Anal fistulas are generally common among those who have had an anal abscess. Treatment is usually necessary to reduce the chances of infection in an anal fistula, as well to alleviate symptoms.

An anal fistula is defined as a small tunnel with an internal opening in the anal canal and an external opening in the skin near the anus. Anal fistulas form when an anal abscess, that's drained, doesn't heal completely.

Different types of anal fistulas are classified by their location.
In order of most common to least common, the various types include:

  • 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.

Diagnosis

It is usually simple to locate the external opening of an anal fistula, meanwhile locating the internal opening can be more challenging. It is important to be able to find the entire fistula for effective treatment. 

People who may have experience with recurring anal abscesses may have an anal fistula . The external opening of the fistula is usually  red, inflamed, oozes pus, and is sometimes mixed with blood.

The location of the external opening gives a clue to a fistula's likely path and sometimes the fistula can actually be felt. However, locating its visual path often requires various tools, and often times it may not be seen until surgery.

Tools often used in diagnosis 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 is potentially complicated or in an unusual place, these tools may also be used:

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

Tools used to rule out other disorders such as ulcerative colitis or Crohn's disease include:

  • 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 

Treatment

Treatment is delicately performed to reduce the risk of affecting bowel emptying, due to the anal fistulas' proximity to the anal sphincter muscles. The best approach requires that each patient is assessed individually.

Treatment of an anal fistula is attempted with as little impact as possible on the sphincter muscles. It will often depend on the fistula's location and complexity, and the strength of the patient's sphincter muscles.

Fistulotomy

In a fistulotomy the surgeon first probes to find the fistula's internal opening. Then the tract is cut open and is scraped and its contents are flushed out, then its sides are stitch to the sides of the incision in order to lay open the fistula.

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), is treated by usually laying open just the segment where the tracts join and the remainder of the tracts are removed.

The surgery may be performed in more than one stage if a large amount of muscle must be cut. The surgery may need to be repeated if the entire tract can't be found.

Advancement Rectal Flap

A surgeon may core out the tract and then cut a flap into the rectal wall to access and remove the fistula's internal opening then stitches the flap back down. This is often done to reduce the amount of sphincter muscle to be cut.

Seton Placement

A seton (silk string or rubber band) is used 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 may also aid in the drainage of the fistula.

Fibrin Glue or Collagen plug

In some cases, fibrin glue, made from plasma protein, may be used to seal up and heal a fistula as opposed to cutting it open. The glue is injected through the external opening after clearing the tract and stitching the internal opening closed. A plug of collagen protein may also be used to seal and close the fistula tract.