Abscesses and fistulas are commonly classified according to the path taken by the burrowing infection relative to the levator sling and the external anal sphincter (Fig. 100.1A). The most common type of abscess is the perianal abscess, which results from infection tracking down the intersphincteric plane to the perianal skin. These small, relatively superficial abscesses are very close to the anal verge and usually can be drained in the office; any resulting fistula will traverse part of the internal
sphincter and can be opened without fear of incontinence. When the infection tracks laterally across the internal and external sphincters into the ischiorectal fat, an ischiorectal abscess results. Less commonly, infection tracks cephalad, and pus accumulates above the levator sling.

Drainage of a perirectal abscess is generally performed with the patient in the lithotomy position. After administration of general anesthesia, a careful rectal examination is performed. Feel the tissues lateral to the rectum between your thumb (outside the anal canal) and forefinger (within the anal canal) to determine their thickness. Often, even though a mass is not palpable, a thickening in one area may be apparent on careful examination. Prep the area with povidone-iodine solution (Betadine). Perform a proctoscopy if you have not already done so as part of the initial evaluation. Make an incision through the skin over the abscess, as close as possible to the anal verge. (This will help to ensure that, if a fistula results, the tract will be short.) If you are unsure where the abscess is located, aspirate with an 18-gauge needle and syringe before drainage and confirm the presence of pus. Deepen the incision using a hemostat until the cavity of the abscess is encountered. Carefully insert a finger and break up all loculations to drain the entire cavity well (Fig. 100.1B). Irrigate the cavity and pack it with clean packing. If necessary, excise some of the skin edges to provide easier access to a deep cavity.

Submucous abscesses should then be drained into the rectum, rather than externally. To do this, place a retractor in the anal canal, dilating the anus to expose the abscess. Confirm its location by aspiration with a needle and syringe. Incise the mucosa overlying the abscess and allow it to drain into the rectum. If the cavity is large, place a Penrose drain in the abscess cavity to keep it open. Generally, such a drain will be passed within 1 to 2 days.