Anus and perianal area
Inflammatory diseases
Fistula

Author: Arvind Rishi, M.D. (see Authors page)
Editor: Toby Cornish, M.D.

Revised: 26 September 2017, last major update April 2014

Copyright: (c) 2002-2017, PathologyOutlines.com, Inc.

PubMed Search: Anus perianal fistula [title]

Cite this page: Rishi, A. Fistula. PathologyOutlines.com website. http://pathologyoutlines.com/topic/anusanalfistula.html. Accessed December 14th, 2017.
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, Dis Colon Rectum 2005;48:575, Proc R Soc Med 1970;63:108), radiation proctitis, iatrogenic and fulminant ulcerative colitis
  • Anal canal adenocarcinoma rarely develops in background of chronic anal fistula (Singapore Med J 2012;53:843, Intern Med 2013;52:445)
Terminology
  • 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
Epidemiology
Diagrams / tables

Images hosted on other servers:

Parks classification of fistula in ano

Perianal abscesses

Fistulae and ischiorectal abscess

Fistula classification

Areas of involvement

Common sites of anal fistulae

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
Laboratory
  • 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

Images hosted on other servers:

Endoanal USG

Inflammatory infiltrate

Treatment
Clinical images

Images hosted on other servers:

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

Images hosted on other servers:

Nonspecific inflammatory reaction (not from fistula)

Videos


Fistula in ano

Additional references