Anal fistula is an infected tunnel that runs between the skin and the anal opening. It is usually the result of an infection that originates in an anal gland and causes an abscess, a swollen pocket of infected tissue and fluid.
The tunnel formed by the fistula connects with the infected gland. The small glands in the anus produce mucous and sometimes become infected. The infections that do not heal in the correct way can lead to an anal fistula.
The different types of fistula is classified by their location, they are listed in order of 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.