An anal fissure is a crack in the lining of the anal canal (the bottom of the back passage). This can occur at any age, but usually occurs in young adults. Fissures may be caused by constipation and passage of hard stool, or diarrhoea and passage of frequent stool. However, the majority seem to develop without any particular cause. The symptoms of a fissure are pain, especially when passing a bowel motion, and some bleeding. Occasionally, people experience discharge of an abscess in association with a fissure.