Colonoscopy and Sigmoidoscopy for IBS

Reviewed by: HU Medical Review Board | Last reviewed: May 2023

Colonoscopy and sigmoidoscopy are medical procedures that allow a doctor to look at a person’s large intestine (colon). In both, a flexible, narrow tube with a small camera at the end is inserted into a person’s rectum to view the inside of their colon. These exams can help doctors understand whether a person’s symptoms are caused by irritable bowel syndrome (IBS) or something else.1,2

When are these procedures used?

Doctors use these procedures to check for problems in the colon, including inflammation. They can also use them to screen for colon polyps and colorectal cancer. And they may use them to help diagnose IBS if other tests have not proved helpful.1,2

When a person complains of symptoms such as diarrhea, constipation, bloating, or abdominal pain and gas, a doctor may order a colonoscopy or sigmoidoscopy. The exam will help them determine whether the cause is IBS or a different medical issue.1,2

Colonoscopy and sigmoidoscopy may be used in cases of:2

  • Changes in bowel activity, such as constipation or diarrhea
  • Stomach pain
  • Bleeding from the anus
  • Blood in the poop (stool), or black, tarry stool
  • Unexplained weight loss
  • Abnormal findings from other diagnostic tests such as a lower GI series (barium enema)
  • Screening for colon polyps and colorectal cancer

What to expect with a sigmoidoscopy versus a colonoscopy

The colon is about 5 feet long in adults and is the last section of the digestive system. This is where liquid from waste materials is absorbed to form stool.3

The colon must be cleaned out prior to both procedures. This is called bowel preparation, or bowel prep. This generally requires taking a liquid medicine that will clean out your bowels. You must also stick to a clear liquid diet – not eating any solid foods – for a period of time before the procedure. Your doctor will tell you how long is appropriate.2,3

If you are scheduled for either procedure, you will receive additional bowel prep instructions from your doctor. For example, you may be asked to stop taking certain medicines before the exam. Make sure to follow these instructions carefully.2,3

During the exam itself, you will receive medicine given intravenously (with an IV) to sedate you. Most people do not feel anything during the procedure. The doctor places the scope into the anus, and air is delivered through the scope into the colon to make it easier to see.2,3

The doctor may decide to take a tissue sample (biopsy) or remove any growths on the lining of the colon or rectum (polyps). They may take photos of the inside of the colon using the scope as well.2,3

The difference between sigmoidoscopy and colonoscopy

To understand the difference between these 2 exams, you need to understand the anatomy of the colon. The colon contains the following main parts:3

  • Ascending colon – This is the first part of the colon. It attaches to the small intestine and extends upward on the right side of the body.
  • Transverse colon – This part extends from the ascending colon horizontally across the body to the left side.
  • Descending colon – This section extends from the transverse colon downward on the left side of the body.
  • Sigmoid colon – This section is S-shaped and connects the descending colon with the rectum, which ends in the anus.

The 2 procedures check for many of the same things, but they are slightly different. A sigmoidoscopy focuses on the sigmoid part of the colon. A sigmoidoscopy is less invasive and requires less bowel prep than a colonoscopy.2,3

During a colonoscopy, the entire length of the colon is screened for abnormalities. A longer tube with a camera at the end, called a colonoscope, is used.2

Additional tests for diagnosing IBS

A diagnosis of IBS is usually made through a variety of tests. These tests help your doctor determine whether symptoms are caused by IBS or something else. Other tests they may use include a physical exam with a symptom history, stool analysis, blood tests, breath tests, and a lower GI series.4

Colonoscopy and sigmoidoscopy are procedures that are usually done when other tests have been inconclusive or to rule out other conditions. Talk with your doctor if you think you need one of these exams. They will be able to recommend what is best based on your symptoms and medical history.4

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