What is irritable bowel syndrome (IBS)?

Irritable bowel syndrome (IBS) is one of the most common ailments of the  bowel (intestines) and affects an estimated 15% of persons in the US. The term,  irritable bowel, is not a particularly good one since it implies that the bowel  is responding irritably to normal stimuli, and this may or may not be the case.  The several names 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.

Some gastrointestinal  diseases can be seen and diagnosed with the naked eye, such as ulcers of the  stomach. Thus, ulcers can be seen at surgery, on x-rays, and at endoscopies.  Other diseases cannot be seen with the naked eye but can be seen and diagnosed  with the microscope. For example, celiac disease and collagenous colitis are  diagnosed by microscopic examination of biopsies of the small bowel 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 would be  Helicobacter pylori infection of the stomach. Many patients with mild upper  intestinal symptoms who were thought to have 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, 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 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 amount of  research on functional disorders has been focused mostly on the esophagus and  stomach (such as dyspepsia), perhaps because these organs are 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.

Most  individuals are surprised to learn they are not alone with symptoms of IBS. In  fact, irritable bowel syndrome (IBS) affects approximately 10-20% of the general  population. It is the most common disease diagnosed by gastroenterologists  (doctors who specialize in medical treatment of disorders of the stomach and  intestines) and one of the most common disorders seen by primary care  physicians.

Sometimes irritable bowel syndrome is referred to as spastic  colon, mucous colitis, spastic colitis, nervous stomach, or irritable  colon.

Irritable bowel syndrome, or IBS, is generally classified as a  "functional" disorder. A functional disorder refers to a disorder or disease  where the primary abnormality is an altered physiological function (the way the  body works), rather than an identifiable structural or biochemical cause. It  characterizes a disorder that generally can not be diagnosed in a traditional  way; that is, as an inflammatory, infectious, or structural abnormality that can  be seen by commonly used examination, x-ray, or blood test.

Irritable bowel syndrome is understood as a multi-faceted disorder.  In people with IBS, symptoms result from what appears to be a disturbance in the  interaction between the gut or intestines, the brain, and the autonomic nervous  system that alters regulation of bowel motility (motor function) or sensory  function.

Irritable bowel syndrome is characterized by a group of  symptoms in which abdominal pain or discomfort is associated with a change in  bowel pattern, such as loose or more frequent bowel movements, diarrhea, and/or  constipation.

Treatment options are available to manage IBS—whether  symptoms are mild, moderate, or severe.

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