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Chapter: Medicine and surgery: Gastrointestinal system

Fistula-in-ano - Disorders of the rectum and anus

A fistula is an abnormal communication between one epithelial surface and another.

Fistula-in-ano

 

Definition

 

A fistula is an abnormal communication between one epithelial surface and another. A fistula-in-ano connects the anal canal to the perianal skin.

 

Aetiology

 

Most anal fistulae have no obvious cause. Associations include inflammatory bowel disease, tuberculosis and carcinoma of the rectum.

 

1.     Low anal fistula is the commonest form with a communication from the anal canal below the level of the anal crypts to the perianal skin.

 

2.     High anal fistulas have a track which extends above the pectinate line below the anorectal ring. The muscle fibres of the internal and external anal sphincter surround the rectum. In both low and high fistulas the track of the fistula may pass through the fibres of both sphincters or descend in the intersphincteric space.

 

3.     Anorectal fistula

 

·        Pelvirectal fistula is a direct communication between the rectum and the skin bypassing the anal canal and passing through the levator ani muscle.

 

·        Ischiorectal fistula is similar to a low fistula, but with additional extension upwards towards the rectum without penetrating the levator ani muscle.

 

Pathophysiology

 

Goodsall’s rule states that if the fistula lies in the anterior half of the anal area then it opens directly into the anal canal, while if a fistula lies in the posterior half of the canal then it tracks around the anus laterally and opens into the midline posteriorly.

Clinical features

 

Patients often present with an abscess, the incision of which completes the fistula. Patients with a completed fistula present with a discharging sinus that causes localised pruritus and excoriation.

 

Investigations

 

Proctoscopy may reveal the internal opening with a flexible probe used to demonstrate the track. Sigmoidoscopy is required to exclude associated rectal diseases.

Management

 

Primary fistulas are laid open to granulate and epithelialise. In pelvirectal fistulas such an incision would divide the anorectal ring causing incontinence. These and secondary fistulae are treated conservatively.

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