What is an anal fistula?
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:
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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:
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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:
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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:
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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.