An anal fistula is almost always the result of a previous abscess. Just inside the anus are small glands. When these glands get clogged, they may become infected and an abscess can develop. A fistula is a small tunnel that forms under the skin and connects a previously infected anal gland to the skin on the buttocks outside the anus.
Surgery is necessary to cure an anal fistula. Fistula surgery usually involves opening up the fistula tunnel. Often this will require cutting a small portion of the anal sphincter, the muscle that helps to control bowel movements. Joining the external and internal openings of the tunnel and con-verting it to a groove will then allow it to heal from the inside out.