Irritable bowel syndrome (IBS) is an intestinal disorder characterized by abdominal pain, gassiness, bloating, and constipation or diarrhea. It is also called spastic or nervous colon. Because people with IBS have a colon that appears to be normal—that is, there are no structural or biochemical abnormalities explaining the symptoms—IBS is called a "functional" disorder: The system just doesn't work right. IBS affects 1 in 5 adults in the United States, making it one of the most common ailments diagnosed by doctors. Women are more frequently affected than men.
Only about 10 percent of people with symptoms of IBS go to a doctor for evaluation or treatment. In spite of this, IBS accounts for nearly 3.5 million physician visits in the United States annually, and it is the most common diagnosis by gastroenterologists.
The cause of IBS is not known, although a combination of factors—including hypersensitivity of the intestinal tract and an imbalance of neurotransmitters, the chemicals that send messages between nerve cells—is thought to play a role. There is no cure. The good news is that IBS does not lead to serious diseases such as cancer and doesn't shorten a person's life span. Most people are able to control their symptoms by modifying their diet, finding ways to better manage stress, and sometimes by taking medications.
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Irritable bowel syndrome affects the large intestine, or colon, which is 6 feet long and about 3-5 inches in diameter. It connects the small intestine with the rectum and anus.
Digestive products enter the colon from the small intestine, and may sit there for several days while water and salt are absorbed through the colon's walls. Muscles in the intestine contract and push stool along toward the rectum, eventually forming a bowel movement.
In people with irritable bowel syndrome, the colon seems to be more sensitive than in most people, so that muscle contractions are triggered more easily and pain and constipation or diarrhea result.
No one knows what causes irritable bowel syndrome. The intestines of people with IBS don't look any different than those of people without the disorder, although they do seem to react more strongly to foods and stresses.
Research has shown that people with IBS have problems with motility, the rhythmic muscle contractions that push fecal matter through the large intestine. In one study, people who had IBS with constipation had almost no contractions in 24 hours, compared with six to eight per day in healthy volunteers. IBS patients with diarrhea, by contrast, had as many as 25 contractions a day.
Either problem can cause symptoms of IBS. The longer it takes for the muscle contractions to move the stool through the colon, the harder the stool becomes—and the tougher it is to pass. By contrast, water won't be absorbed from the stool if muscle spasms push it through too rapidly, leading to diarrhea. Gas can also get trapped along the way, causing discomfort. Sometimes people with IBS pass mucus with their bowel movements. People with this disease also appear to be extra sensitive to distention of the colon. Studies using balloons to distend the colon have shown that patients with IBS experience pain and feel bloated at balloon pressures and volumes that are significantly lower than those that cause symptoms in people without the condition.
Recent studies also have shown that neurotransmitters, chemicals that pass signals between nerves, are particularly active in the gastrointestinal tracts of people with the condition. One neurotransmitter in particular, serotonin, appears to be especially active in the blood and colon of people with IBS. These high levels of serotonin may stem in part from life stresses: The colon has many connections to the brain and central nervous system, which responds to stress and emotional upsets. And more than half of patients seen by a physician for IBS report stressful life events just before or at the same time as the onset of symptoms.
Infection may also play a role in the causation of IBS. The theory is that infections of the intestinal lining prompt an immune-system response that makes the intestine hypersensitive.
Some people who report IBS-like symptoms actually have mild celiac disease. People with celiac disease can't digest gluten, a protein in wheat and other grains. When exposed to gluten, their immune systems damage the small intestine. Doctors can use a blood test and/or biopsies from the intestines to determine whether a person has celiac disease.
Since doctors don't know what causes IBS, it's hard for them to pinpoint risk factors for the disorder. Still:
- Women are far more likely to be affected than men (60 to 70 percent of patients are women).
- Recent life stresses may put people at risk. More than half of patients who are seen by a physician for IBS report stressful life events coinciding with or preceding the onset of symptoms.
- Previous colonic infection
Last reviewed on 7/28/09
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