gastroenterologist doctors wellington

About colonoscopy


Colonoscopy is a test where a flexible tube with a digital camera on its end is passed through the anus and around the large bowel after all the stool has been flushed out by the patient drinking a laxative solution. It is used to look for cancer, polyps, and other bowel conditions, and can be used to remove polyps.

Colonoscopy saves lives!  Studies clearly show that colonoscopy is the most effective screening test used in medicine to reduce burden of disease and save lives.  The great strength of colonoscopy is that it prevents cancer by allowing removal of the polyps that cause bowel cancer, and also allows your doctor to predict your future risk as well.  

Colonic polyps

Polyps are growths on the wall of the bowel, some of which can turn into cancer over many years.  There are three types of polyp: hyperplastic, adenoma and serrated.

Polyps are the commonest finding during a colonoscopy, present in about 60% of people. They are growths of abnormally-arranged and often abnormal cells on the innermost layer of the bowel wall. They can have a variety of appearances, but a small polyp will often look much like a wart on the skin.

Most polyps can be grouped into three broad types: hyperplastic, adenomas, and serrated.

Small hyperplastic polyps are very commonly found in the rectum and sigmoid colon, and have no clinical significance. They are essentially a ‘normal’ finding.

Adenomas are also common, present in about half of New Zealanders over the age of 50, and are of more significance because a small percentage of these have the potential to progress in size, accumulate genetic mutations, and become cancerous. For this reason, if adenomas are found at your colonoscopy, a follow-up colonoscopy might need to be arranged to see whether you grow more, and if you do then you may need regular colonoscopies to reduce your risk of bowel cancer.

Serrated polyps have been recently identified as being significant in the development of certain types of colorectal cancer. These are often flat and inconspicuous, and are easily missed if the endoscopist is not careful in his or her examination. This type of polyp appears to be much more easily seen underwater compared to in an air-filled colon as it doesn’t flatten out so much, and usually has a readily visible mucus cap on it. Like with adenomas, if serrated polyps are found at colonoscopy then further colonoscopies will probably be advised.

How are polyps removed?

Polyps are removed by a process called 'polypectomy'. Small metal loops can be passed down through the colonoscope to cut off the polyps either like a cheese slice or using electricity. A large polyp may need to be removed by a process called endoscopic mucosal resection (EMR) which requires greater specialist expertise.

Polyps develop on the innermost layer of the bowel wall, the mucosa. Beneath the mucosa is a thicker soft layer called the submucosa, and beneath this is the outer muscle layer, the muscularis propria. When removing a polyp the aim is to remove the mucosal polyp by cutting into the submucosa, but not so deep as to damage the muscularis propria which could cause a hole ("perforation"), and therefore a bowel leak.

Most polyps can be removed safely by using a "cold snare" (see the picture series to the right), which acts much like a cheese slice, removing a reasonably superficial sample with no risk of perforation, and no risk of late bleeding (days or weeks after the procedure).

Thicker polyps will not slice through with a cold snare so require an electric knife (electrocautery), a "hot snare". Due to the heat and cutting effect this presents a greater risk of damage to the muscularis propria so more care is required. Sometimes a fluid ‘cushion’ may be injected into the submucosa layer first to thicken it and protect the muscularis propria. When electrocautery is used there is a risk of late bleeding from the polyp site due to delayed damage to large blood vessels in the submucosa.

If a large polyp needs to be removed in several pieces because of its size, this is referred to as an endoscopic mucosal resection (EMR). Traditionally this has required substantial submucosal ‘cushion’ injection, a lot of skill, and carries a not insignificant risk of bowel perforation in some parts of the colon where the bowel wall is very thin.

Underwater EMR - a new way to remove large polyps safely

Underwater EMR is a new technique to remove large polyps from the bowel wall that appears to be easier and more effective than the traditional method.

