EtiologyAn 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 Perianal Fistulas:
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 fistulae.
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 opening.
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:
If the external opening is anterior to an imaginary line drawn horizontally through the anal canal, the fistula usually runs directly into the anal canal. 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 incontinence.
As with most anorectal disorders, follow-up care includes sitz baths, analgesics for pain, stool bulking agents, and good perianal hygiene.