Irritable bowel syndrome (IBS) is a very common gastrointestinal condition which is present in up to 17% of the general population. The classic gastrointestinal symptoms of IBS are chronic or recurrent abdominal pain and/or discomfort and associated alterations in bowel habits. However, many individuals with IBS also suffer from non-gastrointestinal symptoms. Rheumatologic symptoms, such as skin rashes, muscle contraction headache and myalgias, have been reported in two-thirds of IBS patients. Previous studies have found that IBS typically overlaps with fibromyalgia syndrome (FM) in the same patient, suggesting a common cause. FM occurs in up to 60% of patients with irritable bowel syndrome (IBS). Up to 70% of patients with a diagnosis of FM have symptoms of IBS. Following arthritis, FM is the most common condition seen in community rheumatologic practice. It is a common pain disorder in which the presence of multiple muscular tender points is associated with characteristic symptoms of generalized muscle aching, stiffness, fatigue and an abnormal sleep pattern. The diagnosis of FM is presently defined by criteria established by the American College of Rheumatology.

Although IBS is a chronic visceral (bowel) pain condition and FM is a chronic somatic (muscle) pain disorder, IBS and FM have common clinical characteristics: (1) the majority of patients associate stressful life events with the onset or exacerbation of symptoms, (2) the majority of patients complain of disturbed sleep and fatigue, (3) psychotherapy and behavioral therapies are efficacious in treating symptoms, and (4) certain medications can treat symptoms of both conditions. It has been suggested that IBS and FM have a common cause and that specific physiologic alterations may be responsible for the symptoms seen in these two conditions.

The UCLA Neuroenteric Disease Program is presently conducting research studies comparing the physiologic responses to visceral (bowel) or somatic (muscle) stimulus in individuals with IBS alone, FM alone, and both IBS and FM (IBS + FM). By comparing the responses of these three groups, we can learn about more about how chronic visceral and somatic pain are processed in these conditions.

The investigators at the UCLA Neuroenteric Disease Program have recently completed somatic perception studies in female patients with IBS alone, both IBS + FM, and healthy individuals. The responses to pressure which was placed on particular muscle tender points was measured. Like FM patients, patients with IBS + FM demonstrate somatic hyperalgesia (increased somatic pain sensation). However, patients with IBS alone have somatic hypoalgesia (decreased somatic pain sensation). This study demonstrates that while IBS patients have increased sensitivity to visceral pain, they are less sensitive to somatic pain.

UCLA studies evaluating visceral sensation have also been performed in these patients using a balloon catheter placed in the rectum and lower large intestine (colon). The balloon catheter is connected to a computerized pump which inflates the balloon and thus reaches specific pressure levels in the bowel. The patient’s sensations in response to the balloon inflation can be measured. In a recent study published in the journal Gastroenterology, the investigators at the UCLA Neuroenteric Disease Program showed that IBS patients have an altered perceptual response to irritation of the gastrointestinal tract by repetitive distention (inflation of balloon catheter). IBS patients have an increased sensitivity to balloon distention in the rectum, while healthy individuals do not. These findings suggest a compromised ability to activate the body’s own pain inhibition systems to counter-act the irritation of the gastrointestinal tract. The results of this study were also discussed in a previous issue of this newsletter (volume 1, issue 1, Spring 1997). Patients with both IBS + FM also demonstrate an altered perceptual response to balloon inflation in the gastrointestinal tract, but to a lesser degree than patients with IBS alone. These data suggest that IBS have altered perceptual responses to visceral and somatic pain/discomfort and the coexistence of FM alters these perceptual responses.

To determine if an alteration in how the brain processes visceral and somatic pain information in IBS and FM, the investigators at the UCLA Neuroenteric Disease Program are comparing the brain responses to visceral and somatic stimuli in patients with IBS alone, IBS + FM, and FM alone. Brain responses are assessed by positron emission tomography (PET) which can measure blood flow to brain areas in response to a particular stimulus. During this study, a visceral stimulus (balloon inflation in the rectum) and a somatic stimulus (pressure applied to a somatic tender point) are given. Results have shown that in response to a rectal balloon inflation, IBS patients exhibit activation in a portion of the brain which is called the prefrontal cortex. This brain area is involved in the attention and arousal to a particular stimulus and also in memory retrieval of past pain experiences. Patients with IBS + FM have activation in this same brain region to a somatic stimulus but not to a rectal stimulus. Most of the IBS + FM patients reported greater muscular pain due to FM than abdominal pain from IBS at the time of the PET study. These findings suggests that brain’s mechanisms of attention/arousal are activated in response to visceral pain in IBS, but in response to somatic pain in patients with coexistent FM.

In summary, clinical characteristics and a significant overlap of symptoms suggest that the functional syndromes IBS and FM may have a common etiology. Visceral and somatic perception studies and PET imaging have demonstrated that each of these conditions have specific responses to painful stimuli and that patients with both IBS and FM may have responses to somatic and visceral stimuli that are uniquely different from that of IBS alone and FM alone. Further studies including PET, visceral perception tests, and sleep studies are being completed in patients with IBS and/or FM. Hopefully, these studies will improve the understanding of chronic visceral and somatic pain conditions and lead to more effective treatment.

If you would like to learn more about our research program, or if you are interested in participating in one of our ongoing IBS and fibromyalgia studies, please contact our clinical trials office at (310) 268-3432.