A colonoscopy is a fibre-optic scope examination of the rectum and colon which together make up the large bowel. There are a number of important conditions affecting the large bowel, in particular cancer and inflammation (otherwise known as colitis) that need to be diagnosed in a timely and accurate way. The best method by far is with colonoscopy. The length of the large bowel in the average person is about a metre, with the rectum taking up the last 15cm. The specialist uses a set of controls on the colonoscope that manipulate the head of the scope through the whole colon and around the numerous corners to where it joins the last part of the small bowel. Further passage of the scope beyond this point is not usually required as conditions such as cancer rarely affect the small bowel - and from a practical point of view it's not really possible anyway.
The indications for performing a colonoscopy are quite specific and are dictated by guidelines. Certainly any symptoms that suggest disease arising within the bowel - these include blood in the faeces, a sudden and persistent change in bowel habit, and unexplained abdominal pain - demand investigation. A positive bowel cancer screening test (a faecal occult blood test or FOBT) is another important reason - not only to exclude the possibility of cancer but also to identify polyps as a possible cause of the bleeding. The latter are relatively common and are the benign (non-cancerous) precursors of most bowel cancers which is why it's important to identify them and to remove them - this can be done at the time of colonoscopy. However, it is important to bear in mind that there are different types of polyps and only a minority of those that are identified and removed at colonoscopy are in fact a potential cancer risk - this can only be confirmed by subsequent assessment by a pathologist.
A screening colonoscopy, i.e. performed when there are no actual symptoms, should be considered in individuals who are at greater risk of developing bowel cancer, in particular those with a close relative, i.e. parent or sibling, who has had the disease themselves. Guidelines are once again specific with regard to timing - either at age 55 or 10 years junior to the affected relative whichever comes first. Contrary to popular belief, there is absolutely no evidence to support a routine colonoscopy just because you have reached the age of 50.
Similarly, for reasons stated previously, most patients who are found to have polyps do not require repeat procedures, and nor do their relatives, as in the majority of cases there is no associated cancer risk. Persuading patients to the contrary represents dramatic over-prescription.
The procedure is generally very safe but, like all investigations and procedures, it is not entirely risk free. The main complication, whilst rare, is perforation of the bowel which requires an operation and almost always a temporary colostomy (bag) to sort it out. If you would like more information about your individual need for a colonoscopy feel free to discuss this with your colorectal specialist.