Anus & perianal area

Other nonneoplastic


Editorial Board Member: Maryam Kherad Pezhouh, M.D., M.Sc.
Deputy Editor-in-Chief: Catherine E. Hagen, M.D.
Fahire Goknur Akarca, M.D.
Kwun Wah Wen, M.D., Ph.D.

Last author update: 18 August 2022
Last staff update: 18 August 2022

Copyright: 2002-2024,, Inc.

PubMed Search: Anus fistula

Fahire Goknur Akarca, M.D.
Kwun Wah Wen, M.D., Ph.D.
Page views in 2023: 19,744
Page views in 2024 to date: 3,460
Cite this page: Akarca FG, Khorsandi N, Wen KW. Fistula. website. Accessed February 26th, 2024.
Definition / general
  • Anal fistulas are epithelialized tracts created between an external opening in the perianal skin and an internal opening in the anal canal
  • Primary opening usually leads to skin or may end blindly in perianal soft tissue (most commonly ischiorectal fossae)
Essential features
  • Benign epithelialized communication between anal canal and perianal skin that is most commonly idiopathic
  • Other names: fistula in ano, cryptoglandular (anal) fistula
  • Parks classification of anal fistulae based on anatomical location (Br J Surg 1976;63:1):
    • Intersphincteric: fistula located in intersphincteric plane between external and internal anal sphincters
      • Tract begins at dentate line and ends at anal verge
    • Transsphincteric: tract communicates 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
ICD coding
  • ICD-10:
  • ICD-11:
    • DB50.1 - anal fistula
    • DB50.2 - anorectal fistula
    • DB50.Y - other specified fissure or fistula of anal regions
    • DB50.Z - fissure of fistula of anal regions, unspecified
  • Most anal fistulas are idiopathic
  • 15 - 38% of anal fistulas develop from anal abscesses
  • Occurs in males at a rate 2 - 4 times higher than females
  • Mean age of occurrence is 40 years with a range of 20 - 60 years (Dis Colon Rectum 2009;52:217)
  • Recent smoking increases risk of development of anal fistula (Dis Colon Rectum 2005;48:575)
  • Anal canal and external perianal region
  • Infection and occlusion of the anal glands by gut specific bacteria leads to abscess formation → inflammation and granulation tissue → epithelialized connection between anal canal and external perianal area
Diagrams / tables

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Perianal abscesses

Fistulae and ischiorectal abscess

Fistula classification

Clinical features
  • Typical presentation includes itching, drainage, discomfort and possible pain with defecation
  • Small opening outside the anus, with or without visible drainage, may be seen
    • Around the opening, there may be hypertrophied tissue, which is suggestive of a developed tract; this is sometimes palpated on the digital rectal exam
  • There may be associated clinical features secondary to the causative agent (inflammatory bowel disease, radiation therapy, etc.) (StatPearls: Anorectal Fistula [Accessed 8 July 2022])
  • Examination of the anal area (fistula probe, anoscopy / proctoscopy, ultrasound, MRI) is essential to evaluate for the location of the primary opening
  • Presence of fistula opening in the anal area with the clinical features (listed above) confirms the diagnosis
  • No specific laboratory tests
  • Serological studies for inflammatory bowel disease, microbial cultures for infectious organisms and metabolic profile for associated comorbidities can be used for causative agents
Radiology description
  • Radiologic imaging is important in determining the patient's surgical treatment
  • MRI clearly shows the relationship of fistulas to the pelvic diaphragm (levator plate) and the ischiorectal fossae; this relationship has important implications for surgical management and outcome and has been classified into 5 MRI based grades:
    • If the ischioanal and ischiorectal fossae are unaffected, disease is likely confined to the sphincter complex (simple intersphincteric fistulization, grade 1 or 2); outcome following simple surgical management is favorable
    • Involvement of the ischioanal or ischiorectal fossa by a fistulous track or abscess indicates complex disease related to transsphincteric or suprasphincteric disease (grade 3 or 4); correspondingly more complex surgery may be required that may threaten continence or may require colostomy to allow healing
    • If the track traverses the levator plate, a translevator fistula (grade 5) is present and a source of pelvic sepsis should be sought (Radiographics 2000;20:623)
Radiology images

