What Is the Difference between a Sigmoidoscopy and Colonoscopy?

Sigmoidoscopy and colonoscopy both provide a doctor with a view of the inside of the colon, with the colonscopy looking at the entire colon and the sigmoidoscopy a view of only the bottom third. A doctor may recommend sigmoidoscopy and colonoscopy to screen a patient for colon cancer, evaluate a patient with a digestive tract disorder, or perform a procedure like polyp removal. Sigmoidoscopies take less time and are less invasive, while the colonoscopy is more thorough and may be more appropriate for certain cases.

Preparation for both sigmoidoscopy and colonoscopy is the same. The patient may need to stop taking certain medications like blood thinners and must spend the day before the procedure on a special diet. The doctor may also prescribe laxatives. The goal of the medication and diet is to clear out the colon so the doctor will be able to get a good picture. This process is often uncomfortable for patients, but necessary for a successful test.

On the day of the sigmoidoscopy or colonoscopy, the patient will go to a hospital or clinic. For a colonoscopy, the patient may be placed under general anesthesia or sedation. Patients scheduled for a sigmoidoscopy may receive light sedation to keep them comfortable. The patient lies in a position that will expose the rectum, and the doctor introduces a long, flexible tube with attached camera. The doctor advances the tube up the colon to evaluate it, and also looks at the colon on the way out.

In sigmoidoscopy and colonoscopy exams, the doctor looks for any signs of abnormalities like patches of tissue that appear different, polyps, pockets, and so forth. If the doctor sees something that looks suspicious, it may be necessary to take a sample. The doctor could recommend a follow-up examination with a rigid tube to allow for polyp removal in some cases. Any material removed during the test can be evaluated by a pathologist to determine if it is cancerous.

The most important difference between sigmoidoscopy and colonoscopy is that the sigmoidoscopy test only looks at one third of the colon, while the colonoscopy provides a complete view. If a doctor has particular concerns about the risk for colon cancer, the colonoscopy may be a better choice for a patient. This test takes longer and is more invasive, but can achieve better results. If any cancerous material is present along the length of the colon, the colonoscopy is more likely to catch it.