An anal fistula is a medical term for an infected tunnel that develops between the skin and the muscular opening at the end of the digestive tract (anus).

Most anal fistulas are the result of an infection that starts in an anal gland. This infection results in an abscess that drains spontaneously or is drained surgically through the skin next to the anus. The fistula then forms a tunnel under the skin and connects with the infected gland.

Types of Anal fistula

Depending on their relationship with the internal and external sphincter muscles, fistulae are classified into five types:

  1. Extrasphincteric fistulae begin at the rectum sigmoid colon and proceed downward, through the levator ani muscle and open into the skin surrounding the anus. Note that this type does not arise from the dentate line (where the anal glands are located). Causes of this type could be from a rectal, pelvic or supra levator origin, usually secondary to Crohn’s disease or an inflammatory process such as appendiceal or diverticular abscesses.
  2. Suprasphincteric fistulae begin between the internal and external sphincter muscles, extend above and cross the puborectalis muscle, proceed downward between the puborectalis and levator ani muscles, and open an inch or more away from the anus.
  3. Transphincteric fistulae begin between the internal and external sphincter muscles or behind the anus, cross the external sphincter muscle and open an inch or more away from the anus. These may take a ‘U’ shape and form multiple external openings. This is sometimes termed a ‘horseshoe fistula.
  4. Intersphincteric fistulae begin between the internal and external sphincter muscles, pass through the internal sphincter muscle, and open very close to the anus.
  5. Submucosal fistulae pass superficially beneath the submucosa and do not cross either sphincter muscle.

Symptoms and Causes of Anal Abscesses

Symptoms of Anal fistula:

  • Skin maceration
  • Pus, Serous fluid and/or (rarely) feces discharge — can be bloody or purulent
  • Pruritis Ani— Itching
  • Depending on the presence and severity of infection:
  • Pain
  • Swelling
  • Tenderness
  • Fever
  • Unpleasant odor
  • Thick discharge, which keeps the area wet

Causes of Anal fistula:

  • Just inside your anus are several glands that make fluid. Sometimes, they get blocked or clogged. When that happens, a bacteria buildup can create a swollen pocket of infected tissue and liquid. Doctors call this an abscess.
  • If you don’t treat the abscess, it’ll grow. Eventually, it’ll make its way to the outside and punch a hole in the skin somewhere near your anus so the gunk inside it can drain. The fistula is the tunnel that connects the gland to that opening.
  • Most of the time, an abscess causes a fistula. It’s rare, but they can also come from conditions like tuberculosis, sexually transmitted diseases, or an ongoing illness that affects your bowels.

Risk Factors of Anal fistula

Negligence of any anal related symptoms, like burning, bleeding, itching further may develop an infection into anal glands and can lead to abscess formation which in turn leads to fistulae.