Anal Fistula

Types

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

• Intersphincteric fistula. The fistula tract begins in the space between the internal and external sphincter muscles and opens very close to the anal opening.
• Transphincteric fistula. The fistula tract begins in the space between the internal and external sphincter muscles or in the space behind the anus. It crosses the external sphincter and opens an inch or two outside the anal opening. These fistulas 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 fistula 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 fistula 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.