Colonoscopy is the minimally invasive endoscopic examination of the large colon and the distal part of the small bowel with a fiber optic camera on a flexible tube passed through the anus. It may provide a visual diagnosis (e.g. ulceration, polyps) and grants the opportunity for biopsy of suspected lesions. Virtual colonoscopy, which uses 3D imagery reconstructed from computed tomography (CAT) scans, is also possible, as a totally non-invasive medical test, although it is not standard and still under investigation. Furthermore, this imaging technique does not allow for therapeutic maneuvers such as polyp removal or biopsy. If a growth or polyp is detected using CT colonography, a standard colonoscopy would still need to be performed. Colonoscopy is similar but not the same as sigmoidoscopy. The difference between colonoscopy and sigmoidoscopy is related to which parts of the colon each can examine. Sigmoidoscopy allows doctors to view only the final 60 cm of the colon , while colonoscopy allows a complete examination of the colon , which can measure well over six feet (two metres) in overall length.

Uses

Indications for colonoscopy include gastrointestinal hemorrhage, unexplained changes in bowel habit or suspicion of malignancy. Colonoscopies are often used to diagnose or rule out 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 to do a colonoscopy, usually along with an EGD (gastroscopy), 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 polyps (which are easily removed during the colonoscopy procedure), diverticulosis, inflammatory bowel disease (Crohn's disease, ulcerative colitis), or colon cancer.

Due to the high mortality associated with colon cancer and the high effectivity and low risks associated with colonoscopy, it is now also becoming a routine screening test for people 50 years of age or older. 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.

Preparation

The colon must be free of solid matter for the test to be performed properly. For one to three days, the patient is required to follow a low fibre or clear fluid only diet. Then, on the day before the colonoscopy, the patient is given a laxative preparation (such as Bisacodyl, sodium picosulfate, sodium phosphate solution, or a solution of polyethylene glycol and electrolytes) and large quantities of fluid.

The investigation

During the procedure the patient is often given sedation intravenously, employing agents such as midazolam or fentanyl. Although meperidine (Demerol) may be used as an alternative to fentaynl, the concern of seizures has related this agent as second line behind the combination of midazolam and fentanyl. The average person will receive a combination of these two drugs, usually between 1-4 mg iv midazolam, and 25 to 100 mg iv fentanyl. Sedation practices vary between practitioners and nations; in some clinics in Norway, sedation is rarely administered. Some endocoscopists are experimenting with, or routinely use, alternative or additional methods such as nitrous oxide and propofol, which have advantages and disadvantages relating to recovery time (particularly the duration of amnesia after the procedure is complete), patient experience, and the degree of supervision needed for safe administration.

The first step is usually a digital rectal examination, to examine the tone of the sphincter and to determine if preparation has been adequate. The endoscope is then passed though the anus up the rectum, the colon (sigmoid, descending, transverse and ascending colon , the cecum), and ultimately the terminal ileum. The endoscope has a movable tip and multiple channels for instrumentation, air, suction and light. The bowel is occasionally insufflated with air to maximize visibility. Biopsies are frequently taken for histology.

In most experienced hands, the endoscope is advanced to the junction of where the colon and small bowel join up (cecum) in under 10 minutes in 95% of cases. Due to tight turns and redundancy in areas of the colon that are not "fixed", loops may form in which advancement of the endoscope creates a "bowing" effect that causes the tip to actually retract. These loops often result in discomfort due to stretching of the colon and its associated mesentery. Maneuvers to "reduce" or remove the loop include pulling the endoscope backwards while torquing the instrument. Alternatively, body position changes and abdominal support from external hand pressure can often "straighten" the endoscope to allow the scope to move forward. In a minority of patients, looping is often cited as a cause for an incomplete examination.

For screening purposes, a closer visual inspection is then often performed upon withdrawal of the endoscope over the course of 20 to 25 minutes. Lawsuits over missed cancerous lesions have prompted recent institutions to better document withdrawal time as rapid withdrawal times may be a source of potential medical legal liability. This is often a real concern in private practice settings where high throughput of cases have been postulated as a financial incentive to complete colonoscopies as quickly as possible.

Suspicious lesions may be cauterized, treated with laser light or cut with an electric wire for purposes of biopsy or complete removal polypectomy. Medication can be injected, e.g. to control bleeding lesions. On average, the procedure takes 20-30 minutes, depending on the indication and findings. With multiple polypectomies or biopsies, procedure times may be longer. As mentioned above, anatomic considerations may also affect procedure times.

After the procedure, some recovery time is usually allowed to let the sedative wear off. Most facilities require that you have a person with you to help get you home afterwards (again, depending on the sedation method used).

One very common aftereffect from the procedure is a bout of flatulence and minor wind pain caused by air insufflation into the colon during the procedure.

An advantage of colonoscopy over x-ray imaging or other, less invasive tests, is the ability to perform therapeutic interventions during the test. If a polyp is found, for example, it can be removed by one of several techniques. A snare can be place around a polyp for removal. Even if the polyp is flat on the surface it can often be removed. For example, the following show a polyp removed in stages.

Risks

This procedure has a low (0.2%) risk of serious complications. Potential serious complications include a tear or hole in the lining of the colon called a perforation. Rates of perforations is on the order of less than 1 in 2000 colonoscopies. Bleeding complications may be treated immediately during the procedure. Delayed bleeding may also occur at the site of polyp removal up to a week after the procedure and a repeat procedure can then be performed to treat the bleeding site. Less serious complications would include cardiopulmonary complications such as temporary drop in blood pressure and oxygen saturation. These developments often are the result of overmedication and easily reversed. In rare cases, more serious cardiopulmonary events such as a heart attack or stroke may occur but these often occur in critically ill patients with multiple risk factors.

Information referenced from Wikipedia.