Sigmoidoscopy vs. colonoscopy: What’s the difference?

Both sigmoidoscopy and colonoscopy examine the lining of the colon, but the colonoscopy can show more of the colon.
Both sigmoidoscopy and colonoscopy examine the lining of the colon, but the colonoscopy can show more of the colon.

Both sigmoidoscopy and colonoscopy are nonsurgical procedures that involve use of a tube (endoscope) with a light and camera to examine parts of the large intestine (colon).  

A sigmoidoscopy is a procedure in which a flexible tube connected to a fiberoptic camera is used to examine the inner lining of the rectum and the lower part of the colon. The difference between a sigmoidoscopy and colonoscopy is that a colonoscopy examines the upper part of the colon as well. Colonoscopies are usually preferred over sigmoidoscopies since they allow a doctor to check both the upper and lower parts of the large intestine in one procedure.

In a flexible sigmoidoscopy, the endoscope is a flexible tube, while in a rigid sigmoidoscopy the tube is firm. Flexible sigmoidoscopy is more commonly used as a diagnostic tool because it’s more comfortable for the patient and it is easier to perform procedures such as biopsy and polypectomy (removal of a polyp, or small growth in the lining of the intestine). A rigid sigmoidoscopy is useful for some colorectal surgical procedures.

A sigmoidoscopy or a colonoscopy usually take approximately 10-30 minutes to complete, unless there are complications or further investigations with biopsies or polyp removal is necessary.

Both sigmoidoscopy and colonoscopy require preparation to clear out the colon. Most sigmoidoscopy and colonoscopy preparations involve drinking large amounts of cleansing solutions (such as MiraLAX), along with laxatives, enemas, and possibly several days of a clear liquid diet prior to the procedure. During the procedure, sedatives are usually given to relax the patient and reduce pain.

What is the preparation for sigmoidoscopy vs. colonoscopy?

Sigmoidoscopy

In order to obtain accurate results, the rectum and the lower colon must be completely clean of stool. Your doctor will give you detailed instructions on how to cleanse your colon. In general, this requires the use of one or two enemas prior to the procedure and may also call for a laxative and some dietary modifications. Under special circumstances, such as the presence of significant diarrhea, the preparation may be waived.

In general, you can continue to take your regular medicines. You should, however, inform your doctor of all the prescription and non-prescription medications you are taking as well as any allergies you may have. Certain drugs increase the risk of bleeding if biopsies are performed; these include aspirin, blood thinners such as warfarin (Coumadin), and NSAIDs such as Motrin and Advil. Your doctor may ask you to stop these medications for several days before the procedure. You should also alert your doctor if you have an artificial heart valve, hip or knee prosthesis, or have a disease of the heart valves such as mitral stenosis, aortic stenosis, or mitral regurgitation. Patients with these conditions may need antibiotics prior to colonoscopy, or dental procedures to prevent infection of the heart valves or the prosthesis.

Colonoscopy

If the procedure is to be complete and accurate, the colon must be completely cleaned, and there are several different colonoscopy preparations. Patients are given detailed instructions about the cleansing preparation. In general, this consists of drinking a large volume of a special cleansing solution or several days of a clear liquid diet and laxatives or enemas prior to the examination. These instructions should be followed exactly as prescribed or the procedure may be unsatisfactory (visualization of the lining of the colon may be obscured by residual stool), and it may have to be repeated, or a less accurate alternative test must be performed in its place.

Instructions may also be given to avoid certain foods for a couple of days prior to the procedure, such as stringy foods, foods with seeds, or red Jell-O.

Most medications should be continued as usual, but some may interfere with the examination. It is best if the colonoscopist is informed of all current prescription and over-the-counter medications. Aspirin products, blood thinners such as warfarin (Coumadin), arthritis medications, insulin, and iron preparations are examples of medications that may require special instructions. The colonoscopist will also want to be aware of a patient's allergies and any other major illnesses. The colonoscopist should be alerted if, in the past, patients have required antibiotics prior to surgical or dental procedures to prevent infections.

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What happens during that endoscopy and colonoscopy procedures?

Sigmoidoscopy

Flexible sigmoidoscopy is generally well tolerated and rarely causes any significant pain. There may be a sensation of fullness, bloating, pressure, or cramping during the procedure. In most instances, you will be lying on your left side while the instrument is advanced through the rectum and the colon under direct vision on a TV monitor. As the instrument is withdrawn, a careful examination is made of the lining of the colon. The procedure usually takes only 5 to 15 minutes.

If the doctor finds an area in the colon that needs further evaluation, a biopsy (small sample of tissue) can be obtained and sent to the pathology department for examination under a microscope. If a polyp is found, the doctor may remove the polyp at the same time. Polyps are small growths on the inner lining of the colon and the rectum. Most polyps are benign (not cancerous), but some polyps are precancerous. Patients with precancerous polyps are usually asked to return for a colonoscopy after a more vigorous colon cleansing. Colonoscopy is a longer version of flexible sigmoidoscopy where the doctor examines the entire length of the colon. For more, please read the Colonoscopy article. The advantage of colonoscopy over flexible sigmoidoscopy is the ability to find and remove polyps in the parts of colon that are beyond the reach of the flexible sigmoidoscope. Removal of all of the precancerous polyps during colonoscopy has been shown to prevent colon cancer.

