Sometimes perianal abscesses do not heal properly and leave an abnormal track running from the inside of the anus or rectum to the nearby skin; this is called a fistula.  Anal fistulas can be complicated because they might incorporate parts of the anal sphincter mechanism which can make treatment difficult with the risk of faecal incontinence.  An anal fistula should ideally be managed by a specialist colorectal surgeon.  Some patients with conditions like Crohn's disease (a type of inflammatory bowel disease) are particularly prone to anal fistula formation.