Unexplained diarrhea may be the most reliable symptom of small intestinal bacterial overgrowth (SIBO) in at-risk patients, according to a new clinical practice update from the American Gastroenterological Association.
“In those predisposed to SIBO due to anatomical, pathological, pharmacological or other changes that promote stasis or recirculation of colonic contents and/or impaired resistance to bacteria, SIBO will lead to diarrhea and can progress to a full-blown malabsorption syndrome” marked by steatorrhea and vitamin deficiencies, wrote Eamonn M.M. Quigley, MD, of Houston Methodist Hospital and Weill Cornell Medical College in Houston together with his fellow experts in Gastroenterology. But malabsorption is uncommon in patients whose SIBO is not caused by structural abnormalities, and gastrointestinal symptoms are “weakly predictive at best” if patients lack clear risk factors for SIBO, the experts cautioned.
The growing availability of breath testing has fueled diagnoses of SIBO, which the lay press often implicates in various disorders even though SIBO has no clear clinical or laboratory definition. Recent progress in techniques to measure bacterial populations and their metabolic products “should provide much needed clarity,” but for now, a SIBO diagnosis simply means that a patient’s presenting symptoms or laboratory findings are attributed to bacterial changes in the small intestine, the experts wrote.
Detecting SIBO also remains challenging. Most patients have normal results on routine laboratory tests, and there is not enough evidence to support testing for inflammatory markers such as fecal calprotectin. Patients with SIBO may have increased folate levels because of bacterial production of folic acid. Vitamin B12 and other nutrient deficiencies also occur but are less common. The preferred diagnostic method is culture of a duodenal aspirate, and recent research supports a cutoff value of greater than 103 CFUs of coliform bacteria per mL. Breath testing is less invasive but “more complex than simply measuring hydrogen,” the experts stressed. Methane-producing microorganisms (methanogens) suppress hydrogen on a breath test (fortunately, standard breath tests measure methane). Furthermore, a positive methane breath test also has been linked to constipation-predominant irritable bowel syndrome (IBS). Recent studies also suggest that lactulose breath testing is more sensitive than glucose for identifying SIBO in patients with IBS.
Antibiotic therapy is the treatment mainstay but remains largely empiric. The goal is to improve SIBO symptoms, not eradicate bacteria from the small intestine. Ideally, the antimicrobial regimen should cover both aerobic and anaerobic bacteria, but clinicians should be mindful of the risks of chronic broad-spectrum antibiotic exposure. In studies, a single 7- to 10-day antibiotic course improved symptoms in approximately 45%-90% of patients with SIBO (rates of breath test response were lower). For patients with IBS and SIBO, rifaximin (which is poorly absorbed) produced encouraging results in two phase 3 studies, but most patients did not receive breath testing, the experts noted. Patients with recurrent SIBO symptoms may need multiple courses of antibiotics with specific regimens rotated to help prevent resistance. “Decisions on management should be individualized and also [should factor in] such risks as diarrhea, Clostridiodes difficile infection, intolerance, and cost,” the experts wrote. “It is not necessary to repeat diagnostic tests for SIBO following antibiotic therapy [if] gastrointestinal symptoms respond.”
Dr. Quigley disclosed financial ties to 4D Pharma, Alimentary Health, Allergan, Biocodex, Biomerica, Ironwood, Salix, Takeda, Vibrant, and Zealand. He also disclosed patents with and equity in Alimentary Health. Both of his coauthors also disclosed ties to various pharmaceutical companies.
SOURCE: Quigley EMM et al. Gastroenterology. 2020 Jun 1. doi: 10.1053/j.gastro.2020.06.090.