It is not clear why patients develop IBS. Sometimes it occurs after an infection of the intestines. This is called postinfectious IBS. There may also be other triggers.
The intestine is connected to the brain. Signals go back and forth between the bowel and brain. These signals affect bowel function and symptoms. The nerves can become more active during stress, causing the intestines to be more sensitive and squeeze (contract) more.
IBS can occur at any age, but it often begins in the teen years or early adulthood. It is twice as common in women as in men.
About 1 in 6 people in the U.S. have symptoms of IBS. It is the most common intestinal problem that causes patients to be referred to a bowel specialist (gastroenterologist).
Symptoms range from mild to severe. Most people have mild symptoms. Symptoms are different from person to person.
The main symptoms of IBS are abdominal pain, fullness, gas, and bloating that have been present for at least 3 days a month for the last 3 months. The pain and other symptoms will often:
Be reduced or go away after a bowel movement
Occur when there is a change in how often you have bowel movements
People with IBS may switch between constipation and diarrhea, or mostly have one or the other.
People with diarrhea will have frequent, loose, watery stools. They will often have an urgent need to have a bowel movement, which may be hard to control.
Those with constipation will have a hard time passing stool, as well as fewer bowel movements. They will often need to strain and will feel cramps with a bowel movement. Often, they do not release any stool, or only a small amount.
For some people, the symptoms may get worse for a few weeks or a month, and then decrease for a while. For other people, symptoms are present most of the time.
People with IBS may also lose their appetite.
Signs and tests
Most of the time, your doctor can diagnose IBS based on your symptoms, with few or no tests. Eating a lactose-free diet for 2 weeks may help the doctor check for a possible lactase deficiency.
There is no test to diagnose IBS. Tests may be done to rule out other problems:
Blood tests to see if you have celiac disease or a low blood count (anemia)
Stool cultures to check for an infection
Some patients will have a colonoscopy. During this test, a flexible tube is inserted through the anus to examine the colon. You may need this test if:
Symptoms began later in life (over age 50)
You have symptoms such as weight loss or bloody stools
You have abnormal blood tests (such as a low blood count)
Other disorders that can cause similar symptoms include:
Colon cancer (cancer rarely causes typical IBS symptoms, unless symptoms such as weight loss, blood in the stools, or abnormal blood tests are also present)
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; and George F. Longstreth, MD, Department of Gastroenterology, Kaiser Permanente Medical Care Program, San Diego, California. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.