Microscopic colitis is inflammation of the bowel that is only visible using a microscope. Microscopic colitis is a type of inflammatory bowel disease (IBD), which refers to a group of conditions that causes inflammation in the bowel due to an excessive build-up of white blood cells in the bowel lining. Microscopic colitis is less severe than other types of IBD because it does not lead to cancer and rarely requires surgery. However, microscopic colitis can cause considerable pain and discomfort.
The term bowel refers to any part of the small or large intestine. The large intestine includes the colon and the rectum, and together they are about 5 feet long. The small intestine can be 12 to 20 feet long. Colitis means inflammation of the colon. Microscopic colitis is inflammation of the colon and rectum.
Microscopic colitis has two main forms: collagenous colitis and lymphocytic colitis. The symptoms of and treatment for both are identical. Some scientists believe the two forms may be different presentations of the same disease. Slight differences in the way intestinal tissues appear when seen with a microscope set them apart. In both forms, an increase in white blood cells can be seen within the intestinal epithelium—the layer of cells that lines the intestine. Increased white blood cells are a sign of inflammation. But with collagenous colitis, the layer of collagen beneath the epithelium appears thicker than normal. Collagen is a structural protein in bones and cartilage. In the intestines, collagen anchors the intestinal epithelium to underlying layers of tissue. The thicker collagen layer seen with collagenous colitis may result from inflammation.
Microscopic colitis can affect anyone but is more common in people ages 45 and older. Rates of microscopic colitis are similar to other forms of IBD, affecting about nine people in 100,000.1 Although microscopic colitis affects both men and women, collagenous colitis is much more common in women.
Chronic watery and nonbloody diarrhea is the main symptom of microscopic colitis. Episodes of diarrhea can last for weeks, months, or years. Most cases are interrupted by similarly long periods of remission—times when diarrhea goes away.
Scientists believe diarrhea associated with microscopic colitis is caused by the intestinal epithelium’s reduced ability to absorb electrolytes—salts and minerals in the body. The resulting electrolyte imbalance decreases the colon’s ability to absorb fluid and increases fluid secretion into the colon.
The cause of microscopic colitis is unknown. Many scientists believe it is an abnormal immune response triggered by something in the gastrointestinal (GI) tract—the large, muscular tube that extends from the mouth to the anus and digests food. Normally, the immune system is triggered by germs, but sometimes it reacts to harmless bacteria, pollen, food, or even the body’s own cells. The belief that something in the GI tract causes microscopic colitis is supported by evidence that the colon, when empty for a long time, recovers from inflammation. Keeping the colon empty is accomplished through a surgical procedure called an ileostomy, which diverts digestive waste away from the colon to an opening in the abdomen. The belief is further supported by the fact that inflammation returns when the ileostomy is reversed and the normal digestive route through the colon is restored.
Scientists believe one’s genes may make a person more likely to develop microscopic colitis. Although a gene unique to microscopic colitis has yet to be found, dozens have been linked to other forms of IBD. Scientists have proposed several possible substances that might trigger microscopic colitis.
Harmful and harmless bacteria. Some people get microscopic colitis after being sick with certain harmful bacteria, including Yersinia enterocolitica, Campylobacter jejuni, and Clostridium difficile. Other people test negative for these and other harmful bacteria, but their condition improves with antibiotic treatment, suggesting normally harmless bacteria in the colon may trigger microscopic colitis in some people.
Medications. No medications have been proven to cause microscopic colitis but several have been linked to it, including:
Food. Certain foods appear to trigger microscopic colitis in some people. Although no specific foods have been identified, following a caffeine- or lactose-free diet sometimes improves symptoms.