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, virtual
colonoscopy does not allow for therapeutic maneuvers such as polyp removal or
biopsy nor visualization of lesions smaller than 5 millimeters. If a growth or
polyp is detected using CT colonography, a standard colonoscopy would still need
to be performed. Colonoscopy can remove polyps smaller than one millimeter. Once
polyps are removed, they can be studied with the aid of a microscope to
determine if they are precancerous or not. 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 two feet of the
colon, while colonoscopy allows an examination of the entire colon, which
measures four to five feet in length.
Indications for colonoscopy include gastrointestinal hemorrhage, unexplained
changes in bowel habit or 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 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.
The patient may be asked to skip aspirin for up to five days before the
procedure to avoid the risk of bleeding if a polypectomy is performed during the
procedure. 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 fiber or clear fluid only diet. Then, on the day before the colonoscopy, the
patient is either given a laxative preparation (such as Bisacodyl, sodium
picosulfate, or sodium phosphate) and large quantities of fluid or whole bowel
irrigation is performed using a solution of polyethylene glycol and
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 fentanyl, the concern of seizures has relegated
this agent to second choice for sedation 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 µg 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 patients have a person with them to help
them 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.
1. Polyp is identified.
2. A sterile solution is injected under the polyp to lift it away from deeper
3. A portion of the polyp is now removed.
4. The polyp is fully removed.
This procedure has a low (0.2%) risk of serious complications.
The most serious complication is a tear or hole in the lining of the colon
called a gastrointestinal perforation, which is life-threatening and requires
immediate major surgery for repair; however, the rate of perforation is less
than 1 in 2000 colonoscopies.
Bleeding complications may be treated immediately during the procedure by
cauterization via the instrument. 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. Even more rarely, splenic rupture can
occur after colonoscopy because of adhesions between the colon and the spleen.
As with any procedure involving anesthesia, other complications would include
cardiopulmonary complications such as temporary drop in blood pressure and
oxygen saturation, usually the result of overmedication and easily reversed. In
rare cases, more serious cardiopulmonary events such as a heart attack, stroke,
or even death may occur; these are extremely rare except in critically ill
patients with multiple risk factors.