Anal fistula or fistulain ano is an abnormal communication between the anorectal canal and the perianal skin that is lined with granulation tissue.
It is almost always a consequence of an anorectal abscess that was drained. While the abscess represents the acute phase of the disease, fistula represents the chronic phase.
How does it develop?
Infection of glands in the intersphincteric space of the anal canal is thought to underlie both acute anorectal abscesses and anal fistulas —the ‘cryptoglandular hypothesis’
How are fistulas classified?
A variety of classification systems have been described, but the most useful and widely accepted classification is that described by Parks. This classification system is based on the relation between the primary track—the main tunnel that constitutes the fistula—and the sphincter muscles around the anal canal.
Do not traverse the external sphincter, run along the longitudinal muscle layer between the internal and external sphincters towards the perineal skin.
Fistulas track upwards through the intersphincteric space and then arch downwards and cross the levator muscle, reaching the ischiorectal fossa and then the skin.
The track crosses the external sphincter into the ischiorectal fossa before heading down to the perineum
Have a more proximal origin, and cross the levator muscles to reach the ischiorectal fossa. There is no involvement of the anal canal or the anal sphincter complex.
Considering the origin of the disease, anal fistulas may be classified as:
MRI is considered the “gold standard” for imaging fistula anatomy.
The cumulative incidence of anal fistula in patients with Crohn’s disease is 20-25%.
Fistulas are often complex and multiple; this makes the treatment difficult. In this cases use of the anti-tumour necrosis factor α antibody, infliximab is considered then first line treatment.
Surgical options are considered if medical treatment fails, but because of the poor rate of wound healing in active Crohn’s disease, a defunctioning colostomy is a more common strategy.
Tuberculosis may be the cause of anal fistula in some cases.
Tuberculosis should be suspected in patients who fail to respond to standard treatment or who develop recurrent fistulas. Diagnosis is made through the histological finding of granulomatous disease and the positive identification of acid fast bacilli.
Antituberculous drugs are the first line treatment.