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Anorectal infections are common but vary widely in their presentation. An understanding of the anorectal anatomy is essential to make a diagnosis and plan proper treatment. Failure to diagnose and treat an extensive abscess may be life threatening. It is imperative to obtain a surgical consultation if one is unsure of the extent of an abscess.

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The peak age of incidence of anorectal infections is in the third or fourth decade of life. Perianal abscesses are two to three times more common in men than women.1 Male predominance is even more pronounced in the pediatric population.2 In one series, all patients under 2 years of age were males, while 60 percent of the children greater than 2 years were males.2 The increased incidence of perianal infection in males may be related to androgen conversion in the anal glands.3 In infants, deep anal crypts are associated with perianal abscesses.4

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Abscesses may completely resolve after the incision and drainage procedure. However, 50 percent recur or develop a chronic epithelialized tract or fistula-in-ano. Abscesses and fistulas are different sequelae of the same process.5

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Knowledge of the anatomy of the region is important to understand the pathophysiology of anorectal infections (Figures 93-1 and 93-2). Columnar epithelium transitions to squamous epithelium where there are vertical folds of tissue called the columns of Morgagni at the level of the dentate line. Semilunar folds of epithelium called anal valves connect the inferior borders of the anal columns. At the base of each anal valve is an anal crypt, into which opens the ducts of the anal glands. The anal glands secrete mucous to aid in evacuation of feces. The anal glands are located in the space between the internal and external anal sphincter muscles. Most anorectal infections begin in this intersphincteric space due to blockage and resultant infection of the anal glands.6

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Figure 93-1
Graphic Jump Location

The topical anatomy of the anal canal.

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Figure 93-2
Graphic Jump Location

The major supporting structures of the anal canal.

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The spread of an infection is determined by the anatomy of the anorectal region. There are five anatomic spaces into which an infection can spread (Figure 93-3).5 The perianal space is located at the area of the anal verge. The ischiorectal space, which is continuous with the perianal space, extends from the levator ani muscle to the perineum. The intersphincteric space lies between the internal and external anal sphincter muscles. It connects inferiorly with the perianal space and superiorly with the rectal wall. The supralevator (or pelvirectal) space is located superior to the levator ani muscle and is bounded superiorly by the peritoneum. The rectum forms its medial border and the pelvic wall forms the lateral boundary. ...

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