Anorectal Fistula



  • An open communication between anal canal and perirectal skin
  • Anorectal fistulas typically form from an abscess of the anal crypt glands.
  • In patients with perianal Crohn disease, perirectal abscesses with spontaneous drainage can evolve into fistulous tracts.
  • The classification of fistulas grades severity and guides treatment.
  • Five subtypes:
    • Submucosal or superficial: The fistula tracks beneath the submucosa and does not involve the sphincter mechanism (not classified under original Park classification).
    • Intersphincteric: The fistula travels along the intersphincteric plane between the internal and external anal sphincters (Park type 1).
    • Transsphincteric: The fistula traverses through the internal and external sphincter (type 2).
    • Suprasphincteric: The fistula originates at the dentate line and loops over the external sphincter to the ischiorectal fossa (type 3).
    • Extrasphincteric (rare): high in the anal canal (proximal to dentate line), does not involve sphincter complex (type 4)
  • Fistulas also classified as low or high based on location:
    • Low fistulas involve the distal 1/3 of the external sphincter muscle.
    • High fistulas involve more of the external sphincter.
  • Fistulas may be simple or complex:
    • Simple fistulas are low and include superficial, intersphincteric, or low transsphincteric fistulas. They also involve only one communicating tract and are not associated with inflammatory bowel disease (IBD) or other organs (bladder or vagina).
    • Complex fistulas are higher along the gastrointestinal (GI) tract, have multiple tracts, involve other organs, are recurrent, or are associated with IBD or radiation.
  • System(s) affected: GI; skin/exocrine
  • Synonym(s): fistula-in-ano; anal fistula


  • True prevalence unknown
  • Mean age of presentation for anal abscess and fistula is ~40 years.
  • Males are twice as likely to develop an abscess and/or fistula.
  • Lifetime risk of developing anorectal fistulas is 20–30% in Crohn disease patients.

Etiology and Pathophysiology

  • Inspissated debris in an obstructed anal crypt gland results in suppuration and abscess formation. Tracking along the path of least resistance in the perianal and perirectal spaces leads to fistula formation.
  • Abscess rupture or drainage leads to an epithelialized tract or fistula formation in ~1/3 of patients.
  • Patients undergoing pelvic radiation are predisposed to fistula formation.
  • Immunocompromised patients with primary perianal actinomycosis can (rarely) develop fistula-in-ano.
  • Anorectal mucosal laceration due to rectal foreign bodies or trauma can cause abscess and fistula formation.

Risk Factors

  • Crohn disease
  • Pelvic radiation
  • Perianal trauma; previous anorectal abscess
  • Pelvic carcinoma or lymphoma
  • Ruptured anal hematoma
  • Abscess formation due to acute appendicitis, salpingitis, or diverticulitis
  • Tuberculosis (rare); syphilis; lymphogranuloma venereum due to Chlamydia trachomatis infection
  • Immunocompromised state (actinomycosis)

General Prevention

  • Perianal hygiene
  • Prevention or prompt treatment of anorectal abscess; management of commonly associated conditions or risk factors

Commonly Associated Conditions

  • Anorectal abscess
  • Crohn disease
  • Diabetes
  • Chronic steroid treatment or immunosuppression

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