The prevalence of SIBO and its relationship to several clinical conditions are unclear because of controversies related to both its detection and definition. Moreover, the clinical manifestations of SIBO overlap with those of many other gastrointestinal disorders. High clinical suspicion should be given to individuals with underlying disorders known to disrupt the protective elements that exist to prevent SIBO. The prevalence of SIBO varies depending on the population studied and the diagnostic methods used. In healthy individuals, SIBO has been described in 0–12 % using the glucose breath test and 20–22 % using the lactulose breath test. SIBO appears to be more prevalent in the elderly. In neonates and the elderly, SIBO may lead to significant morbidity or even death; however, exact mortality rates directly linked to SIBO are not available.

The most important factors contributing to the pathogenesis of SIBO are intestinal dysmotility, anatomical alterations of the gastrointestinal tract that predispose to stagnation of intestinal contents (e.g., large duodenal diverticula, prior gastric bypass surgery, resection of the ileocecal valve), low gastric acid production, and advanced age. To a lesser extent, bile, proteolytic pancreatic enzymes, and the innate immune system also protect against SIBO. Based on these factors, Table 11.1 presents the most common conditions associated with SIBO. It is not always easy, however, to detect a specific predisposing factor. SIBO may develop either in the presence of a specific pathogenetic mechanism (e.g., diabetes, blind loop syndrome, total gastrectomy, immunodeficiency states) or due to a combination of the aforementioned mechanisms. For example, chronic pancreatitis is associated with decreased intestinal motility and decreased production of pancreatic enzymes; advanced age is associated with gastric atrophy and intestinal dysmotility; obesity is related to altered gut flora and small bowel motility; and cirrhosis and nonalcoholic steatohepatitis are related to hypomotility and increased intestinal permeability.

Originally, SIBO was a condition thought to be associated with diarrhea, malnutrition, vitamin deficiencies, and weight loss in the presence of conditions that induce stagnation in the intestinal lumen; however, this “classic” presentation is now uncommon. Indeed, the majority of SIBO patients present with nonspecific symptoms such as abdominal pain or discomfort, flatulence, bloating, and a change in bowel habits in the absence of an obvious risk factor. Children with SIBO may present with diarrhea and abdominal pain, while malnutrition and vitamin deficiencies are usually absent. In contrast, elderly SIBO patients may be asymptomatic, or they may present with unexplained weight loss and malnutrition. SIBO symptoms may develop through different mechanisms that include gas production, induction of diarrhea, and malabsorption, as shown in Table 11.2.

While SIBO has traditionally been linked to diarrhea and malabsorption, there is evidence that SIBO is implicated in the development of disorders with no pathognomonic biochemical, immunological, and/or histological findings. Indeed, during the last decade, evidence has accumulated to support a role of SIBO for the development of irritable bowel syndrome (IBS). Table 11.3 summarizes the epidemiological, clinical, and translational evidence for this association. The controversy regarding the role of SIBO in IBS primarily originates from the accuracy of breath testing as the method to diagnose SIBO. As discussed in the SIBO diagnosis section below, hydrogen breath tests have lower accuracy for the detection of SIBO compared to the culture of proximal small intestinal contents, considered the diagnostic “gold standard.” A recent systematic literature review, however, showed that no test is appropriately validated for the diagnosis of SIBO.

Symptoms of diarrhea, weight loss, bloating, and flatulence in patients with a coexisting predisposition to SIBO, regardless of whether malabsorption has been demonstrated, should prompt the clinician to consider testing for bacterial overgrowth, especially if the patients have failed to respond to other empiric treatments. It is likely that SIBO is commonly overlooked in patients without known predisposing factors and in patients who have nonspecific symptoms. Table 11.4 shows the armamentarium of tests used to diagnose SIBO. In the absence of a “gold standard,” the most practical method of evaluating SIBO has been suggested to be a “test, treat, and assess outcome” approach with breath testing being used to help discriminate patients that may benefit from antibiotics by the normalization of the breath test after treatment.