In 2010/2011, Dr Binmoeller at Interventional Endoscopy Services in San Francisco noted that when the colon is in a relaxed state filled only with water, ultrasound examination of the wall layers showed that the submucosa layer remained very thick, and would not require submucosal ‘cushion’ injection to protect the muscularis propria. This led to the technique of ‘underwater EMR’ (UEMR), which allows seamless removal of large polyps quickly and simply while doing a water-assisted colonoscopy. Data from Dr Binmoeller’s group and my own experience (see poster on right) also suggests that UEMR reduces the risk of incomplete excision of large polyps, which can occur in up to 25% of cases performed by traditional EMR. An incompletely removed polyp is a risk for cancer, and means repeat colonoscopies until it is removed completely.

Watch a video on the underwater EMR technique.

Sedation for colonoscopy

Colonoscopies in NZ are commonly performed using sedation drugs such as midazolam, fentanyl and sometimes propofol to reduce discomfort.  These drugs can cause after-effects for several hours following the colonoscopy and driving is not permitted for 18 hours.  Water-assisted colonoscopy as offered by Dr Cameron reduces the need for these drugs, and most patients in his care do not require any at all.  

When the technique of colonoscopy was first developed it was an unsedated procedure. Over time, in most western countries, sedation use has become routine, and some colonoscopists have gone to the extreme of using a very powerful sedative called propofol administered by an anaesthetist. It is, however, extremely uncommon for someone to actually need this level of sedation if the colonoscopy is performed by a colonoscopist with good technique as there should be only minimal discomfort if any. In fact, it has been shown that if a colonoscopist uses heavy sedation he or she uses more forceful movements with the colonoscope as there is no feedback from the patient, and this does increase the risk of injury to the bowel or surrounding tissues.

The use of water-assistance has allowed me to offer ‘sedation on demand’ to the majority of patients I perform colonoscopy on. This means that pain relief/sedative drugs are not given at the beginning of the procedure, but can be administered at any point of the procedure if the patient finds it uncomfortable. If the procedure is completed without drugs then the patient is unencumbered for the rest of the day and can even drive home, which is not allowed by law if pain relief/sedative drugs are given. More than 2/3 of patients manage colonoscopy without sedation in my care. There are some patients who for various reasons cannot tolerate or do not want a colonoscopy without sedation, but even in these patients the amount of drug needed is reduced. I am very happy to discuss your sedation requirements before the procedure to provide the optimum colonoscopy experience for you.

What are the risks of colonoscopy?

Colonoscopy is a safe procedure and the specific risks in your particular situation will be discussed with you by your colonoscopist.

As colonoscopy is performed by a human and not a machine, some of the risks of a colonoscopy are due to the performance of the human operator.  It is now widely understood that the average colonoscopist misses significant polyps (>1cm in size) during colonoscopy 2 to 6% of the time.  For poorly performing colonoscopists that number might be higher.  This is a problem for the patient because these are the polyps that can grow into cancer, and might lead to a cancer occurring before the next scheduled colonoscopy.  This is why medical facilities that perform colonoscopy need to have good quality control systems in place to try and reduce that miss rate by ensuring that the colonoscopists adhere to important technical standards such as performing complete colonoscopies most of the time, and pulling the colonoscope out slowly enough that he or she can identify all the polyps that are present.  One quality standard that is often not adhered to in the private medical system is limiting the number of colonoscopies performed on any one 'list': a colonoscopist gets tired like anyone else, and when that occurs his or her attention will reduce, which increases the miss rate for polyps.  A colonoscopist should generally not be performing more than 5 or 6 colonoscopies in a 'list'.

Colonoscopy is a safe procedure. It is rare for a complication to occur during the colonoscopy, and the risks only go up significantly if large polyps need to be removed. In this scenario there is a risk of making a hole in the bowel wall (‘perforation’), or causing bleeding from the site the polyp was removed from. This bleeding can occur any time up to about three weeks after the procedure. Almost always any bleeding that occurs is of minor degree only. The risks of removing large polyps by colonoscopy are always substantially lower than the risks of them being removed by surgery if performed by an experienced therapeutic endoscopist such as Dr Cameron.

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