What is an anal fistula?

A fistula is an abnormal connection between two organs or two tissue surfaces. An anal fistula (fistula-in-ano) is a connection between the anal canal and the skin. In order to understand the cause, treatment, and complications of therapy for fistula-in-ano, an understanding of the anatomy of the anal canal is necessary.

Anatomy

138The anal canal is the end (terminal) portion of the gastrointestinal tract. Two rings of muscle, the internal anal sphincter and external anal sphincter, surround the anal canal.

The internal anal sphincter is composed of smooth muscle and is not under voluntary control. The external anal sphincter is composed of skeletal muscle and is under voluntary control. Together these two muscles are very important in the maintenance of continence.

Approximately one to two centimeters inside the anal canal, the lining of the anal canal changes. There is a line that marks this change called the dentate line. Anal glands located between the layers of the internal and external anal sphincters empty into the anal canal at the level of the dentate line.

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