Anal Fistula

An anal fistula is an abnormal communication between the innermost lining of the anal canal and the perianal skin.

Anal fistulae are formed as a result of an infection of the anal glands which are located between the two layers of the anal sphincters. The glands usually secrete mucus to lubricate stool. When the outlet of these glands becomes blocked, an abscess can form which can eventually point to the skin surface. The communication between the opening on the skin where the pus drains and the internal opening where the blockage occurred previously forms the anal fistula tract.

After drainage of such perianal abscesses, about 50% of wounds heal without any further problem. The other 50% of patients will either experience recurrence of the abscess or persistent discharge from the wound. This is usually related to an underlying anal fistula.

The persistent discharge from an anal fistula can give rise to persistent “wetness” in the perianal area, discomfort and itchiness. In addition, recurrent abscesses may lead to significant pain and provides a source for systemic spread of infection.

Surgery is considered essential in the treatment of anal fistulae. A perianal abscess should be drained in a timely fashion. Repair of the fistula is an elective procedure. The type of procedure performed depends on the anatomy of the anal fistula tract.

For a simple fistula, i.e. the fistula tract only incorporates superficial lower part of anal sphincters, an anal fistulotomy can be performed. This involves defining the tract with a probe and then dividing the anal muscles down onto the probe. In most instances, the risk of faecal incontinence is low as only the most superficial muscle fibres are divided.

For a complex fistula, i.e. one that incorporates significant amount of anal sphincters or involving the upper half of anal muscles, a seton is usually inserted as a temporary measure to control the sepsis. A definitive operation will then be contemplated at a later stage.

There are many operations described for the definitive treatment of complex fistulae. Dr Chew’s current preference is anal sphincter advancement flap. Anal sphincter advancement flap is an operation that Dr Chew designed in 2005 and the initial results were published in an American journal titled Diseases of the Colon and Rectum in 2007. Please go to to read the journal article.