning of the anal canal (1 to 2 cm long) and is in continuity with the perianal skin at its distal 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 (perianal region), 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 in longitudinal depth. Infection and inflammation of these crypts and glands become the source of anal sepsis, as characterized by the development of cryptitis, fissures, abscesses, and fistulas.

The anal wall, from its mucosal lining to the intersphincteric plane, which separates the internal from the external sphincters, 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, undergoing a transition just proximal to the dentate line 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; its dilated veins in this area are referred to as the internal hemorrhoidal plexus. Likewise, the inner circular muscle layer of the rectum thickens considerably as it terminates distally in the anorectum to form the internal sphincter muscles, while 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 plane (Fig 78-1).

Additional sphincteric support is provided by an outer layer of voluntary skeletal muscles, the external sphincters, which are divided into three parts: deep, superficial, and subcutaneous. The external sphincters are actually a caudal extension of the puborectalis muscle, which interacts with the levator ani muscle forming the pelvic floor. The puborectalis, the proximal external sphincters, and the internal sphincters form the ring of muscles that one palpates when performing a digital examination of the anorectum.

Lateral to the external sphincters is the ischiorectal space, and superior to the levator ani is the pelvirectal space, where deep, life-threatening infections can occur. EXAMINATION OF THE PATIENT

No matter how much historical information is obtained, no definitive diagnosis can be made without a careful examination of the anus and rectum, including anoscopy and, if necessary, proctoscopy.

The lateral, or Sims, position, performed with the patient lying on his or her left side with the left leg extended and the right knee and hip flexed, is probably the most commonly used approach for performing a routine digital rectal examination and is the preferred position for elderly or pregnant patients. From the Sims position, one should elevate the upper right buttock to provide better exposure of the perianal area; if needed, endoscopic examination of the anus and distal rectum can be performed with the patient in this position. In debilitated patients, one may have to perform the examination with the patient in a supine, lithotomy position.

Examining a patient placed in the knee-chest position requires a cooperative patient who is not too ill or in too much distress. Ihis provides for a thorough inspection of the perianal area and is convenient for anoscopy and proctoscopy. Ihighs should be at right angles to the table with the feet extended over the end of the table.

A digital examination should always be performed before doing any endoscopic procedure. No bowel preparation is needed to perform an anoscopic examination. After performing a digital examination and determining that the patient will tolerate passage of an anoscope, introduce a well-lubricated, lighted anoscope, remove the obturator, and gently rotate it 360° to view the anorectum circumferentially.

It is usually difficult to perform a proper sigmoidoscopic examination in an emergency department (ED) setting. Ordinarily, the lower bowel must be prepped; a natural bowel movement, spontaneous or induced 1 to 2 h before examination, is usually sufficient preparation. In some acute situations, such as trying to determine the source of lower GI bleeding or obtaining cultures in a case of suppurative proctitis, emergency proctoscopy may be performed. A rigid sigmoidoscope should be utilized, with the patient placed in a Sims position. An inexperienced endoscopist should not attempt to pass the sigmoidoscope beyond the rectosigmoid junction, where the lumen is greatly angulated, because of the risk of perforation.

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