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Anorectal abscess classification

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anorectal abscess classification perianal ischiorectal intersphincteric supralevator ASCRS

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anorectal abscess classification

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Anorectal abscesses are classified by the anatomical space involved:
  1. Perianal abscess
    • Located in superficial tissue at the anal verge.
    • Most common type.
    • Usually presents as a tender, fluctuant swelling near the anus.
  2. Ischioanal (ischiorectal) abscess
    • Extends through the external sphincter into the fat-filled ischioanal fossa.
    • Produces a deeper, often larger and more diffuse buttock/perianal swelling.
  3. Intersphincteric abscess
    • Located between the internal and external anal sphincters.
    • May be difficult to detect externally and can cause deep anal pain.
  4. Supralevator abscess
    • Located above the levator ani muscle.
    • Can result from upward extension of an intersphincteric abscess, or from pelvic disease such as Crohn disease, diverticulitis, pelvic sepsis, or malignancy.
    • Least common of the main types and requires careful imaging and drainage planning.
  5. Submucosal abscess
    • Lies beneath the anorectal mucosa.
    • Uncommon.
  6. Horseshoe abscess
    • Not a separate primary space, but a pattern of circumferential extension around the anal canal or rectum, often involving both ischioanal fossae through the deep postanal space.
A traditional surgical text lists approximate frequencies: perianal 60%, ischiorectal 20%, intersphincteric 5%, supralevator 4%, and submucosal 1% (Fischer's Mastery of Surgery, 8th ed., p. 4246). The ASCRS classification summary commonly emphasizes the first four categories and horseshoe extension.
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