Difference Between Colonoscopy and Sigmoidoscopy

Colonoscopy and sigmoidoscopy are very similar investigations. Sigmoidoscopy allows visualisation of only the distal part of the colon while colonoscopy allows visualisation of the entire large bowel and the distal small bowel, as well. Both investigations involve passing a camera through the anus. Both procedures can be used to take biopsies, conduct small therapeutic procedures, and make a visual diagnosis of bowel conditions. Here, the two investigation methods, colonoscopy and sigmoidoscopy, and the differences between them are discussed in details.


Colonoscopy involves passing a camera or a flexible fibre optic cable through the anus. Many medical associations recommend routine use of colonoscopy to screen for colon cancers. Evidence suggests that the risk of colon cancer is low for the next 10 years if a good colonoscopy does not detect cancers. For a good colonoscopy, the large bowel should be free of solids. The patient should take only clear liquid up to three days before undergoing a colonoscopy. The day before the procedure a laxative-preparation should be administered to clear the bowel. Suppositories only clear the distal portion of the gut while preparations like polyethylene glycol clear the whole large bowel. On the day of the procedure, the patient is sedated with fentanyl or midazolam (most commonly). First the doctor performs a digital rectal examination to assess the adequacy of preparation. Then the camera is passed through the anus up to the caecum and then into the terminal ileum. The camera has many channels for air, suction, light and instruments. Moderate inflation of bowel with air may be needed for better visualization. This may give the patient a feeling of impending bowel motion. Almost always biopsies are taken for histological analysis. The doctors may alter the body position of the patient or press on the abdomen with a hand to guide the colonoscopy properly. On average, the procedure finishes in about 20 to 30 minutes. After the procedure, it takes a little while for the sedation to go away. About one hour may be needed for proper recovery.

A common side effect of colonoscopy is flatulence. The air used to inflate the large bowel for proper visualization comes out as flatulence. The clear advantage of colonoscopy over other less invasive imaging studies is it allows the surgeon to conduct many therapeutic procedures while visually examining the large bowel. Colonoscopy provides a colourful clear picture of lesions in the large bowel as opposed to the monotonic pictures of an MRI or CT. Complications are rare in colonoscopy. Dehydration due to laxatives, perforation of bowel, bowel inflammation resulting in diarrhoea, and flatulence are known complications.


There are two types of sigmoidoscopies. The flexible sigmoidoscopy is useful to visualize the sigmoid colon up to the splenic flexure of the large intestine. Rigid sigmoidoscopy is best for assessment of ano-rectal diseases. The preparation and procedure are the same as in colonoscopy. Procedures such as biopsy, ligation, cauterization, and section can be performed during sigmoidoscopy.

What is the difference between Sigmoidoscopy and Colonoscopy?

• Laxative suppositories may be adequate because only the most distal part of the colon is visualized in sigmoidoscopy while full bowel clearance is needed in colonoscopy.

• Colonoscopy allows visualization up to the terminal ileum while sigmoidoscopy does not.

• Sigmoidoscopy does not need as much sedation as in colonoscopy. Sigmoidoscopy needs less recovery time than colonoscopy.


Read more:

1. Difference Between Colonoscopy and Endoscopy

2. Difference Between Endoscopy and Gastroscopy

3. Difference Between Ileostomy and Colostomy