Anorectal Abscesses

Abscesses are common in the perianal and perirectal regions, as are fistulas, which are common sequelae. Almost all begin with involvement of an anal crypt and its gland. From there, the infection can progress to involve any of the potential spaces that are normally filled with fatty areolar tissue and have little inherent resistance to the progression of infection. These spaces, which can become infected alone or in combination with each other, are as follows: the perianal space, the intersphincteric space, the ischiorectal space, the deep postanal space (connecting the ischiorectal space on each side posteriorly), and the supralevator or pelvirectal space (Fig 78:4 and Fig 78-5).

Deep Postanal Space

FIG. 78-4. Illustration of mechanism for anorectal abscess and fistula formation.

Postanal Abscess
FIG. 78-5. Anatomical classification of common anorectal abscesses.

The perianal abscess is the most common anorectal abscess and occurs when pus spreads caudally between the internal and external sphincters. It presents close to the anal verge, post midline, as a superficial tender mass, which may or may not be fluctuant. In contradistinction, ischiorectal abscesses tend to be larger, indurated, and to present more laterally, on the medial aspect of the buttocks. Deeper perirectal abscesses may not manifest cutaneous signs, but rectal pain and tenderness are invariably present. The isolated perianal abscess not associated with deeper, perirectal abscess(es) is the only type of anorectal abscess that can be adequately treated under local anesthesia in an ED setting.

Ischiorectal and other deep abscesses pose a different problem. The ischiorectal fossa forms a large potential space on either side of the rectum, communicating behind it through the deep postanal space, and, in males, has extensions anteriorly above the perineal membrane to the prostate. Infections in this area are insidious and extensive and can point in an area some distance from the anal verge. These abscesses can be large, and yet only a diffuse, nonfluctuant, tender "mass" is palpable either through the rectal wall or the overlying skin. If only induration is present, endorectal ultrasonography 3 and/or needle localization under anesthesia may be needed to confirm the diagnosis.

Most abscesses in the anorectal area are the result of obstruction of an anal gland that opens in the base of an anal crypt and normally drains into the anal canal. When obstruction occurs, the gland orifice is blocked, resulting in infection and abscess formation. An element of cryptitis can frequently be identified by anoscopic examination. A variety of diseases are associated with the development of fistulous abscesses, including Crohn's disease, carcinoma of adjacent organs, Hodgkin disease, tuberculosis, and gonococcal proctitis.

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