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Anorectal infections are common problems presenting to the Emergency Department. Understanding anorectal anatomy is essential to make a diagnosis, institute proper treatment, and anticipate complications. Failure to diagnose and treat an extensive abscess may be life threatening. Surgical consultation is imperative if the extent of an abscess is unclear.

Anorectal infections occur mostly in the third or fourth decade of life. Perianal abscesses (Figure 135-1) are two to three times more common in men than women.1,2 Male predominance is even more pronounced in the pediatric population.3 In one series, all patients under 2 years of age and 60% greater than 2 years were males.3 The increased incidence of perianal infection in males may be related to androgen conversion in the anal glands.4 Deep anal crypts are associated with perianal abscesses in infants.5

FIGURE 135-1.

A perianal abscess. (Used with permission from Hornez E, et al: Surgical proctologic emergency in isolated sea-based environment: how is it performed in the French navy. Mil Med 2013; 178(4):e498-e502.)

Abscesses may completely resolve after proper incision and drainage. Approximately 50% recur or develop a chronic epithelialized tract or fistula-in-ano. Abscesses and fistulas are different sequelae of the same process.6


Knowledge of the anatomy of the region is important to understand the pathophysiology of anorectal infections (Figures 135-2 and 135-3). Columnar epithelium transitions to squamous epithelium at 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 the ducts of the anal glands drain. The anal glands are positioned circumferentially in the anal canal and secrete mucus to aid in the evacuation of feces. The anal glands are 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.7

FIGURE 135-2.

The anatomy of the anal canal.

FIGURE 135-3.

The major structures of the anal canal.

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 135-4).6 The perianal space is located at the area of the anal verge. The ischiorectal space is continuous with the perianal space and extends from the levator ani muscle to the perineum. The submucosal space can be visualized with the use of anoscope. The intersphincteric space lies between the internal and external anal ...

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