Table of Contents
Definition / general | Terminology | Epidemiology | Diagrams / tables | Clinical features | Laboratory | Radiology description | Radiology images | Treatment | Clinical images | Gross description | Microscopic (histologic) description | Microscopic (histologic) images | Videos | Additional referencesCite this page: Rishi A, Cornish TC. Fistula. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/anusanalfistula.html. Accessed March 7th, 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, 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
- Intersphinteric: fistula located in intersphincteric plane between external and internal anal sphincters
Epidemiology
- Develop from anal abscess in 25 - 40% cases (Ann Chir Gynaecol 1984;73:219)
- Male to female ratio 1.8:1
- Mean age: 38 - 45 years
Diagrams / tables
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
Treatment
- Surgical resection is the primary form of treatment (Indian J Surg 2012;74:217, Can J Gastroenterol 2011;25:675, Indian J Surg 2012;74:301)
- Medical treatment is directed toward the causative disease; no proven response for fistula itself
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
Videos
Fistula in ano
Additional references