Anorectal fistula

What is an anorectal fistula?

An anorectal or anal fistula is an abnormal, infected, tunnel-like passageway that forms from an infected anal gland. Sometimes an anal fistula works its way from an internal gland to the outside of the skin surrounding the anus. On the skin, this looks like an open boil.

What causes an anorectal fistula?

Usually the condition starts with an anal abscess, filled with pus. Even with treatment to drain the abscess, a persistent tunnel can develop between the infected area and the surface of the skin.

Patients with Crohn’s disease have a higher chance of developing anal fistulas due to their increased risk of infection in the gastrointestinal tract.


Anorectal fistulas tend to be painful.

Symptoms include

  • Pain and swelling in the anal area
  • Red, irritated skin around the anus
  • A boil-like opening near the anus, possibly with:
    • Pus discharge
    • Bleeding
  • Fever

If you are experiencing any symptoms of an anorectal fistula you should see a health care provider.


If your doctor suspects you have an anal fistula, the first thing they will do is look for an opening or boil on the skin surrounding the anus. They will also look for signs of an abscess. If a fistula cannot be seen on the skin, your doctor may perform anoscopy to look for the cause of your symptoms.

If you have a visible anal fistula, additional tests may still be required to determine the best treatment method. Tests that can help doctors determine the location, size, and severity of an anal fistula are anoscopies, ultrasounds, and MRIs.  


Surgery is needed to treat an anal fistula. Your surgeon will determine how much of the anal sphincter muscle is affected by the fistula in order to minimize the effect the surgery will have on the opening and closing of the anus.

A fistulotomy opens the length of the fistula tunnel, allowing it to heal from the bottom up. It can be performed if little of the sphincter muscle is involved.

When the fistula involves a significant amount of the anal sphincter, the surgeon will consider other approaches, including the placement of a small band that promotes drainage of the fistula to improve symptoms.

Last reviewed: 
August 2018

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