<p> Anal fistula (plural fistulae), or fistula-in-ano, is a chronic
abnormal communication between the epithelialised
surface of the anal canal and (usually) the perianal skin.
An anal fistula can be described as a narrow tunnel with its
internal opening in the anal canal and its external opening
in the skin near the anus. Anal fistulae commonly occur in
people with a history of anal abscesses. They can form
when anal abscesses do not heal properly.
Anal fistulae originate from the anal glands, which are
located between the internal and external anal sphincter
and drain into the anal canal. If the outlet of these glands
becomes blocked, an abscess can form which can
eventually extend to the skin surface. The tract formed by
this process is a fistula.
Abscesses can recur if the fistula seals over, allowing the
accumulation of pus. It can then extend to the surface
again - repeating the process
Depending on their relationship with the internal and external sphincter
muscles, fistulae are classified into five types:
Extrasphincteric fistulae begin at the rectum or 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 supralevator origin, usually secondary to
Crohn's disease or an inflammatory process such as appendiceal or
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.
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