Small intestinal bacterial overgrowth
Small intestinal bacterial overgrowth is excessive bacterial colonization of the proximal small bowel, driven by stasis from strictures, blind loops, or dysmotility, or by loss of gastric-acid and immune defenses, producing bloating, diarrhea, and fat and B12 malabsorption. Diagnosis rests on a carbohydrate breath test or jejunal aspirate culture, and treatment corrects the underlying cause while a course of rifaximin or another antibiotic reduces the bacterial burden.
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What this article covers
- Overview
Definition: small intestinal bacterial overgrowth (SIBO) is excessive colonization of the proximal small bowel by bacteria, typically colonic-type organisms, that the small intestine normally keeps sparse.
- Epidemiology
Underlying disease drives prevalence: SIBO is common wherever intestinal stasis or blind loops occur, including after bariatric surgery, in Roux-en-Y gastric bypass and biliopancreatic diversion with…
- Risk factors
Small-bowel stasis from structural disease: small-bowel diverticula, strictures (including Crohn disease), and postoperative blind loops such as enteroenterostomy or a gastrojejunocolic fistula.
- Pathophysiology
Failure of normal defenses: peristalsis (the most important defense), gastric acid, and small-bowel IgA normally keep the proximal small intestine at low bacterial counts; when one or more of these is impaired, bacteria proliferate.
- Clinical features
Chronic diarrhea: the most common presenting symptom, often watery and nonbloody.
- Differential diagnosis
Irritable bowel syndrome: bloating and altered bowel habits without malabsorption, B12 deficiency, or a structural predisposing lesion, though SIBO and IBS-D symptoms overlap and can coexist (irritable bowel syndrome).
- Diagnosis
Establish the diagnosis with a positive carbohydrate breath test or jejunal aspirate culture in the appropriate clinical context: typical symptoms (abdominal discomfort, flatulence, bloating…
- Management
Treat the underlying anatomic or motility cause first when identifiable: surgical correction of a blind loop or stricture, medical therapy for Crohn disease, or optimized glycemic control in diabetic…
- Complications
Fat-soluble vitamin deficiency: deficiencies of vitamins A, D, E, and K from chronic steatorrhea, with vitamin K deficiency risking coagulopathy and vitamin D deficiency contributing to metabolic bone disease.
- Prognosis
Generally favorable with treatment of the underlying cause: symptoms and malabsorption typically improve substantially with antibiotic therapy, and resolution of steatorrhea and bloating often occurs…
- Prevention
Correct reversible anatomic and motility causes: definitive surgical repair of a blind loop, resection or dilation of a symptomatic stricture, and euglycemic control (target hemoglobin A1c under 7%)…
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