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 patient’s 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.