When and why

A sigmoidoscopy can help to find out the cause of persistent diarrhoea or of abnormal bleeding from the lower bowel. It is commonly carried out when the results of stool cultures have not been able to identify the cause of diarrhoea.

A rigid sigmoidoscope is the usual tool for rectal biopsy. It can only be passed approximately 15cm into the bowel and looks mainly at the rectum. Occasionally a flexible sigmoidoscope (or colonoscope) is used; this is a flexible tube containing optical fibres.

A laxative is given prior to the procedure to ensure that the views of the bowel are not obscured; a bowel wash-out is usually not required. Small pieces of tissue (biopsies) are normally taken from areas of the lining of the bowel which look abnormal and are sent off to the laboratory to be examined under a microscope.

How it will help

The doctor can see whether there is any inflammation or ulceration of the rectum or colon or lesions which might be the cause of bleeding, such as Kaposi's sarcoma. The biopsy specimens will help diagnose the cause of the diarrhoea which could be cytomegalovirus (CMV) colitis, Cryptosporidium, Mycobacterium avium intracellulare (MAI) infection or Microsporidium.