A colonoscopy is an examination of the large bowel or colon using a colonoscope, which is a flexible tube that combines the features of magnification and illumination. It is two metres long and enables the doctor to view the entire colon.
For the lower third of the colon, an instrument called a sigmoidoscope may be used, which is rigid but shorter than the colonoscope and enables an examination of the bowel for polyps, tumours and other diseases. The instrument used for seeing inside the rectum and anus is called a proctoscope.
Before having a colonoscopy, the patient will be given a specific low residue diet for three days, followed by a laxative or an enema, and then be instructed to drink only clear fluids for the day before the test is performed. A solution (eg. sodium phosphate) must also be drunk in the day before the test to thoroughly cleanse the bowel. This is the part of the procedure that most patients complain about.
It is unusual to feel any discomfort during the procedure as an effective sedative is given. The examination takes about 20 to 30 minutes, and any polyps found will be removed during the procedure.
As with any procedure, there are risks. Perforation of the bowel may occur in about one in every 2000 procedures, and this will require an open operation to close the hole. Bleeding into the bowel, particularly after the removal of a polyp, is another rare complication.
Even in the best circumstances, about 5% of cancers present will be missed.
One in every 16000 patients dies from the complications of a colonoscopy.