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.



What causes IBS?
As discussed previously, IBS is believed to be due to the abnormal function (dysfunction) of the muscles of the organs of the gastrointestinal tract or the nerves controlling the organs. The nervous control of the gastrointestinal tract, however, is complex. A system of nerves runs the entire length of the gastrointestinal tract from the esophagus to the anus in the muscular walls of the organs. These nerves communicate with other nerves that travel to and from the spinal cord. Nerves within the spinal cord, in turn, travel to and from the brain. (The gastrointestinal tract is exceeded in the numbers of nerves it contains only by the spinal cord and brain.) Thus, the abnormal function of the nervous system in IBS may occur in a gastrointestinal muscular organ, the spinal cord, or the brain.

The nervous system that controls the gastrointestinal organs, as with most other organs, contains both sensory and motor nerves. The sensory nerves continuously sense what is happening within the organ and relay this information to nerves in the organ’s wall. From there, information can be relayed to the spinal cord and brain. The information is received and processed in the organ’s wall, the spinal cord, or the brain. Then, based on this sensory input and the way the input is processed, commands (responses) are sent to the organ over the motor nerves. Two of the most common motor responses in the intestine are contraction or relaxation of the muscle of the organ and secretion of fluid and/or mucus into the organ.

As already mentioned, abnormal function of the nerves of the gastrointestinal organs, at least theoretically, might occur in the organ, spinal cord, or brain. Moreover, the abnormalities might occur in the sensory nerves, the motor nerves, or at processing centers in the intestine, spinal cord, or brain. Some researchers argue that the cause of functional diseases is abnormalities in the function of the sensory nerves. For example, normal activities, such as stretching of the small intestine by food, may give rise to abnormal sensory signals that are sent to the spinal cord and brain, where they are perceived as pain.

Other researchers argue that the cause of functional diseases is abnormalities in the function of the motor nerves. For example, abnormal commands through the motor nerves might produce a painful spasm (contraction) of the muscles. Still others argue that abnormally functioning processing centers are responsible for functional diseases because they misinterpret normal sensations or send abnormal commands to the organ. In fact, some functional diseases may be due to sensory dysfunction, motor dysfunction, or both sensory and motor dysfunction. Still others may be due to abnormalities within the processing centers One area that is receiving a great deal of scientific attention is the potential role of gas produced by intestinal bacteria in patients with IBS. Studies have demonstrated that patients with IBS produce larger amounts of gas than individuals without IBS, and the gas may be retained longer in the small intestine. Among patients with IBS, abdominal size increases over the day, reaching a maximum in the evening and returning to baseline by the following morning. In individuals without IBS, there is no increase in abdominal size during the day.

There has been a great deal of controversy over the role that poor digestion and/or absorption of dietary sugars may play in aggravating the symptoms of IBS. Poor digestion of lactose, the sugar in milk, is very common as is poor absorption of fructose, a sweetener found in many processed foods. Poor digestion or absorption of these sugars could aggravate the symptoms of IBS since unabsorbed sugars often cause increased formation of gas.

Although these abnormalities in production and transport of gas could give rise to some of the symptoms of IBS, much more work will need to be done before the role of intestinal gas in IBS is clear. For more information about intestinal gas, please read the Intestinal Gas article.

Dietary fat in healthy individuals causes food as well as gas to move more slowly through the stomach and small intestine. Some patients with IBS may even respond to dietary fat in an exaggerated fashion with greater slowing. Thus, dietary fat could–and probably does–aggravate the symptoms of IBS.


What are IBS symptoms?
The primary purpose of the gastrointestinal tract is to digest (break down) and absorb (take into the blood stream) food. In order to fulfill this purpose, food must be ground, mixed, and transported through the intestines, where it is digested and absorbed. In addition, undigested and unabsorbed portions of the food must be eliminated from the body.

In functional diseases of the gastrointestinal tract, the grinding, mixing, digestion, and absorption functions are disturbed to only a minor degree. These functions are essentially maintained, perhaps because of a built–in over–capacity of the gastrointestinal tract to perform these functions. The most commonly affected function in these diseases is transportation. In the stomach and small intestine, the symptoms of slowed transportation are nausea, vomiting, abdominal bloating (the sensation of abdominal fullness), and abdominal distention (enlargement). The symptom of rapid transportation usually is diarrhea. The interpretation of symptoms, however, may be more complicated than this. For example, let’s say that a person has abnormally rapid emptying of the stomach. The sensing of this rapid emptying by the intestinal sensory nerves normally brings about a motor nerve response to slow emptying of the stomach and transportation through the small intestine. Thus, rapid emptying of the stomach may give rise to symptoms of slowed transportation.

In the colon, abnormally slowed or rapid transportation results in constipation or diarrhea, respectively. In addition, there may be increased amounts of mucus coating the stool or a sense of incomplete evacuation after a bowel movement.

As discussed previously, normal sensations may be abnormally processed and perceived. Such an abnormality could result in abdominal bloating and pain. Abnormally processed sensations from the gastrointestinal organs also might lead to motor responses that cause symptoms of slowed or rapid transportation.

Slowed transportation of digesting food through the small intestine may be complicated, for example, by bacterial overgrowth. In bacterial overgrowth, gas–producing bacteria that are normally restricted to the colon move up into the small intestine. There, they are exposed to greater amounts of undigested food than in the colon, which they turn into gas. This formation of gas can aggravate bloating and/or abdominal distention and result in increased amounts of flatus (passing gas, or flatulence) and diarrhea.

The gastrointestinal tract has only a few ways of responding to diseases. Therefore, the symptoms often are similar regardless of whether the diseases are functional or non–functional. Thus, the symptoms of both functional and non–functional gastrointestinal diseases are nausea, vomiting, bloating, abdominal distention, diarrhea, constipation, and pain. For this reason, when functional disease is being considered as a cause of symptoms, it is important that the presence of non–functional diseases be excluded (ruled out). In fact, the exclusion of non–functional diseases usually is more important in evaluating patients who are suspected of having functional disease. This is so, in large part, because the tests for diagnosing functional disease are complex, not readily available, and often not very reliable. In contrast, the tests for diagnosing non–functional diseases are widely available and sensitive (able to diagnose most cases).

Source: http://www.medicinenet.com/