• Clinical science

Anal abscess and fistula


Anal abscesses are the acute manifestation of a purulent infection in the perirectal area, while anal fistulas are the chronic manifestation of such infections. An anal abscess is a pus-filled cavity that most commonly develops from an infected anal crypt gland following obstruction and bacterial overgrowth. Less common causes for the formation of anorectal abscesses are inflammatory bowel disease, acute gastrointestinal infections (e.g., appendicitis), radiation-induced proctitis, or malignancy. An anorectal abscess may heal spontaneously following drainage into the anal canal. In about 30–60% of cases, anal abscesses progress into fistulas, which are ductal connections between the abscess and the anal canal or the perianal skin. Complications of abscesses and fistulas involve chronic tissue damage, fecal incontinence, and sepsis. Patients with an anal abscess present with anorectal pain, a palpable tender mass on digital rectal examination, and fever in more advanced cases. Patients with anal fistulas may present with a visible perianal site draining pus and discomfort during defecation. Imaging studies such as CT, MRI, or anal ultrasonography are only needed for extended abscesses or complex fistulas. Definitive management of an anal abscess and fistula involves surgical treatment. Abscesses are incised and drained, followed by open wound healing. The standard treatment option for anal fistulas is fistulotomy.


  • Sex: > (2:1)
  • Age: mean of 40 (range 20–60 years)
  • Incidence: approx. 100,000 people per year in the US
  • ∼ 50% of patients with anorectal abscess develop fistulas.


Epidemiological data refers to the US, unless otherwise specified.




Anal abscesses and fistulae may be classified according to their variations in anatomical position and distribution.

  • Abscesses
    • Perianal (most common): abscess beneath the perianal skin, does not transverse the external sphincter
    • Ischiorectal: abscess below the levator ani muscle
    • Intersphincteric: abscess between the internal and external sphincters
    • Supralevator (least common): abscess above the levator ani muscle
  • Fistulas
    • (Park's classification)
      • Intersphincteric (Park's Type I)
        • Course: along the anatomical space between the internal and external sphincter
        • Opening: perianal skin (anoderm) and/or rectum
      • Transsphincteric (Park's Type II)
        • Course: penetrates the external sphincter ani muscle into the ischiorectal fossa
        • Opening: perianal skin (anoderm)
      • Suprasphincteric (Park's Type III)
        • Course: ascends caudally within the intersphincteric space, penetrates the levator ani muscle and follows caudally towards the ischiorectal fossa
        • Opening: perianal skin (anoderm)
      • Extrasphincteric (Park's Type IV)
      • Subcutaneous/subanodermal/submucosal
        • Course: beneath the perianal/anal skin/rectal mucosa
        • Opening: perianal skin (anoderm)/anal canal/rectum


  • Typical development
    • Obstruction of anal glands by thick debris → stasis and bacterial overgrowth abscess formation
    • Abscess may extend into adjacent perirectal spaces → possible fistula formation , bacteremia and sepsis
  • Rare forms of development: Pathophysiology and localization depend on the specific comorbidities (e.g., Crohn's disease); also, see “Less common causes” under etiology above.


Clinical features

  • Abscesses:
    • Perianal abscess
      • Dull perianal discomfort and pruritus
      • Erythematous, subcutaneous mass near the anus found by manual inspection
    • Perirectal abscess
      • Rectal or perirectal drainage (bloody, purulent, or mucoid)
      • Severe pain, fever, and chills
    • Pain exacerbation with sitting and defecation
  • Fistulas:
    • Purulent drainage (from anal canal or surrounding perianal skin)
    • Pain during defecation
  • Digital rectal examination: fluctuant, indurated mass, pain with pressure



  • CT/MRI or anal ultrasonography: confirmatory tests for deeper abscesses
  • Further testing: to identify possible fistulae (risk of recurrence) and comorbidities (malignancy, IBD)
    • Endoscopy
    • MRI
    • Fistula probe (with methylene blue)
  • Anal manometry (as a preoperative evaluation)
  • Localization of pathology in lithotomy position

Invasive examinations are painful and can only be tolerated by the patient while under anesthesia or with adequate pain relief!



  • Early surgical incision and drainage
  • 4/5 of the sphincter muscles may be incised without any loss of bowel continence because of the presence of the puborectal sling
  • Obtain pus for culture
  • Postoperative
    • Sitz baths
    • Analgesics and stool softeners


Postoperative complications