Irritable Bowel Syndrome (IBS)

1. Irritable Bowel Syndrome (IBS) Slideshow Pictures
2. Digestive Disease Myths Slideshow Pictures
3. Diverticulitis (Diverticulosis) Slideshow Pictures

Medical Author:

Jay W. Marks, MD

Jay W. Marks, MD

Jay W. Marks, MD, is a board-certified internist and gastroenterologist. He graduated from Yale University School of Medicine and trained in internal medicine and gastroenterology at UCLA/Cedars-Sinai Medical Center in Los Angeles.

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Medical Editor:

Bhupinder Anand, MD

Medical Editor:

William C. Shiel Jr., MD, FACP, FACR

William C. Shiel Jr., MD, FACP, FACR

Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

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• Irritable bowel syndrome (IBS) is a functional disease, that is, a disease in which the intestine (bowel) functions abnormally.
• Theories of the cause of IBS include abnormal input from intestinal sensory nerves, abnormal processing of input from the sensory nerves, and abnormal stimulation of the intestines by the motor nerves.
• The primary symptoms of IBS are constipation, diarrhea, and abdominal pain. Secondary symptoms include abnormal passage of stool, abnormal form of stool, increased amounts of mucus in the stool, and a subjective feeling of abdominal distention (bloating).
• IBS is diagnosed on the basis of typical symptoms (Rome Criteria) and the absence of other intestinal and non-intestinal diseases that may give rise to the symptoms. Testing in IBS is directed primarily at excluding the presence of other intestinal diseases and non-intestinal diseases.
• Treatment of IBS consists primarily of medications to control constipation, diarrhea, and abdominal pain. Anti-depressant medication and psychological treatments also may be used. It is not clear if dietary alterations have much effect on the symptoms of IBS except for increases in dietary fiber, which may improve constipation.
• Although it has been hypothesized that IBS may be caused by intestinal bacteria, specifically by small intestinal bacterial overgrowth, there is little rigorous scientific support for the hypothesis. On the other hand, there are a limited number of rigorous scientific studies demonstrating that probiotics and antibiotics improve the symptoms of IBS.
• Future advances in the treatment of IBS depend on a clearer understanding of its cause(s).

Irritable bowel syndrome (IBS) is one of the most common ailments of the bowel (intestines) and affects an estimated 15% of people in the US. The term, irritable bowel, is not a particularly accurate one since it implies that the bowel is responding irritably to normal stimuli, and this may or may not be the case. The several terms used for IBS, including spastic colon, spastic colitis, and mucous colitis, attest to the difficulty of getting a descriptive handle on the ailment. Moreover, each of the other names is itself as problematic as the term IBS.

IBS is best described as a functional disease. The concept of functional disease is particularly useful when discussing diseases of the gastrointestinal tract. The concept applies to the muscular organs of the gastrointestinal tract; the esophagus, stomach, small intestine, gallbladder, and colon. What is meant by the term, functional, is that either the muscles of the organs or the nerves that control the organs are not working normally, and, as a result, the organs do not function normally. The nerves that control the organs include not only the nerves that lie within the muscles of the organs but also the nerves of the spinal cord and brain to which they are connected.

Some gastrointestinal diseases can be seen and diagnosed with the naked eye, such as ulcers of the stomach when visualized by certain methods. Thus, ulcers can be seen at surgery, on X-rays, and at endoscopy. Other diseases cannot be seen with the naked eye but can be seen and diagnosed under the microscope. For example, celiac disease and collagenous colitis are diagnosed by microscopic examination of biopsies of the small intestine and colon, respectively. In contrast, gastrointestinal functional diseases cannot be seen with the naked eye or with the microscope. In some instances, the abnormal function can be demonstrated by tests, for example, gastric emptying studies or antro-duodenal motility studies. However, these tests often are complex, are not widely available, and do not reliably detect the functional abnormalities. Accordingly, by default, functional gastrointestinal diseases are those involving the abnormal function of gastrointestinal organs in which abnormalities cannot be seen in the organs with either the naked eye or the microscope.

Occasionally, diseases that are thought to be functional are ultimately found to be associated with abnormalities that can be seen. Then, the disease moves out of the functional category. An example of this is Helicobacter pylori infection of the stomach. Many patients with mild upper intestinal symptoms who were thought to have "functional" abnormal function of the stomach or intestines have been found to have an infection of the stomach with Helicobacter pylori. This infection can be diagnosed by seeing the bacterium and the inflammation (gastritis) it causes under the microscope. When the patients are treated with antibiotics, the Helicobacter pylori, gastritis, and symptoms disappear. Thus, recognition of Helicobacter pylori infection removed some patients' diseases from the functional category.

The distinction between functional disease and non-functional disease may, in fact, be blurry. Thus, even functional diseases probably have associated biochemical or molecular abnormalities that ultimately will be able to be measured. For example, functional diseases of the stomach and intestines may be shown ultimately to be caused by reduced levels of normal chemicals within the gastrointestinal organs, the spinal cord, or the brain. Should a disease that is demonstrated to be due to a reduced chemical still be considered a functional disease? I think not. In this theoretical situation, we can't see the abnormality with the naked eye or the microscope, but we can measure it. If we can measure an associated or causative abnormality, the disease probably should no longer be considered functional.

Despite the shortcomings of the term functional, the concept of a functional abnormality is useful for approaching many of the symptoms originating from the muscular organs of the gastrointestinal tract. This concept applies particularly to those symptoms for which there are no associated abnormalities that can be seen with the naked eye or the microscope.

While IBS is a major functional disease, it is important to mention a second major functional disease referred to as dyspepsia, or functional dyspepsia. The symptoms of dyspepsia are thought to originate from the upper gastrointestinal tract; the esophagus, stomach, and duodenum (the first part of the small intestine). The symptoms include upper abdominal discomfort, bloating (the subjective sense of abdominal fullness without objective distension), or objective distension (swelling, or enlargement). The symptoms may or may not be related to meals. There may be nausea with or without vomiting and early satiety (a sense of fullness after eating only a small amount of food).

The study of functional disorders of the gastrointestinal tract often is categorized by the organ of involvement. Thus, there are functional disorders of the esophagus, stomach, small intestine, colon, and gallbladder. The research on functional disorders is focused mostly on the esophagus and stomach (such as dyspepsia), perhaps because these organs are the easiest to reach and study. Research into functional disorders affecting the small intestine and colon (for example, IBS) is more difficult to conduct and there is less agreement among the research studies. This probably is a reflection of the complexity of the activities of the small intestine and colon and the difficulty in studying these activities. Functional diseases of the gallbladder, like those of the small intestine and colon, also are more difficult to study.

Recently, experts in the field of functional gastrointestinal disorders have met to begin organizing an approach to the functional disorders, specifically by setting definitions for the various functional diseases (the Rome I, II, and III criteria). The definition for IBS has been narrowed greatly; as discussed later in this article.

Picture of the organs and glands in the abdomen