IBS – Pathophysiology
Since there has been no identifiable cause to point to and lab tests are inconclusive, there are many theories that try to explain the disturbance in the body in people with IBS. Most of these theories are centered on abnormalities within the transit system of the abdomen through the rest of the gut (intestine, colon), hypersensitivity (especially to pain), and psychological factors.
Studies have been done comparing colon function in normal individuals to those suffering with IBS. Some of these studies have shown that there is a difference in patients with IBS and those that do not have IBS in the colon’s response to food. These studies showed a pattern of abnormal colonic movements in IBS patients more associated with diarrhea. It is suggested that electrical activity within the intestines is abnormal, resulting in abnormal stimulation of the colonic walls.
In the small intestine, abnormal movement patterns, when slowed or delayed, cause constipation in those more likely to get this type (constipation-predominant IBS) and if the movements are accelerated, it causes diarrhea (diarrhea-predominant IBS).
Some other theories suggest that the disorder is within the muscles of small and large intestines itself and that there is a hyperresponsiveness to normal stimulation. So some people believe that the problem lies with abnormal electrical activity and others propose that the electrical activity within the intestines is fine, that the problem lies with the intestinal smooth muscles being hyperresponsive. Those who believe that smooth muscles are the problem point to other problems like bladder dysfunction, which has been seen in 50% of patients with IBS but much less (13%) in those without IBS.
Hypersensitivity to Pain
Some experts believe the problem is with the hypersensitivity to pain some patients with IBS experience. Clinical studies have demonstrated that inflating the colon near the rectum stimulates pain at a lower threshold than normal patients. That is, a patient with IBS will feel pain sensations before a person without IBS does. Also, patients with IBS describe dermatomal (or regional) distribution of pain. It is believed that the hypersensitivity is due to neurons (nerve cells) in the spinal cord that are hyperexcitable.
CNS – Gut Abnormalities
There is a lot of interaction between the brain and the gut. There are many neurotransmitters in the brain and along the gut. These transmitters are important in determining: 1) how fast food passes in the GI tract, 2) controlling pain, 3) controlling emotions, and 4) confirming immunity. When this axis (CNS-gut) does not work properly, the symptoms of IBS can be seen.
Although it has not been proven that psychiatric problems cause IBS, it is well documented that IBS has been found to be more common in persons with psychiatric disorders. These patients have more relapses and worse, illness. The psychiatric disorders often associated with IBS include major depression, anxiety disorders, panic disorders, and personality disorders. These disorders are not believed to cause IBS but rather affect the person’s view of their illness.
Stress leads to GI problems and in fact, 77% of patients diagnosed with a psychiatric Axis I disorder have disturbances with their gastrointestinal functions. It has also been reported that patients with a history of physical abuse or sexual abuse have increased prevalence of IBS. Research continues at this time to see if IBS leads to the psychiatric disturbance or if the psychiatric problems lead to IBS.