An anal fistula is a small tunnel (tract) with an internal opening in the anal canal and an external opening in the skin near the anus. It forms when an anal abscess that's drained (either on its own or via surgery) doesn't heal completely.

Anal fistulas are classified by their location in relation to the anal sphincter muscles. They are listed from most common to least common:

Intersphincteric fistula - The tract begins in the space between the internal and external sphincter muscles and opens very close to the anal opening.
Transphincteric fistula - The tract begins in the space between the internal and external sphincter muscles or in the space behind the anus. It then crosses the external sphincter and opens an inch or two outside the anal opening. These can wrap around the body in a U shape, with external openings on both sides of the anus (called a horseshoe fistula).
Suprasphincteric fistula - The tract begins in the space between the internal and external sphincter muscles and turns upward to a point above the puborectal muscle, crosses this muscle, then extends downward between the puborectal and levator ani muscle and opens an inch or two outside the anus.
Extrasphincteric fistula - The tract begins at the rectum or sigmoid colon and extends downward, passes through the levator ani muscle and opens around the anus. These fistulas are usually caused by an appendiceal abscess, diverticular abscess or Crohn's disease.

Anal fistulas are fairly common in people who have had an anal abscess. It's important to treat an anal fistula to reduce the chance of reinfection. Effective treatment also relieves accompanying symptoms. Patients treated for anal fistulas at Mayo Clinic benefit from skilled surgeons experienced in treating the most complex fistulas. Patients also have access to newer treatments, such as fibrin glue.

How is an anal fistula diagnosed?

A history of recurring anal abscess with drainage is a clue that a person may have an anal fistula. The external opening usually appears as a red, inflamed spot that oozes pus, sometimes mixed with blood. Detecting the external opening of an anal fistula is usually easy, while finding the internal opening can be more complicated. Knowing the fistula's complete path is critical for effective treatment.

The location of the external opening gives a clue to a fistula's likely path and sometimes the fistula can actually be felt as a hard cord. In many cases, however, visualizing its path may take various tools, and sometimes the path may not be clear until surgery.

Tools doctors may use include:

Fistula probe - An instrument specially designed to be inserted through a fistula.
Anoscope - A small instrument to view the anal canal.

If a fistula appears particularly complicated or in an unusual place, doctors may also use:

Diluted methylene blue dye - Injected into a fistula to trace its path.
Fistulography - Injection of a contrast solution into a fistula and then X-raying it
Magnetic resonance imaging

To rule out other disorders such as ulcerative colitis or Crohn's disease, doctors may use:

Flexible sigmoidoscopy - A thin, flexible tube with a lighted camera inside the tip allows doctors to view the lining of the rectum and sigmoid colon as a magnified image on a television screen
Colonoscopy - Similar to sigmoidoscopy, but with the ability to examine the entire colon or large intestine

How is an anal fistula treated?

Because anal fistulas pass through the anal sphincter muscles, treatment must be delicately performed to reduce the risk of affecting bowel emptying. Each patient must be individually assessed for the best approach. The goal of treatment is to cure the fistula with as little impact as possible on the sphincter muscles. The plan will depend on the fistula's location and complexity, and the strength of the patient's sphincter muscles.

• Fistulotomy
The surgeon first probes to find the fistula's internal opening. Then the surgeon cuts the tract open, curettes it (scrapes and flushes out its contents), then stitches its sides to the sides of the incision so the fistula is laid open (flattened out).

To treat a more complicated fistula, such as a horseshoe fistula (where the tract extends around both sides of the body and has external openings on both sides of the anus), the surgeon may lay open only the segment where the tracts join and remove the remainder of the tracts.

If a significant amount of the sphincter muscle must be cut, the surgery may be performed in more than one stage. It may also need to be repeated if the entire tract can't be found.

• Seton Placement
The surgeon uses a seton (silk string or rubber band) to either:

Create scar tissue around part of the sphincter muscle before cutting it with a knife
Allow the seton to slowly cut all the way through the muscle over the course of several weeks

The seton can also help the fistula drain.

• Fibrin Glue or Collagen Plug
In some cases, a doctor may use fibrin glue, made from plasma protein, to seal up and heal a fistula rather than cutting it open. The doctor injects the glue through the external opening after clearing the tract and stitching the internal opening closed. The fistula tract can also be sealed with a plug of collagen protein and then closed.