Anus & perianal area

Other nonneoplastic


Topic Completed: 1 April 2014

Minor changes: 15 October 2021

Copyright: 2002-2021,, Inc.

PubMed Search: Anus perianal fistula [title]

Arvind Rishi, M.D., M.B.B.S.
Toby C. Cornish, M.D., Ph.D.
Page views in 2020: 8,159
Page views in 2021 to date: 10,740
Cite this page: Rishi A, Cornish TC. Fistula. website. Accessed December 3rd, 2021.
Definition / general
  • Abnormal fistulous (hollow like a pipe) communication that opens within anal canal, usually at or above dentate line (primary opening)
  • Primary opening usually leads to skin or may end blindly in perianal soft tissue (most commonly ischiorectal fossae)
  • Multiple secondary openings may branch from primary opening
  • Most common causes are Crohn's disease (complex fistulae with irregular edges), infections (tuberculosis [lung disease usually present] and lymphogranuloma venereum) and rectal foreign bodies
  • Uncommon causes are actinomycosis (rare perianal manifestation, mostly in immunocompromised hosts), radiation proctitis, iatrogenic and fulminant ulcerative colitis (Dis Colon Rectum 2005;48:575, Proc R Soc Med 1970;63:108)
  • Anal canal adenocarcinoma rarely develops in background of chronic anal fistula (Singapore Med J 2012;53:843, Intern Med 2013;52:445)
  • Parks classification of anal fistulae based on anatomical location:
    • Intersphinteric: fistula located in intersphincteric plane between external and internal anal sphincters
      • Tract begins at dentate line and ends at anal verge
    • Transsphincteric: tract goes through external sphincter to ischiorectal fossa
      • External opening is located on skin
    • Suprasphincteric: tract originates higher in anal gland crypt, extends through all sphincter muscles and ends in ischiorectal fossa
    • Extrasphincteric: tract located very high and proximal to dentate line and extends through levator muscles and entire sphincter apparatus
Diagrams / tables

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Parks classification of fistula in ano

Perianal abscesses

Fistulae and ischiorectal abscess

Fistula classification

Clinical features
  • Rectal examination may reveal a firm area or a cord-like thickening under the mucosa
  • Skin with fistulous opening may reveal granulation tissue or discharging abscess
  • There may be associated clinical features secondary to the causative agent
  • Examination of lower gut (anoscopy and proctoscopy) is indicated to evaluate for the location of the primary opening
  • There are no specific laboratory studies for anal fistula
  • Most studies are directed to confirm the causative agent, in the form of serological studies for inflammatory bowel disease, microbial cultures for infectious organisms and metabolic profile for associated comorbidities
  • Immunosuppression should be excluded in cases with longstanding infected fistulae
Radiology description
  • Fistulography: inject contrast through internal opening, followed by radiological imaging to delineate the fistulous tract
  • MRI is recommended for complex and recurrent lesions and has a high concordance rate with operative findings
  • CT scan is more efficient in detecting fistula associated with perirectal disease but requires contrast and provides poor delineation of relationship of fistula with muscle
Radiology images

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Endoanal USG

Inflammatory infiltrate

Clinical images

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Horseshoe fistula in ano

Multiple fistula tracts

Gross description
  • Pathologist most commonly receives a fistula resection specimen which looks either linear or completely maloriented and may have epithelial lining at one of its edges
  • The lining may be skin or anorectal mucosa
  • May be helpful to blunt probe the fistula from the anal mucosal aspect (the primary opening)
  • May be challenging to find the primary opening due to chronicity and scarring; suggest looking at slightly stretched aspect of adjoining mucosa or viewing a small area with dye that was used to track the fistula during the surgery
  • There may be many branched secondary openings and therefore a gentle probing of an unfixed specimen may yield better information
  • Probing should be followed by longitudinal dissection of fistula with pediatric or finer scissors along the inserted probe
  • Reviewing operative notes or preoperative radiological studies may be helpful
  • It is also helpful to photograph the specimen in an unfixed state and pay attention to the mucosa adjoining the fistula to look for friable areas or ulcers in a setting of inflammatory bowel disease
  • Usually there is limited mucosa; if mucosal ulcers are present, then submit the entire area of friability and ulceration to exclude dysplasia associated with inflammatory bowel disease
  • Must sample ulcerated areas to exclude a rare malignancy
Microscopic (histologic) description
  • Histological features vary based on etiology, duration of disease and presence of infection
  • Fistulous tract with mostly fibroconnective tissue with variable scarring, neutrophilic microabscess, inflammatory granulation tissue with reactive endothelial cells, fibroblastic proliferation, granulomas, histiocytic response and foreign body type giant cells
  • Fibroblastic proliferation may be exuberant and mitotically active, resembling pseudosarcoma
  • Focal squamous lining is uncommon
  • Must exclude viral cytopathic effects especially cytomegalovirus (Gastrointest Endosc 1998;47:87, Can J Infect Dis Med Microbiol 2012;23:e41)
  • May obtain special stains for myobacteria and fungal organisms
Microscopic (histologic) images

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Nonspecific inflammatory reaction (not from fistula)


Fistula in ano

Additional references
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