An anal fistula is a small tunnel that runs from an opening inside the anal canal to an outside opening in the skin near the anus. An anal fistula often results from a previous or current anal abscess. As many as 50 percent of people who have an abscess get a fistula, but a fistula can also occur independently.

If the small glands just inside the anus become blocked, an infection may result. When the infection is serious, an abscess (a pocket filled with pus) may form. A fistula may form to drain the abscess. Crohn’s disease, cancer, trauma and radiation increase the risk of infections and fistulas.

An abscess may cause pain, redness or swelling around the anal area or canal. Other common signs include running a fever, having chills, and feeling ill or tired. Patients with fistulas have similar symptoms, as well as drainage from an opening near the anus. A fistula is suspected if these symptoms tend to recur in the same area every few weeks.

Most anal abscesses or fistulas are diagnosed and managed based on a physical exam. Occasionally, imaging studies such as ultrasound, CT scan or MRI assist in diagnosing and managing deeper abscesses, and may be used to visualize the fistula.

For most patients, an abscess can be drained surgically in a simple procedure. Surgery is nearly always needed to treat an anal fistula. If the fistula is not too deep, the fistula track will be opened to allow healing from the bottom up. The complexity of the surgery depends upon how much the sphincter muscle is affected.

Antibiotics alone are not effective in treating an anal abscess or fistula.

Unfortunately, despite proper treatment and complete healing, an abscess or a fistula can recur.