Passage forming a connection between the anal mucous membrane and the skin around the anus, usually caused by an abscess directly above the anus. Rectal fistulas can be caused by other conditions such as ulcerative colitis, Conn’s syndrome, tuberculosis of the rectum or a tumour. A fistula is characterized by an opening in the skin producing a discharge (pus, for example). It is not usually very painful, but pain can be increased by defecation. An X-ray using a contrast medium can be used to make the fistula visible. Treatment is by opening or removing the fistula under general anaesthetic.