A sigmoidoscopy is a tool physicians use to screen for colon and rectal cancer. You can think of a sigmoidoscopy as the little brother of a colonoscopy, which is the most familiar and widely used procedure for colon cancer screening.

Like a colonoscopy, sigmoidoscopy examines the colon using a lighted tube and camera. While a colonoscopy looks at the entire length of the colon, a sigmoidoscopy only examines the lower third, called the sigmoid colon or descending colon (the top third is the ascending colon and the middle is the transverse colon). Sigmoidoscopies can detect inflamed tissues and abnormal growths that could develop into tumors and ulcers. Physicians can perform a sigmoidoscopy in their office quickly and easily, so it's less expensive than a full colonoscopy.


Before undergoing a sigmoidoscopy, patients must have one or more enemas two hours prior to the procedure to remove solid wastes from the lower colon. An enema flushes water or a laxative into the anus to clean the colon. Some patients may be required to empty their entire gastrointestinal tract by adhering to a clear liquid diet for one to three days before the procedure. This is comparable to the preparation for a colonoscopy. In some cases, patients may also take a laxative or have another enema just before the procedure.


Unlike colonoscopy, patients undergoing a sigmoidoscopy remain awake. The patient lies on his left side and tucks his knees up to his chest. The physician first conducts a digital rectal exam by inserting a gloved finger into the rectum to check for blockages and to dilate the anus before inserting the scope. After insert inserting a lighted scope and camera, the physician views the images from the camera on a computer screen to check for polyps or abnormalities. If the patient has polyps, the physician can insert a tool to remove them or take sample tissue for further testing.

Patients do not typically feel pain during a sigmoidoscopy, although some patients report feeling a sense of pressure, bloating, cramping or the need to move their bowels. The procedure only takes about 20 minutes.

How effective is it?

A study by the National Cancer Institute found that the colonoscopy is more effective than a sigmoidoscopy in detecting cancer. The sigmoidoscopy misses two-third of existing tumors in women, twice as many as the procedure misses in men.