Microscopic Colitis
05.14.10
Microscopic colitis refers to inflammation of the colon that is only visible when the colon’s lining is examined under a microscope. The appearance of the inner colon lining in microscopic colitis is normal by visual inspection during colonoscopy or flexible sigmoidoscopy.
The diagnosis of microscopic colitis is made when a doctor, while performing colonoscopy or flexible sigmoidoscopy, takes biopsies (small samples of tissue) of the normal-appearing lining, and then examines the biopsies under a microscope. There are two types of microscopic colitis; lymphocytic colitis and collagenous colitis. In lymphocytic colitis, there is an accumulation of lymphocytes (a type of white blood cell) within the lining of the colon.
In collagenous colitis, there is an additional layer of collagen (scar tissue) just below the lining. Some experts believe that lymphocytic colitis and collagenous colitis represent different stages of the same disease. The inflammation and the collagen probably interfere with absorption of water from the colon, and cause the diarrhea. The primary symptom of microscopic colitis is chronic, watery diarrhea. Patients with microscopic colitis can have diarrhea for months or years before the diagnosis is made.
This chronic diarrhea of microscopic colitis is different from the acute diarrhea of infectious colitis, which typically lasts only days to weeks. Some patients with microscopic colitis also may report mild abdominal cramps and pain. Blood in the stool is unusual. The prevalence of microscopic colitis in the U.S. is not clearly known.
It is estimated that 10-20% of patients with chronic diarrhea may have microscopic colitis. It is this author’s experience, that the condition is becoming more common in recent years. It is not clear, however, whether there is an actual increase in the frequency of microscopic colitis or whether doctors are just better at diagnosing it.
Microscopic colitis most commonly occurs in middle aged to elderly patients and is more common among women than men. The cause(s) of microscopic colitis is unknown. Some doctors suspect that microscopic colitis is an autoimmune disorder similar to the autoimmune disorders that cause chronic ulcerative colitis and Crohns disease.
The diagnosis of microscopic colitis is made by performing biopsies from different regions of the colon during colonoscopy or sigmoidoscopy. The abnormalities of the colon’s lining in microscopic colitis occur in a patchy distribution (areas of normal lining may exist adjacent to areas of abnormal lining).
For this reason, multiple biopsies should be taken from several different regions of the colon in order to accurately make a diagnosis. The patchy nature of microscopic colitis also is the reason why flexible sigmoidoscopy often is inadequate in diagnosing the condition because the abnormalities of microscopic colitis may be absent from the sigmoid colon (the colonic segment that is closest to the rectum and is within the reach of a sigmoidoscope) in 30-40% of the patients with microscopic colitis.
Thus, biopsies of other regions of the colon accessible only with colonoscopy may be necessary for diagnosing microscopic colitis.
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