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Written by Professor Jonathan Rhodea, consultant gastoenterologist
What is a colonic polyp?
- A polyp is a benign (non-cancerous) growth of the lining of the colon (large bowel).
- It can be anything from 2mm up to 5cm or more in diameter.
- Commonly, the abnormal cells form a small ball (about the size of a pea) on the end of a stalk of normal cells.
- The type of cell that forms the polyp varies and is important in determining its potential for developing into a cancer.
Types of polyp
Metaplastic polyps versus adenomatous polyps
The most common sort of polyp is a metaplastic polyp (in which cells change from one normal type to another). These usually do not grow much more than 5mm in diameter and have almost no risk of becoming malignant (cancerous). These polyps can be very similar in appearance to adenomatous polyps, the next most common type, which do have the potential to become malignant.
About 50 per cent of people aged 60 will have at least one adenomatous polyp of 1cm diameter or greater. Familial polyposis coli (familial adenomatous polyposis or FAP ) involves multiple adenomatous polyps, often in their hundreds. This condition carries a very high risk of colon cancer.
Other rarer types of polyps include:
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Juvenile polyps: these are usually solitary polyps called hamartomas that affect 1 to 2 per cent of older children or adolescents. A single polyp carries no significant cancer risk but when these polyps are inherited and usually multiple (about one third of patients), the colon cancer risk is about 10 per cent. In this case, regular surveillance after excision (cutting out) of all polyps is required.
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Peutz-Jeghers polyps: found in Peutz-Jeghers syndrome, in association with freckling of the lips, are also of the hamartomatous type. These usually present in early adult life and carry a low but definite risk of malignancy, probably around five per cent per polyp, so they need excision. The number of polyps per individual is very variable and ranges, from as few as one or two to as many as 20 or more. Peutz-Jeghers polyps can also occur in the small intestine and can then be difficult to diagnose because they are beyond the reach of conventional fibre-optic endoscopes (internal telescope instruments). Such polyps tend to present with symptoms of obstruction (bowel blockage) or abdominal pain. Diagnosis is usually made with barium X-rays (taken after the patient swallows barium liquid to show up the inside of the intestine). Treatment will usually be an operation that opens up the abdomen.
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Inflammatory pseudopolyps: can occur as a complication of ulcerative colitis or Crohn's disease of the colon.
They are completely harmless and carry no risk of cancer but they can be confused with adenomatous polyps on examination.
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Cronkhite-Canada syndrome: an exceptionally rare condition, involves multiple colon polyps, hyperpigmentation (darkening of the skin) and nail atrophy (wasting away). The syndrome is not inherited and affects middle-aged or older individuals. It is linked with malabsorption and has been reported to respond to vitamin E therapy.
What causes polyps?
. Most polyps, with the exception of the inflammatory pseudopolyps, result from some form of genetic (DNA) mutation in one of the colon lining cells. Fortunately, several, probably at least five, mutations are needed in the same cell before cancer occurs and most benign polyps probably only have one gene mutated. DNA damage occurs surprisingly often.
Even in a healthy adult's colon, about 10 per cent of the lining cells, on average, contain major abnormalities of the chromosomes (packages of DNA that contain many genes). Fortunately, almost all these cells seem to undergo a form of programmed death called apoptosis, and then fall off harmlessly into the bowel lumen (cavity).
Adenomatous polyps, even those from individuals who do not have familial polyposis, commonly contain mutations that stop the gene working in both copies of the adenomatous polyposis coli (APC) gene, the gene that is mutated in familial polyposis coli.
What symptoms do polyps cause?
Polyps usually cause no symptoms until they grow to 2cm or more in diameter. Then the most common symptom is rectal bleeding. If a polyp is large enough, say 2cm in diameter, it can simulate faeces so the colon undergoes vigorous muscular movements (peristalsis) in a futile attempt to expel the polyp. This can lead to severe colicky pains.
Occasionally, large polyps with a characteristic villous (frondy) appearance will cause profuse watery diarrhoea, which can then result in severe potassium deficiency causing muscle weakness.
How is the diagnosis made?
Polyps can be seen directly during colonoscopy (telescope examination of the whole large bowel starting at the rectum). An experienced doctor can often differentiate metaplastic polyps from the pre-malignant adenomatous polyp by appearance alone.
Nevertheless, most doctors prefer not to take the risk of getting it wrong and either take biopsy samples (small bites of tissue) or cut out the entire polyp (polypectomy). In this case, checking the polyp's cells under the microscope (histology) is not essential providing that the doctor is certain that the polyp has been completely removed.
Symptomless polyps are often found at barium enema examination (an X-ray test done after barium liquid is poured into the rectum) in a patient with bowel symptoms that could be due to irritable bowel syndrome.
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