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. Treatment is necessary to reduce the chances of infection and to alleviate symptoms.
Anal fistulas are classified by their location including:
- 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.
Locating the external opening of an anal fistula is fairly straightforward as it is typically visible and inflamed, oozing with pus, and sometimes mixed with blood. The location of the external opening gives a clue to the likely path of the fistula and sometimes the fistula can actually be felt. However, locating its path visually often requires further investigation. Often the fistula may not be seen until surgery.
Tools often used diagnose a fistula include:
- Fistula probe
An instrument specially designed to be inserted through a fistula
A small instrument to view the anal canal
If the path of an anal fistula is complex or cannot be easily located, the following tests may be done:
- Diluted methylene blue dye injected into the fistula
Injection of a contrast solution into a fistula and then X-raying it
- Magnetic resonance imaging (MRI)
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
Similar to sigmoidoscopy, but with the ability to examine the entire colon or large intestine
Due to the proximity of the anal fistula to the anal sphincter muscles, the surgeon will take all steps avoid impacting normal bowel emptying. The complexity of the surgery depends on the fistula’s location and strength of the patient’s sphincter muscles.
In a fistulotomy, the surgeon first probes to find the fistula’s internal opening. The tract is then cut open, scraped and its contents flushed out. Then its sides are stitched to the sides of the incision 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.
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.