These symptoms may last for a few days or weeks and then cease for months at a time. Also, they may recur with varying degrees of intensity, over a long period. When the irritable bowel syndrome is present, however, the pattern of motility becomes disordered by excessive muscular contractions, which cause the pain, diarrhea and constipation. It is not known what causes the overactivity of the intestinal muscle. Emotional stress is believed to be a factor in a great many cases.
Some researchers think that the syndrome is an allergic response to particular foods.
There also is some evidence that alcohol, caffeine and heavy smoking may worsen the problem. The excessive intestinal spasms that produce the symptoms may be detected in the course of a barium enema, a test in which a chalky liquid mixture is infused into the colon and X-ray photographs are taken. Alternately, or additionally, the doctor may perform a sigmoidoscopic examination in which a hollow tube with viewing instruments is inserted into the anus and passed upward into the colon. These examinations are usually performed to rule out other colon disorders that may produce similar symptoms. Symptoms can interfere with daily living significantly despite lack of structural abnormality of the GI tract.