Colonic bleeding. Endoscope view of a bleeding diverticulum in the sigmoid colon, the part of the large intestine which joins to the rectum. Diverticula are small pouches in the lining of the colon. Their formation is thought to be linked to a low-fibre diet and they are very common in the elderly. Usually they cause no symptoms, although some may cause abdominal cramps and constipation or diarrhoea. Haemorrhaging diverticula such as this cause blood in the faeces (melaena), and persistent blood loss may lead to anaemia. The bleeding usually stops naturally, and a high-fibre diet may also relieve symptoms. Surgery is rarely needed.