Colonoscopy is the endoscopic examination of the large bowel and the distal part of the small bowel with a CCD camera or a fiber optic camera on a flexible tube passed through the anus. It can provide a visual diagnosis (e.g. ulceration, polyps) and grants the opportunity for biopsy or removal of suspected colorectal cancer lesions. It can remove polyps as small as one millimeter or less. Once polyps are removed, they can be studied with the aid of a microscope to determine if they are precancerous or not. It can take up to 15 years for a polyp to turn cancerous. Colonoscopy is similar to sigmoidoscopy—the difference being related to which parts of the colon each can examine. A colonoscopy allows an examination of the entire colon. A sigmoidoscopy allows an examination of the distal portion of the colon, which may be sufficient because benefits to cancer survival of colonoscopy have been limited to the detection of lesions in the distal portion of the colon.
Conditions that call for colonoscopies include gastrointestinal hemorrhage, unexplained changes in bowel habit and suspicion of malignancy. Colonoscopies are often used to diagnose colon cancer, but are also frequently used to diagnose inflammatory bowel disease. In older patients (sometimes even younger ones) an unexplained drop in hematocrit (one sign of anemia) is an indication that calls for a colonoscopy, usually along with an esophagogastroduodenoscopy (EGD), even if no obvious blood has been seen in the stool (feces).
Fecal occult blood is a quick test which can be done to test for microscopic traces of blood in the stool. A positive test is almost always an indication to do a colonoscopy. In most cases the positive result is just due to hemorrhoids; however, it can also be due to diverticulosis, inflammatory bowel disease (Crohn’s disease, ulcerative colitis), colon cancer, or polyps. However polypectomy has become a routine part of colonoscopy, allowing for quick and simple removal of polyps without invasive surgery.
Colonoscopy is one of the colorectal cancer screening tests available to people in the US who are over 50 years of age. The other screening tests include flexible sigmoidoscopy, double-contrast barium enema, computed tomographic (CT) colongraphy (virtual colonoscopy), guaiac-based fecal occult blood test (gFOBT), fecal immunochemical test (FIT), and multitarget stool DNA screening test (Cologuard).
Subsequent rescreenings are then scheduled based on the initial results found, with a five- or ten-year recall being common for colonoscopies that produce normal results. People with a family history of colon cancer are often first screened during their teenage years. Among people who have had an initial colonoscopy that found no polyps, the risk of developing colorectal cancer within five years is extremely low. Therefore, there is no need for those people to have another colonoscopy sooner than five years after the first screening.
Screening is recommended every 10 years beginning at age 50 for adults without increased risk for colorectal cancer. Research shows that the risk of cancer is low for 10 years if a high-quality colonoscopy does not detect cancer, so tests for this purpose are indicated every ten years.
Colonoscopy reduces cancer rates by detecting some colon polyps and cancers on the left side of the colon early enough that they may be treated, and a smaller number on the right side; many of these left-sided growths would likewise have been detected by a safer sigmoidoscopy procedure.
Since polyps often take 10 to 15 years to transform into cancer, in someone at average risk of colorectal cancer, guidelines recommend 10 years after a normal screening test before the next screening test is done.
Colonoscopy is contraindicated in the following situations: