Sigmoidoscopy: Purpose, procedure and how it compares to a colonoscopy

An anoscopy examines the anus, and a colonoscopy examines the colon. So, what does a sigmoidoscopy examine? Hint: the answer is in the name.

A sigmoidoscopy – sometimes called a flexible sigmoidoscopy – is a procedure doctors use primarily to see inside the rectum and sigmoid colon.

You may be wondering what conditions a sigmoidoscopy can help diagnose and how a sigmoidoscopy differs from a colonoscopy. Here, I’ll answer these questions and more.

Why would a doctor do a sigmoidoscopy?

Your doctor may perform a sigmoidoscopy if you have symptoms like lower rectal bleeding, lower abdominal pain or a sudden change in bowel habits.

A sigmoidoscopy uses a flexible scope that is inserted into your anus and then slowly moved through your rectum and into the lowest part of your colon, known as the sigmoid colon. It is done primarily to examine the rectum and the last part of your colon.

What conditions does a sigmoidoscopy help diagnose?

A sigmoidoscopy can help diagnose many conditions, including

• Cancer
• Polyps
• Diverticulosis
• Diverticulitis
• Inflammatory bowel diseases (Crohn’s disease and ulcerative colitis)

Do you need bowel prep before a sigmoidoscopy?

Usually, you will have an enema the night before your appointment and an enema the morning of your appointment. An enema is when you inject fluid into your bottom to help cleanse the rectum and colon of any stool.

Be sure to tell your doctor if you are on blood thinners, as you may need to stop taking them before the procedure.

Your doctor will give you specific instructions to follow before your sigmoidoscopy.  

What happens during a sigmoidoscopy?

A sigmoidoscopy is done as an outpatient procedure performed in the doctor’s office. This means you will not need to stay overnight.

You will not be sedated or given anesthesia, so you will be awake during the procedure.

During a sigmoidoscopy, your doctor will insert a lubricated, flexible scope into your anus and carefully guide it through your rectum and into your colon. Using the scope, your doctor will put air into your colon to help it expand. This makes it easier for your doctor to see inside.

If it doesn’t hurt when you have a bowel movement, this should feel no different. You may feel some slight discomfort, like gas cramps, but it should not be painful. If the discomfort becomes too much, your doctor can withdraw the air or even stop the procedure.  

Most sigmoidoscopies take about five minutes for the doctor to examine inside the colon, but it could take longer if it’s harder to see your colon or if your doctor removes polyps or takes some tissue to biopsy.

Getting results

Your doctor can usually give you the results of your sigmoidoscopy right after the procedure.

Normal

A normal result means that your doctor didn’t find any problems in the tissue or lining of your rectum or sigmoid colon.

Abnormal

If the results are abnormal, this means that your doctor found something abnormal during the exam. Abnormalities could be polyps or signs of other conditions, such as diverticulitis.

If a small polyp or tissue was removed to biopsy, that will be reviewed by a pathologist. Your doctor will share those results with you when available and discuss next steps. If you have not had a colonoscopy in the last one to two years, you will need one so your doctor can examine your entire colon and remove more polyps, if needed.

What is recovery like after a sigmoidoscopy?

You can resume your normal diet and activities the same day as your sigmoidoscopy.  

It’s normal to have a little gas after the procedure, but that should resolve on its own after a few hours.

There is a very small risk of rectal bleeding after a sigmoidoscopy, particularly if you’ve had a biopsy. But that is extremely rare.

What is the difference between a sigmoidoscopy and a colonoscopy?

Both a sigmoidoscopy and a colonoscopy use a scope to examine the colon. But they are very different procedures. Here’s how they differ.

What they look for

A colonoscopy allows doctors to see the lining of the entire colon. A sigmoidoscopy primarily looks at the sigmoid colon, or the lowest part of the colon.

Bowel prep

The bowel prep required for a colonoscopy is more involved than the prep required for a sigmoidoscopy. You’ll likely be on a diet of clear liquids 24 hours before a colonoscopy, as well as drink a prep solution that works as a laxative to clean the stool out of your colon.

Anesthesia

You will be sedated or given anesthesia before a colonoscopy, so you will be asleep during the procedure. You’re awake during a sigmoidoscopy.

Time

Because doctors are looking at more of the colon, a colonoscopy takes longer than a sigmoidoscopy. Typically, a colonoscopy takes between 20 and 30 minutes.

Recovery

Because you are put to sleep during a colonoscopy, you’ll spend about 30 to 45 minutes in a recovery area after the procedure. You will need someone to drive you home, and you should not drive or operate heavy machinery for the rest of the day. Most people are able to return to their normal diet right away and resume normal activities the next day.

Can a sigmoidoscopy replace a colonoscopy for colorectal cancer screening?

No. A colonoscopy is still the recommended screening exam for colorectal cancer. You should get your first colonoscopy at age 45 if you are at average risk for colorectal cancer. If no polyps are found during your initial colonoscopy, then you wouldn’t need your next colonoscopy for another 10 years.

If you’re at higher risk for colorectal cancer, you should begin getting a colonoscopy earlier than age 45.

People at average risk for colorectal cancer who are younger than 45 may opt for a sigmoidoscopy and submit a fecal immunochemical test, or your doctor may perform a sigmoidoscopy to look at a specific part of your colon.  

It’s important to note that if your doctor finds any signs of cancer during a sigmoidoscopy, you will need to have a colonoscopy. A colonoscopy is still the gold standard for colorectal cancer screening, and sigmoidoscopy does not replace that.  

Craig Messick, M.D., is a colon and rectal surgeon at UT MD Anderson.

Request an appointment at UT MD Anderson online or call 1-877-632-6789.