Anal fistula is one of the common anorectal diseases. An anal fistula is a small tunnel (tract) with an internal opening in the anorectal canal and an external opening in the skin near the anus. It usually occurs after an anal abscess.

The diagnosis of anal fistula can be easily made by detecting the external opening of an anal fistula. Anal fistulas are classified by their location in relation to the anal sphincter muscles (intersphincteric transphincteric, suprasphincteric, and extrasphincteric fistula). Doctors may use anoscope and a probe to identify the fistula path. If a fistula appears particularly complicated or in an unusual place, doctors may also do the fistulography and colonoscopy for further evaluation.