Mechanisms of irritable bowel syndrome

An excellent succinct review of the various peripheral mechanisms of irritable bowel syndrome (IBS) has been published (NEJM 2012; 367: 1626-35).  Understanding these mechanisms is crucial in developing and targeting appropriate therapy.

Peripheral factors affecting IBS (Table 1 in review):

  1. Colonic motility –affects up to 45% of diarrhea-predominant IBS and 25% of constipation-predominant IBS.  Specific factors include enteroendocrine cell products (eg. 5-HT, granins), organic acids (eg. bile acids, short chain fatty acids [SCFAs]) impaired bile acid synthesis.
  2. Colonic motor and sensory response to food ingestion. Factors like fat content of meal can contribute to pain, urgency and diarrhea.
  3. Sensing responses in small bowel and colon.  Food stimulation of enteroendocrine cell products may trigger diarrhea, bloating and pain.
  4. Colonic mucosal permeability.  This may lead to malabsorption of carbohydrates or fats and subsequently increased levels of SCFAs.  Also, could trigger immune activation and altered feedback of bile acid synthesis.
  5. Mucosal immune activation.  Previous gastroenteritis along with mast cells, T lymphocytes, and circulating cytokines may be involved factors.
  6. Colonic microbiome.  There may be increased types of some bacteria (eg. firmicutes).  Antibiotics and probiotics could influence the microbiome. Fermentable oligosaccharides, disaccharides, and polyols (FODMAPs) are a group of foods that may trigger IBS symptoms, possibly due to a relationship with the colonic microbiome.

With regard to gluten intolerance, the author notes that the prevalence of celiac disease among IBS is similar to that among controls; however, a subgroup of individuals with diarrhea-predominant IBS respond to a gluten-free diet.

Some genetic factors have been identified which can contribute to IBS:

  • mutation in the guanylate cyclase C secretory pathway
  • mutations that increase the risk of postinfectious IBS
  • genetic variability in bile acid synthesis
  • variation in expression of neurotransmitters and cytokines

Towards the end of the review, the author notes that “IBS is no longer regarded as an idiopathic bowel dysfunction” due to stress.  The specific factors that have been identified will likely be further defined and likely lead to more specific individualized therapy.  Potential treatments:

  • diets -including gluten-free and FODMAPs
  • bile acid sequestrants and 5-HT3 antagonists
  • prokinetics or secretagogues in patients with constipation-predominant IBS
  • probiotics and non-absorbed antibiotics
  • antiinflammatory agents
  • tignt-junction modifiers
  • biofeedback

Previous related blog entries:

Additional references:
  • -J Pediatr 2009; 155: 416.  n=43 children & 56 control pts.  High incidence of abnormal lactulose (65%) breath test with IBS but not control pts (7%).  (lactulose 10g given in 20mL)
  • -Gastroenterol 2009; 137: 766.  Notes relatively weak data supporting use of antispasmotics, probiotics, and antidepressants for IBS.
  • -Am J Gastroenterol 2010; 105: 859-865.  n=466 & 451 controls.  IBS pts with lower incidence of adenomas (7.7.% vs 26%).  9% had diverticulosis (lower).  Microscopic colitis present in 1.5%.
  • -Gastroenterol 2002; 123: 2105-07. & 2108-2131. AGA guidelines for IBS
  • -Gastroenterol 2007; 133: 799.  Natural hx of functional disorders: 20% persist w same Sx, 40% develop other Sx, 40% get better.  Large study from Olmstead county (n=1365)
    • -Clin Gastro & Hep 2005; 3: 397.  managing pts c severe IBS; advocates low dose tricyclics..
  • -Am J Gastro 2003; 98: 412-9.  use of neomycin for SBBO in IBS (43% response). 84% IBS pts c abnl lactulose vs 20% placebo
  • -Gastroenterol 2003; 124: S1152.  only 4 of 33 IBS pts had abnl jejunal samples
  • -J Musculoskel Pain 2001; 9:107-113.  78% of fibromyalgia pts c abnl BHT.

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