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Endoanal USG

Inflammatory infiltrate

Prognostic factors
  • Features of complex fistulas or those that are at high risk of failing traditional treatment with a primary fistulotomy (Surg Clin North Am 2010;90:45):
    • Fistulas that anatomically are suprasphincteric, high transsphincteric (> 30% of sphincter), extrasphincteric according to the Parks classification
    • Recurrent fistulas
    • Fistulas with multiple tracts
    • Females with anterior fistulas
    • Fistulas related to inflammatory bowel disease, infectious diseases, with or without radiation
    • Anal incontinence
    • Rectovaginal fistulas
Case reports
  • 26 year old woman with a complex perianal fistula with actinomyces spp. present in anal cytology (ACG Case Rep J 2017;4:e82)
  • 47 year old man with a 20 year history of multiple chronic perianal fistulas with extensive squamous cell carcinoma arising from perianal fistula (J Crohns Colitis 2013;7:e232)
  • 53 year old man with a 20 year history of anal fistula with implanted rectal adenocarcinoma (Surg Today 2006;36:747)
  • Exam under anesthesia is the most common first step of a suspected fistula, to determine complexity and characteristics
  • Simple fistula is treated with a primary fistulotomy with incision made along fistula tract for curettage and to promote adequate tract drainage and wound healing; this is very curative
  • Any complex fistulas must employ approaches that spare the sphincter based on the anatomy of the fistula and clinical features (Surg Clin North Am 2010;90:45)
Clinical images

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Horseshoe fistula in ano

Multiple fistula tracts

Gross description
  • Typically unoriented, irregular shaped soft tissue excision with an inflamed sinus opening on one surface
  • When the probe followed from the sinus opening, cut surface shows the sinus tract
  • Hemorrhage, abscess with yellow-green pus or scarring may be seen around the sinus tract
Microscopic (histologic) description
  • Histological features vary based on etiology, duration of disease and presence of infection / abscess
  • Fibroconnective tissue usually shows variable scarring with mixed acute and chronic inflammation
  • If it is inflamed, microabscess, inflammatory granulation tissue with reactive endothelial cells, fibroblastic proliferation, granulomas, histiocytic response and foreign body type giant cells can be seen
  • Granulomas and giant cells may require special stains for mycobacteria and fungal organisms
Microscopic (histologic) images

Contributed by Fahire Goknur Akarca, M.D. and Kwun Wah Wen, M.D., Ph.D.
Epithelized anal fistula tract

Epithelized anal fistula tract

Granulation tissue with inflammatory cells Granulation tissue with inflammatory cells

Granulation tissue with inflammatory cells

Giant cells, in association with granulation tissue Giant cells, in association with granulation tissue

Giant cells, in association with granulation tissue


Perianal fistula

Fistula in ano

Sample pathology report
  • Anal fistula, excision:
    • Benign anal mucosa with acute inflammation, consistent with anal fistula (see comment)
    • Comment: The H&E sections show benign anal mucosa with acute inflammation, fat necrosis and foreign body giant cell reaction. The special stains for AFB and GMS are negative for acid fast bacillus and fungal microorganisms, respectively. Overall, the histological findings are consistent with anal fistula.
Differential diagnosis
  • Hidradenitis suppurativa (HS):
    • Chronic, autoinflammatory skin disease in the intertriginous body areas, such as the axillae and inguinal areas but also the buttocks and perianal area
    • Presents with recurring abscesses, inflammatory nodules and sinus tracts
    • When located in the perianal area, it may be difficult to distinguish from the anal fistula (Int J Colorectal Dis 2019;34:1337)
    • Histologic findings are similar with the perianal fistula (chronic inflammation, granulation tissue and epithelized tract)
    • Transperineal ultrasound and anal ultrasound may be helpful to show the internal and external orifices of the perianal fistula and to distinguish it from hidradenitis suppurativa (Postepy Hig Med Dosw (Online) 2012;66:838)
  • Infected cysts (e.g., sebaceous, Bartholin) with draining tracts:
    • Location of the cysts and the absence of a sinus tract helps to differentiate the diagnosis
  • Sinus tracts from trauma or foreign body:
    • Obtaining a clinical history, detailed examination and careful observation allows for detection of a foreign body or trauma history
  • Specific infections (tuberculosis, actinomycosis, lymphogranuloma venereum, gonococcal infection, etc.):
    • Blood cultures are helpful if there is any suspicion of a specific infection
    • Granulomatous inflammations (necrotizing or nonnecrotizing) should be evaluated carefully for infectious causes
Board review style question #1

What is the term for this epithelialized connection between the anal canal and perianal skin seen in the photo?

  1. Anal fistula
  2. Hidradenitis suppurativa
  3. Infected pilonidal cyst
  4. Subcutaneous abscess
Board review style answer #1
A. Anal fistula. An anal fistula is an epithelialized connection between the anal canal and perianal skin.

Comment Here

Reference: Fistula
Board review style question #2
What is the most common cause of anal fistula?

  1. Adenocarcinoma
  2. Fungal infection
  3. Idiopathic
  4. Pelvic radiation
Board review style answer #2
C. Idiopathic. The most common etiology of anal fistulas is idiopathic. Less common causes include inflammatory bowel disease, infections, radiation and trauma, among others.

Comment Here

Reference: Fistula
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