Internal Medicine

Anorectal fistula

Anal/anorectal fistula or fistula-in-ano is an inflammatory tract or connection between the anal canal and the perianal skin.


Anal/anorectal fistula or fistula-in-ano is an inflammatory tract or connection between the anal canal and the perianal skin.


Parks-Gordon classification:

characterized by their tract location relative to the internal and external sphincters
  • Intersphincteric fistula (M/C, 45%): Fistula penetrates through the internal sphincter but spares the external sphincter.
  • Transsphincteric (ischiorectal) fistula (30%): Fistula passes through both the internal and external sphincters
  • Suprasphincteric fistula (20%): Fistula penetrates through internal sphincter and then extends superiorly in the plane between the sphincters to pass above the external sphincter before extending to the perineum. This classification includes horseshoe abscesses.
  • Extrasphincteric fistula (5%, rare): Forms connection from rectum to perineum that extends laterally to the internal and external sphincter
Fistula classification. A Submucosal (low fistula not involving muscle.) B Intersphincteric fistula. C Trans-sphincteric fistula. D Supersphincteric fistulae. E Extrasphincteric fistula. |, 2010 (



  • F: Foreign body
  • R: Radiation
  • I: Infection
  • I: Inflammatory Bowel Disease (IBD) (25% cases associated with Crohn disease)
  • E: Epithelialization
  • N: Neoplasm
  • D: Distal obstruction (cryptoglandular theory)

Anorectal abscess:

Perirectal abscesses and fistulas represent the acute and chronic manifestations of the same disease process, an infected anal gland.
  • Perianal abscess: Infection extends between internal and external sphincter, reach the anal verge
  • Ischiorectal abscess: Abscess ruptures through the external sphincter
  • Supralevator abscess: Abscess extends cephalad in the rectal wall resulting in a high intermuscular abscess and extension of abscess above the levators
  • Deep postanal abscess: Extend to either or both ischiorectal fossae resulting in a horseshoe abscess
High abscesses. 1, supralevator; 2, submucosal/intermuscular. Low abscesses. 3, intersphincteric; 4, ischiorectal; 5, perianal. | Russell K. Pearl, M.D., Department of Surgery, University of Illinois at Chicago.)


Anal fistula is an epithelialized connection between the anal canal and external peri-anal area, is characterized by inflammatory tissue and granulation tissue. The distal obstruction prevents the fistula from healing. Because cells are continually being turned over, there is constant debris in the fistula tract, which causes obstruction and prevents healing.

Simple anal fistulas:

Due to glandular obstruction resulting in an anorectal abscess and, ultimately, a fistula. Characteristics of a simple fistula include a single tract, subcutaneous tract, and those that involve less than 30% of the external sphincter.

Complex anal fistulas:

Anal fistulas involving > 30% of the external sphincter, fistulas with multiple tracts, recurrent fistulas, and those associated with other predisposing factors, including Crohn disease and radiation treatment.

Clinical features


Principle symptom of anorectal abscess
  • Itching, drainage, discomfort, and pain with defecation
An anal fistula at operation. The skin abnormality to the left of the anus is the external opening of the fistula. | (2020) Anal Fistula | Brisbane, Australia. Retrieved November 27, 2020, from

Associated features:

  • Low (intersphincteric, perianal, and ischiorectal) abscesses: Associated with swelling, cellulites, and exquisite tenderness, but few systemic symptoms
  • High (submucosal, supralevator) abscesses: Few local symptoms, but significant systemic (fever, toxicity) symptoms


Goodsall’s rule:

A fistula with the external opening anterior to an imaginary transverse line across the anus has its internal opening at the same radial position and for an external opening posterior to this line, the internal opening is in the midline posteriorly with a horse-shoe track.
Diagram showing fistulous tract as per Goodsall’s rule | Shindhe, P., & Killedar, R. (2018). Substantiating accuracy of Goodsall’s rule in fistula-in-ano (Bhagandar) with anatomical consideration. Indian Journal of Health Sciences and Biomedical Research (KLEU), 11(2), 151–154.


Assists in the identification of fistulous tracts and occult abscesses as well as characterizing proximity of tracts to the internal and external sphincters to coordinate effective planning.
Axial T2-weighted images with fat suppression (STIR) at two different levels (a, b). The dotted arrows demonstrate the inter-sphincteric fistula and its extension to the skin (solid arrow) | Torkzad, M. R., & Karlbom, U. (2010). MRI for assessment of anal fistula. Insights into imaging, 1(2), 62–71.

Differential diagnosis:

Other causes of anal pain
  • Anal fissure
  • Thrombosed hemorrhoids
  • Levator spasm
  • Sexually transmitted disease (STDs)
  • Proctitis
  • Cancer


Abscess management:

The goal of surgical therapy of an abscess is to drain the abscess expeditiously, drain any associated sepsis in adjacent anatomic spaces, identify a fistula tract and either proceed with primary fistulotomy to prevent recurrence (if sphincterotomy is deemed safe) or mark the fistula track with a loose seton for future consideration.

Fistula management:

The goal of surgical therapy of a fistula is to define the anatomy accurately, drain associated sepsis (undrained abscess), eradicate the fistula tract if possible, prevent recurrence, and preserve sphincter integrity and continence. Fistulotomy is preferable to fistulectomy. Excision of the entire fistula tract is not only unnecessary, but also will result in a wider and deeper gap in the sphincter mechanism and worsening fecal incontinence.
  • Fistulotomy (gold standard for acute anal fistula): Entailing of opening the fistula tract and possibly dividing sphincter muscle
  • Endorectal advancement flap (ERAF): Closure of the internal opening of the tract, debridement of the tract, and mobilization of anorectal mucosa to cover the defect
  • Seton drain placement (for complex fistulas and used in a two-stage technique): Initial placement used to gain source control, while the second stage includes fistulotomy
  • Ligation of intersphincteric fistula tract (LIFT) (for treatment of simple and complex fistulas): Identification of internal opening with suture ligation of the intersphincteric portion of the fistula. The tract and infected gland are then excised and the wound debrided with curettage
  • Anal fistula plug (AFP) (not effective): Collagen matrix used to block/plug the internal opening of the fistula tract

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