Ulcerative colitis and Crohn's disease are two different forms of inflammatory bowel diseases. Ulcerative colitis only affects the colon whereas Crohn's disease can affect any part of the gastrointestinal tract from the mouth to the anus. Around 20 to 25% of Crohn's disease only affects the colon (sometimes called Crohn's colitis).
The distinction between ulcerative colitis and Crohn's disease is made on the basis of several factors. One is whether the disease is continuous or has areas of diseased bowel with normal bowel in between (so called skip lesions) which can be seen in Crohn's disease, however Crohn's disease can also be continuous. There are characteristic appearances when the bowel lining is assessed using colonoscopy. The presence of snake like (serpiginous) ulcers and ‘cobblestoning’ (deep linear ulcers giving the bowel lining an appearance like a cobble stoned street) more suggestive of Cron's disease whereas flat continuous ulceration or areas of raised normal lining surrounded by ulcers (known as pseudopolyps) is more suggestive of ulcerative colitis. Some patients may have features suggestive of both conditions.
Looking at biopsies of the colon under the microscope also helps to decide whether the disease is ulcerative colitis or Crohn's disease. A particular feature called granulomas is highly suggestive of Crohn's disease but is only found in around one quarter of biopsies.
The presence of any disease outside of the colon, such as in the small bowel or a fistula (burrowing tract from the surface of the bowel) enables a diagnosis of Crohn's disease to be made.
Unfortunately, around 10% of patient with IBD in the colon have an indeterminate colitis, where the exact distinction cannot be made. Over time, many patients will develop one or more defining feature that enables a firm diagnosis to be made.
From a practical perspective, the absence of a firm diagnosis does not have many significant impacts on treating the colitis. Mesalasine or sulphasalazine compounds, corticosteroids, and immunomodulators like azathioprine (Imuran), 6-mercaptopurine (Puri-Nethol) and methotrexate are used with success in both conditions. When these agents are not sufficient to control the disease, a diagnosis of Crohn's colitis allows access to PBS funded anti-TNF therapy (adalimumab (Humira), infliximab (Remicade)) whilst at the present time, ulcerative colitis is not a PBS indication for funded therapy.
Long term followup of the colon is also very similar between colitis caused by ulcerative colitis and Crohn's colitis with the need for regular surveillance colonoscopy after 8 years from diagnosis to detect early changes that put the patient at a higher risk of bowel cancer.
If all medical therapy fails to control the disease and surgical treatment with colectomy is needed, a definitive diagnosis can usually be made on the whole colon. This is important in determining a patient's suitability or not for a ‘pouch’ (ileal pouch anal anastomosis (IPAA)) procedure or the need to keep a permanent stoma (ileostomy). Pouches are usually not formed in patients with Crohn's disease due to the high complication rate compared with ulcerative colitis patients.
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