- Virtual Colonoscopy
What is a colonoscopy?
Colonoscopy is a procedure that enables an examiner (usually a gastroenterologist) to evaluate the inside of the colon (large intestine or large bowel). The colonoscope is a four-foot long, flexible tube about the thickness of a finger with a camera and a source of light at its tip. The tip of the colonoscope is inserted into the anus and then is advanced slowly, under visual control, into the rectum and through the colon usually as far as the cecum, which is the first part of the colon. Usually, it also is possible to enter and examine the last few inches of the small intestine (terminal ileum).
Why is colonoscopy done?
Colonoscopy may be done for a variety of reasons. The vast majority of colonoscopies are performed as part of screening programs to diagnose colon cancer. When done for other reasons, it is most often done to investigate the cause of blood in the stool, abdominal pain, diarrhea, a change in bowel habit, or an abnormality found on colonic X-rays or computerized axial tomography (CT) scan.
Individuals with a previous history of polyps or colon cancer and certain individuals with a family history of some types of non-colonic cancers or colonic problems that may be associated with colon cancer (such as colonic polyps) may be advised to have periodic colonoscopies because their risks are greater for polyps or colon cancer.
How often should one undergo colonoscopy depends on the degree of the risk for cancer and the abnormalities found at previous colonoscopies. One widely accepted recommendation has been that even healthy people at normal risk for colon cancer should undergo colonoscopy at age 50 and every 10 years thereafter for the purpose of removing colonic polyps before they become cancerous.
What bowel preparation is needed for 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.
Can I take my medications before a colonoscopy? Is there a special diet?
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.
Learn more about: Coumadin
What should I expect during 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.
What if there are abnormalities detected during colonoscopy?
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 should I expect when my colonoscopy is done?
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. Someone else must drive them home since their reflexes and judgment may be impaired for the rest of the day, making it unsafe to drive, operate machinery, or make important decisions. 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.
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.
What are the possible risks, complications, or alternatives to 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.
What is virtual colonoscopy?
An alternative to colonoscopy is virtual colonoscopy. Virtual colonoscopy is a technique that uses CT scanning to obtain images of the colon that are similar to the views of the colon obtained by direct observation through colonoscopy. The images are constructed using the CT images so they do not represent true images. They are virtual images.
In preparation for virtual colonoscopy, the day before the examination, the colon is cleaned-out using laxatives. During the examination a tube is inserted into the anus and is used to inject air into the colon. The CT scans then are performed with the colon inflated, and the scans are analyzed and manipulated to form a virtual image of the colon. When properly performed, virtual colonoscopy can be effective. It can even find polyps "hiding" behind folds that occasionally are missed by colonoscopy.
Nevertheless, virtual colonoscopy has several limitations.
- Virtual colonoscopy has difficulty identifying small polyps (less than 5 mm in size) that are easily seen at colonoscopy though it is true that small polyps are less likely to be cancerous.
- Virtual colonoscopy has great difficulty identifying flat cancers or premalignant lesions that are not protruding, that is, are not polyp-like.
- Virtual colonoscopy does not allow removal of polyps that are found. Thirty to forty percent of people have colon polyps. If polyps are found by virtual colonoscopy, then colonoscopy must be done to remove the polyps. Therefore, many individuals having virtual colonoscopy will have to undergo a second procedure, colonoscopy.
- Virtual colonoscopy exposes individuals to a moderate amount of radiation.
- Virtual colonoscopy does not allow the use of the newer techniques that are being developed to differentiate between abnormal lesions that need to be biopsied or removed and those that don't. (See section "What's new in colonoscopy?" section.)
Because of these limitations, virtual colonoscopy has not replaced colonoscopy as the primary screening tool for individuals at increased risk for polyps or colon cancer. It is currently an option for individuals at normal risk for polyps and colon cancer who cannot or will not undergo colonoscopy.
What's new in colonoscopy?
There are several new developments in colonoscopy. Most of these center around improving the detection of difficult-to-see lesions -- small ones (for example, small polyps) and flat ones -- as well as the ability to determine at the time of colonoscopy whether or not polyps and lesions need to be biopsied or removed because they may contain premalignant or malignant tissue. This is important because many of these lesions are not premalignant or malignant, and a lot of time and money is spent removing them and sending them for microscopic examination unnecessarily.
High resolution images that allow better detection of flat lesions have become standard on most colonoscopes. Magnification of the images also may improve the detection of the lesions.
Narrow-band imaging uses a special wavelength of light that enhances the pattern of tiny blood vessels that lie just below the lining of the colon. The pattern of these vessels is different in normal, premalignant and malignant tissue. Determination of the pattern allows lesions, particularly premalignant and malignant flat lesions, to be identified more easily and also allows a decision to be made as to whether or not the lesion should be biopsied or removed at the time of colonoscopy without waiting for the results of the microscopic examination.
Chromoendoscopy uses dyes (stains) that are sprayed on the colon lining to differentiate normal lining from neoplastic (benign, premalignant, and malignant) tissues and determine which lesions should be removed or biopsied.
Fluorescence endoscopy uses fluorescein-labelled chemicals either sprayed on the lining of the colon or injected intravenously. The chemicals are taken up by abnormal cells (premalignant and malignant) of the colon's lining more than the normal cells, and special lighting make the areas of abnormal cells clearer to see so they can be biopsied or removed completely. Confocal laser endoscopy uses a particular wavelength of light that penetrates the lining of the fluorescein-stained colon for several millimeters. Abnormal cells may be more clearly identified than with fluorescein staining alone.
There are even colonoscopes and accessories that allow a retrograde view of the colon in addition to the antegrade view from the tip of the colonoscope. Thus, images are obtained in two, 180 degree-opposed directions in order to identify lesions that might be hiding behind folds in the lining of the colon that would be missed by a standard, forward-viewing colonoscope. There are even attempts to develop a self-advancing colonoscope.
Most of these newer colonoscopic techniques, with the exception of high resolution imaging, are not standard. Which one(s) will ultimately turn out to be valuable adjuncts to colonoscopy has yet to be determined.
Finally, magnetic resonance imaging (MRI) can be used to examine the colon in a manner similar to CT virtual colonoscopy. The major advantage of MRI is that there is no radiation exposure; otherwise, the limitations are similar to CT virtual colonoscopy.
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UpToDate. Patient information: Colonoscopy (Beyond the Basics).