An anorectal fistula (Fistula-in-Ano) is an abnormal
communication between the anus and the perianal skin.
Anal canal glands situated at the dentate line afford a
path for infecting organisms to reach the intramuscular spaces.
Fistulas can occur spontaneously or secondary to a perianal (or perirectal)
abscess. In fact, following drainage of a perianal abscess, there is an
approximate 50% chance of developing a chronic fistula.
Other fistulae develop secondary to trauma, Crohn's
disease, anal fissures, carcinoma, radiation therapy, actinomycoses,
tuberculosis, and chlamydial infections.
The cryptoglandular hypothesis states that
an infection begins in the anal gland and progresses into the muscular wall of
the anal sphincters to cause an anorectal abscess. Following surgical or
spontaneous drainage in the perianal skin, occasionally a granulation
tissue-lined tract is left behind, causing recurrent symptoms.
Fistulas are named according to the Park's Classification of
Transsphincteric fistulae are the result of
ischiorectal abscesses, with extension of the tract through the external
sphincter. Account for about 25% of all fistulae.
Intersphincteric fistulae are confined to the
intersphincteric space and internal sphincter. They result from
perianal abscesses. Account for about 70% of all fistulae.
Suprasphincteric fistulae are the result of
supralevator abscesses. They pass through the levator ani muscle, over
the top of the puborectalis muscle, and into the intersphincteric space.
Account for about 5% of all fistulae.
Extrasphincteric fistulae bypass the anal canal and
sphincter mechanism, passing through the ischiorectal fossa and levator ani
muscle, and open high in the rectum. Accounts for about only 1% of all
Patients with Fistula-in-Ano present with chronic drainage of
pus or stool from the skin opening. The tracts do not heal on their own,
and surgery to correct the problem is necessary. Patients may also
complain of pain, swelling, or excoriation at the tract site, and a history of
IBD, diverticulitis, previous radiation therapy, steroid therapy, or HIV
infection may suggest a more complex fistula.
Physical exam remains the mainstay of diagnosis. The
physician should observe the entire perineum, looking for an external opening
that appears as an open sinus or elevation of granulation tissue.
Discharge from the tract may be spontaneous or expressible with a digital rectal
examination. Anoscopy is usually required to identify the internal
In order to help the examiner predict the trajectory of the
tract, and probable location of the internal opening, Goodsall's Rule can be
applied. With the patient in the lithotomy position:
the external opening is anterior to an imaginary line drawn horizontally
through the anal canal, the fistula usually runs directly into the anal
If the external opening is
posterior to the line, the fistula usually curves to the posterior midline
of the anal canal.
It should be noted, however, that the further away the external
opening is from the anus, the less reliable Goodsall's rule becomes.
Additionally, the trajectory of a complex fistula is unpredictable.
The laying open technique (fistulotomy) is useful in the
majority of fistulae repairs. In this procedure, a probe is inserted
through the fistula (through both openings), and the overlying skin,
subcutaneous tissue, and sphincter muscle are divided, thereby opening (or
unroofing) the tract. Curettage is used to remove granulation tissue in
the tract base. Care is taken to avoid cutting too large a portion of the
sphincter (which could lead to incontinence). The fistulotomy is allowed
to close by secondary intention.
In patients with complex fistulae, recurrent fistulae, Crohn's
disease, immunocompromised states, or with poor preoperative sphincter
pressures, a seton can be used alone, or in combination with a fistulotomy.
A seton is typically made from a large silk suture, silastic
vessel marker, or rubber band, that is threaded through the fistula tract and
serves three purposes. It allows direct visualization of the tract, it
allows drainage and promotes fibrosis, and it also cuts through the fistula.
That is, with time, as fibrosis occurs above the seton, it gradually cuts
through the sphincter muscle, and exteriorizes the tract. The seton is
tightened during repeat office visits until it is pulled through over 6-8 weeks.
The advantage of using a seton, is that this "staged fistulotomy" allows for
progressive division of the sphincter muscle, avoiding the complication of
As with most anorectal disorders, follow-up care includes sitz
baths, analgesics for pain, stool bulking agents, and good perianal hygiene.