Small Intestinal Bacterial Overgrowth Syndrome

Small intestinal bacterial overgrowth (SIBO) is an under-recognized cause of malabsorption in the geriatric population, characterized by the excessive growth of bacteria in the small bowel, at times bearing a resemblance to colonic flora. SIBO is defined as a condition in which part of the small bowel harbors for a long time bacterial counts over 103 CFU/ml, determined in a duodenal or jejunal aspirate. The spectrum of SIBO is still being elucidated, including its association with functional gastrointestinal disorders. Prevalence of SIBO varies with testing methodology used. While age may be a predisposing factor, far more important is the presence of additional comorbid risk factors such as diabetes mellitus, systemic sclerosis, irritable bowel syndrome, Parkinson’s disease, and stasis syndromes or anatomical abnormalities, such as encountered following intestinal or bariatric surgery, blind loops, small bowel diverticular disease, fistulae, and ileal valve dysfunction. SIBO may be caused by defective protective antibacterial mechanisms, e.g., achlorhydria, pancreatic exocrine insufficiency, immunodeficiency syndromes, malfunction or lack of the ileocecal valve, intestinal obstruction, small intestinal diverticula, fistulae, surgical blind loop, and/or motility disorders (e.g., as in scleroderma, autonomic neuropathy in diabetes mellitus, postradiation enteropathy, and small intestinal pseudo-obstruction). SIBO that occurs more often with a stagnant loop as a result of dysmotility of the small bowel (also termed blind loop syndrome without an anatomical blind loop) or a surgical procedure may occur without an identifiable cause. Manifestations such as bloating, gas, diarrhea, and discomfort are common but non-specific and do not make a definitive diagnosis; bloating is in fact the most common symptom. Several medications including those that impair immune function such as steroids and opioids and the excessive or prolonged use of PPIs are recognized as contributory. Diagnostic methods include aspiration of duodenal or jejunal contents for culture, the gold standard, or the less invasive approach using glucose, lactulose, or hydrogen breath tests, which vary in sensitivity and specificity. Management typically involves short courses of antibiotics; while several antimicrobials are used, rifaximin has shown the greatest promise in management. Dietary patterns may influence response to therapy. Recurrences typically follow months following antibiotic therapy, necessitating repeat courses. Addressing the risk factors is important, although often little can be done to correct anatomical blind loops.