Colonoscopy

Prior to colonoscopy, intravenous fluids are started, and the patient is placed on a monitor for continuous monitoring of heart rhythm and blood pressure as well as oxygen in the blood. Medications (sedatives) usually are given through an intravenous line so the patient becomes sleepy and relaxed, and to reduce pain. If needed, the patient may receive additional doses of medication during the procedure. Colonoscopy often produces a feeling of pressure, cramping, and bloating in the abdomen; however, with the aid of medications, it is generally well-tolerated and infrequently causes severe pain.

Patients will lie on their left side or back as the colonoscope is slowly advanced. Once the tip of the colon (cecum) or the last portion of the small intestine (terminal ileum) is reached, the colonoscope is slowly withdrawn, and the lining of the colon is carefully examined. Colonoscopy usually takes 15 to 60 minutes. If the entire colon, for some reason, cannot be visualized, the physician may decide to try colonoscopy again at a later date with or without a different bowel preparation or may decide to order an X-ray or CT of the colon.

If an abnormal area needs to be better evaluated, a biopsy forceps can be passed through a channel in the colonoscope and a biopsy (a sample of the tissue) can be obtained. The biopsy is submitted to the pathology laboratory for examination under a microscope by a pathologist. If infection is suspected, a biopsy may be obtained for culturing of bacteria (and occasionally viruses or fungus) or examination under the microscope for parasites. If colonoscopy is performed because of bleeding, the site of bleeding can be identified, samples of tissue obtained (if necessary), and the bleeding controlled by several means. Should there be polyps, (benign growths that can become cancerous) they almost always can be removed through the colonoscope. Removal of these polyps is an important method of preventing colon and rectal cancer, although the great majority of polyps are benign and do not become cancerous. None of these additional procedures typically produces pain. Biopsies are taken for many reasons and do not necessarily mean that cancer is suspected.

What are the risks of colonoscopy?

Sigmoidoscopy

Flexible sigmoidoscopy and biopsy are generally safe when the procedure is performed by properly trained individuals. Possible but rare complications include a perforation (making a hole in the wall of the colon) and bleeding from the biopsy site. The former may call for surgery.

Although complications following flexible sigmoidoscopy are rare, it is important to recognize the early signs of any possible complication. Contact your physician or the examiner if you notice any of the following:

Colonoscopy

Complications of colonoscopy are rare and usually minor when performed by physicians who have been specially trained and are experienced in colonoscopy.

Bleeding may occur at the site of biopsy or removal of polyps, but the bleeding usually is minor and self-limited or can be controlled through the colonoscope. It is quite unusual to require transfusions or surgery for post-colonoscopic bleeding. An even less common complication is a perforation or a tear through the colonic wall, but even these perforations may not require surgery.

Other potential complications are reactions to the sedatives used, localized irritation to the vein where medications were injected (leaving a tender lump lasting a day or two), or complications from existing heart or lung disease. The incidence of all of these complications together is less than 1%.

While these complications are rare, it is important for patients to recognize early signs of a complication so that they may return to their physicians or an emergency room. The colonoscopist who performed the colonoscopy should be contacted if a patient notices severe abdominal pain, rectal bleeding of more than half a cup, or fever and chills.

Colonoscopy is the best method available to detect, diagnose, and treat abnormalities within the colon. The alternatives to colonoscopy are quite limited. Barium enema is a less accurate test performed with X-rays. It misses abnormalities more often than colonoscopy, and, if an abnormality is found, a colonoscopy still may be required to biopsy or remove the abnormality. At times, an abnormality or lesion detected with a barium enema is actually stool or residual food in a poorly cleansed colon. Colonoscopy may then be necessary to clarify the nature of the lesion. Flexible sigmoidoscopy is a limited examination that uses a shorter colonoscope and examines only the last one-third of the colon.

Patients will be kept in an observation area for an hour or two post-colonoscopy until the effects of medications that have been given wear off. If patients have been given sedatives before or during colonoscopy, they may not drive, even if they feel alert. Should patients have some cramping or bloating, this can be relieved quickly with the passage of gas, and they should be able to eat upon returning home.

After the removal of polyps or certain other manipulations, the diet or activities of patients may be restricted for a brief period of time.

When will I get the results for an endoscopy vs. colonoscopy?

Sigmoidoscopy

After the procedure, the examiner will explain the findings to you. You may have some residual cramping or bloating because of the air that was instilled into your colon during the procedure. This should quickly disappear with the passage of gas or flatus. It can be expedited by walking about in the room. Under most circumstances, you should be able to resume your regular activities upon leaving the doctor's office or the hospital.

Colonoscopy

Prior to the patient's departure from the coloscopic unit, the findings can be discussed with the patient. However, at times, a definitive diagnosis may have to wait for a microscopic analysis of biopsy specimens, which usually takes a few days.

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Medically Reviewed on 1/30/2020
References
UpToDate. Patient information: Colonoscopy (Beyond the Basics).

Falchuk, Myron, MD. "Patient information: Flexible sigmoidoscopy (Beyond the Basics)." UpToDate.com. Updated Jun 30, 2015.
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