Irritable bowel syndrome (IBS) is defined as abdominal pain or discomfort and altered bowel habits in the absence of any other explanation for the symptoms. Ten to fifteen percent of the US population is affected to some degree, women are affected more often than men, and symptoms are more common in people between 30 and 50 years of age. (Mertz HR. Irritable bowel syndrome. N Engl J Med 2003;349: 2136-46)
The cause of IBS is unknown. Underlying factors involved in the generation of symptoms include altered intestinal motility and increased sensitivity of the gut to various stimuli.
Proposed Mechanisms (Causes) of Irritable Bowel Syndrome (IBS)
- Stress associated with increased levels of cortisol, slowed gastric emptying, and accelerated colonic transit
- Abnormal response to brain-intestine activation signals
- Altered colonic motility and abnormal neuronal function
- Heightened intestinal and colonic response to eating
- Abnormal gas propulsion and expulsion
- Subclinical inflammation with production of prostaglandins, bradykinins, 5-HTP, adenosine, and other factors that affect intestinal motility
- Intolerance to certain dietary substances, such as dairy or wheat
- Aberrant nocturnal “housekeeper stripping wave” of the small intestine, with subsequent small intestinal bacterial overgrowth
(Talley NJ, Spiller R. Irritable bowel syndrome: a little understood organic bowel disease? Lancet 2002;360:555-64 and Pimentel M, et al. Lower frequency of MMC is found in IBS subjects with abnormal lactulose breath test, suggesting bacterial overgrowth. Dig Dis Sci 2002;47:2639-43)
The symptoms of IBS may mimic those of other gastrointestinal conditions. Crohn’s disease, ulcerative colitis, celiac disease, thyroid dysfunction, or even cancer can be mistaken for IBS; usually, though, these diseases exhibit “red flags” (anemia, blood in the stool, fever, weight loss, or onset after 50 years of age) that lead physicians to an alternative diagnosis.
Patients with IBS often consult a physician for abdominal pain, bloating, flatulence, and constipation or diarrhea. If no “red flag” signs or symptoms are present, a diagnosis of IBS can be made on the basis of certain criteria.
Diagnostic Criteria for Irritable Bowel Syndrome
Manning criteria
IBS is diagnosed if three of the following are present:
- Abdominal pain
- Pain relief with defecation
- Increased stool frequency with pain
- Looser stools with pain
- Mucus in stools
- Feeling of incomplete evacuation
Rome I criteria
IBS is diagnosed if the following are present:
- At least three months of continuous or recurrent abdominal pain that is relieved with defecation and/or is associated with a change in stool consistency
- Plus, at least two of the following on at least 25 percent of days:
- Altered stool frequency
- Altered stool form or passage
- Passage of mucus
- Bloating or feeling of abdominal distension
(From Fass R, et al. Evidence- and consensus-based practice guidelines for the diagnosis of irritable bowel syndrome. Arch Intern Med 2001;161:2081-88)
Until recently, IBS was considered a “diagnosis of exclusion”: other bowel disorders had to be ruled out before irritable bowel syndrome could be diagnosed. This usually led to costly and invasive testing. It is now clear that IBS can be confidently identified in individuals who meet criteria and who exhibit no evidence of other GI disease. (Cash BD, et al. The utility of diagnostic tests in irritable bowel syndrome patients: a systematic review. Am J Gastroenterol 2002;97:2812-19)
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