A colonoscopy is a diagnostic procedure used to examine the large intestine (colon) and rectum. It is an accurate way of detecting changes or abnormalities in the bowel, such as polyps which may develop into cancer. Tissue samples, called biopsies, can also be taken during a colonoscopy.
A colonoscopy uses an instrument called a colonoscope, which is a long, thin, flexible tube with a tiny video camera at one end, to examine the inside of the colon.
During the procedure, the colonoscope is inserted into your rectum and gently fed through your colon. Images are transmitted from the camera at the tip of the tube to an external monitor. This enables the doctor to get a detailed view of the inside of your colon. Instruments can be inserted through the colonoscope to take tissue samples or remove polyps. The procedure normally takes 20 to 30 minutes.
A colonoscopy is used to:
- Investigate the possible causes of bleeding from your bottom or blood in your stools, as well as other intestinal problems such as chronic diarrhoea or constipation, or abdominal pain.
- Screen for bowel cancer. This involves checking for the disease even if you don’t have any symptoms. Bowel cancer is treatable if it is caught early before it has had chance to spread. Bowel cancer screening is normally offered to people who are at increased risk of the disease – including the over 50s or those with a family history of the disease – as a preventative measure or a way of picking up the disease in its earliest stages. Private bowel cancer screening can be arranged at any age to provide reassurance and peace of mind.
- Follow up if you have previously had polyps removed or have had bowel cancer.
You might be offered a colonoscopy if you:
- Have a variety of lower gastrointestinal symptoms needing further investigation.
- Have Inflammatory Bowel Disease (Crohn’s Disease or ulcerative colitis).
- Are over the age of 50, as 90% of bowel cancers occur in this age group.
- Have a family history of bowel cancer or polyps.
- Have a family history of Lynch Syndrome, which is an inherited condition that tends to cause cancer in people under the age of 45.
- Diagnostic colonoscopy is for people who have lower gastrointestinal symptoms, or an abnormal stool test or iron deficiency anaemia.
- Screening colonoscopy is for people who have no current symptoms but who are considered at higher risk of bowel cancer with an abnormal FIT (faecal immunochemical) test or iron deficiency anaemia.
There are several different types of colonoscopy:
- A colonoscopy enables the doctor to examine the entire length of the colon. You will be given strong laxatives to take the day before to empty the bowel and a small amount of carbon dioxide or air will be pumped into the bowel during the procedure so that the doctor can get a clear view.
- A CT (or virtual) colonoscopy uses a CT scan to check the colon and rectum. You will be given an injection of medicine to relax the bowel muscle and midway through the scan contrast medium will be injected to show up the bowel more clearly. You will be scanned lying on your front and on your back. It is an effective and non-invasive test, but small polyps can sometimes be missed, particularly in the last portion of the colon. It is not possible to take biopsies or remove polyps during a CT colonoscopy.
- A flexible sigmoidoscopy is used to examine the lower part of the colon and the rectum. As with a colonoscopy it uses a thin, flexible tube with a camera at one end which is inserted via the rectum. However, unlike a colonoscopy it does not cover the entire length of the bowel.
Before the procedure you might be asked to follow a special low fibre diet for a few days, and you will not be able to eat anything for approximately 24 hours before the procedure. You can drink clear liquids but should avoid anything that is red in colour as this may be confused with blood during the procedure.
You will be given a strong laxative to empty your bowel beforehand. You normally take this on the day before the procedure. In some cases, you may also need to use an enema kit. Tell your doctor if you are taking medication for diabetes, high blood pressure or heart disease as you may need to stop these temporarily.
Just before the procedure, you will be given a sedative to help you relax. You will be asked to lie on your left side with your knees drawn up to your chest. The doctor will insert the colonoscope into your anus and pass it gently into your colon.
A small amount of air or carbon dioxide will be pumped into your colon to inflate it, so the doctor has a better view. This may cause a cramping sensation or a feeling like you need to go for a poo. This can be uncomfortable but should ease off quite quickly. If the doctor finds abnormal areas or tissue or polyps, tiny instruments may be passed through the colonoscope to take samples or remove the polyps.
Afterwards it normally takes around an hour for the effects of the sedative to wear off. You will need someone to escort you home as you will not be able to drive or use public transport alone after having a sedative.
You may pass gas for a few hours after the examination as it clears from your colon. You may also notice a small amount of blood when you pass stools. This is normal but you should consult a doctor if it continues or you have fever or abdominal pain.
Natalie has been a consultant gastroenterologist since 2016 and in addition to her private practice she currently holds a post at Imperial College Healthcare NHS Trust, with a specialist interest in gastroenterology and…
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Consultant Gastroenterologist & HPB Physician