IBS and Visceral Hypersensitivity

Visceral hypersensitivity, the experience of pain in internal organs (viscera) at an increased level than what is normally expected, may play a role in irritable bowel syndrome (IBS). This pain sensitivity is usually studied using some variation of balloon distention in the rectum, a procedure in which a balloon is inserted into the rectum and slowly inflated. As an overall research trend, people who suffer from IBS experience discomfort and pain in the rectal area at lower levels of pressure than individuals who do not suffer from IBS. The issue is not a simple one. The following is an overview of current research findings regarding IBS and visceral hypersensitivity. As you read this, please keep foremost in your mind that this is not a personal failing on the part of IBS patients. This hypersensitivity is merely the body’s way of reacting to the experience of intestinal pain.

Research Findings

• IBS patients label rectal stimulation as being painful or uncomfortable at lower levels of stimulation compared to healthy individuals.
• This hypersensitivity to stimulation is also seen when the stimulation occurs in the small intestine or esophagus.
• This hypersensitivity is not seen when the stimulation is directed at the outer body. In fact, the evidence to date indicates that pain tolerance to external stimuli is equal, if not better, in IBS patients compared to healthy subjects.
• Hypersensitivity to visceral stimulation under research conditions is seen in approximately 50% of IBS patients. The fact that it is not seen in all IBS patients prevents the measurement of sensitivity from serving as a diagnostic tool.
• There is some evidence that visceral hypersensitivity is more prevalent in women.
• Hypersensitivity rates were equal in IBS patients regardless of whether they suffered from diarrhea-predominant IBS (IBS-D) or constipation-predominant IBS (IBS-C). The lack of correlation between sensitivity and predominant symptom pattern also prevents hypersensitivity from serving as a diagnostic marker.
• Healthy volunteers who received glycerol, a type of laxative which triggers rapid bowel emptying, exhibited subsequent hypersensitivity. It thus appears that the experience of unusual pain or irritation can set the stage for the development of visceral hypersensitivity.

The Brain-Gut Connection

It appears likely that the visceral hypersensitivity seen in some IBS patients is a result of changes in nervous system functioning on both the level of the intestines and the brain. At the level of the gut, it seems as if nerve pathways in the gastrointestinal tract become sensitized to stimulation, resulting in over-reactivity and resulting in pain amplification. As for the brain, brain imaging studies provide even more clues: In comparing healthy individuals with IBS patients, there are significant differences in the parts of the brain that are activated in response to pain. In individuals who do not have IBS, rectal distension triggers a response in parts of the brain that are associated with modulating pain. In IBS patients, this same rectal stimulation triggers a response in the parts of the brain associated with vigilance and anxiety -- parts of the brain that serve to amplify the sensation of pain.

Is Inflammation Involved?

Preliminary research is beginning to point to the existence of a mild inflammatory process, particularly in post-infectious IBS (IBS-PI) cases. The cells involved in this inflammatory process are very close to GI nerve cells and thus may contribute to the sensitization that result in pain amplification. This is a very new, but exciting avenue in IBS research.

Further Reading:

• Five Reasons Why IBS Is Not All in Your Head
  
• Motility Dysfunction in Irritable Bowel Syndrome
• The Brain Gut Connection in Irritable Bowel Syndrome
• Inflammation and Irritable Bowel Syndrome
• Gut Bacteria and IBS

_Sources:

_{Chang, L. Brain responses to visceral and somatic stimuli in irritable bowel syndrome: a central nervous system disorder? Gastroenterological Clinics of North America 2005 34:271-279.}

_{Delvaux, M. Role of visceral sensitivity in the pathophysiology of irritable bowel syndrome Gut 2002 51:i61-i67.}

_{Jones, J., Boorman, J., Cann, P., Forbes, A., Gomborone, J., Heaton, K., Hungin, P., Kumar, D., Libby, G., Spiller, R., Read, N., Silk, D., & Whorwell, P. British Society of Gastroenterology guidelines for the management of the irritable bowel syndrome Gut 2000 47:ii1-ii19.}

_{Kuiken, S.D., Lindeboom, R., Tytgat, G.N., & Boeckxstaens, G.E. "Relationship Between Symptoms and Hypersensitivity to Rectal Distension in Patients With Irritable Bowel Syndrome" Alimentary Pharmacology & Therapeutics 2005 22:157-164.}

_{Naliboff, B.D., Munakata, J., Fullerton, S., Gracely, R.H., Kodner, A., Harraf, F., & Mayer, E.A. Evidence for two distinct perceptual alterations in irritable bowel syndrome Gut 1997 41:505-512.}