Small intestinal bacterial overgrowth

• SIBO is a pathologically excessive growth of bacteria in the small intestine.
• Core Mechanism: Excessive numbers of bacteria in the small intestine (<10^5 organisms/mL is normal), leading to fermentation of carbohydrates, malabsorption, and mucosal inflammation.

Etiology

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• Disruption of protective mechanisms (gastric acidity, peristalsis, ileocecal valve).
• Anatomic abnormalities:
  • Blind loops (e.g., Billroth II reconstruction, Roux-en-Y gastric bypass). c
  • Diverticula, strictures (Crohn disease, radiation), fistulas.
• Motility disorders:
  • Systemic sclerosis (scleroderma).
  • Diabetic gastroparesis/enteropathy.
  • Chronic intestinal pseudo-obstruction.
• Medications:
  • Chronic PPI or H2RA use (causing achlorhydria).
  • Opioids/narcotics (causing delayed intestinal transit).

Pathophysiology

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• Overgrown bacteria ferment ingested carbohydrates into hydrogen ($H_2$) and methane ($C{H}_4$) gases, leading to bloating, flatulence, and osmotic watery stools.
• Bacteria also deconjugate bile acids, causing fat malabsorption and further contributing to diarrhea.

Clinical features

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• Abdominal bloating, flatulence, and distension.
• Watery diarrhea or steatorrhea (foul-smelling, greasy stools due to bacterial bile acid deconjugation impairing micelle formation).
• Abdominal pain/discomfort.
• Weight loss and signs of nutrient malabsorption. c

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Diagnosis

• Initial/Screening: Carbohydrate breath test (lactulose or glucose). c
  • Positive test: Early peak in hydrogen/methane levels (from bacterial fermentation in the small intestine before the substrate reaches the colon).
• Gold Standard: Jejunal aspirate and culture demonstrating $\ge 10^3$ or $\ge 10^5$ CFU/mL.
  • Rarely performed due to invasiveness, cost, and patchy distribution of overgrowth.
• Key Labs:
  • Macrocytic anemia (Vitamin B12 deficiency due to bacterial consumption of B12-intrinsic factor complexes).
  • Elevated folate levels (produced by overgrowing bacteria).
  • Microcytic anemia (iron deficiency due to mucosal inflammation/malabsorption).
  • Fat-soluble vitamin deficiencies (A, D, E, K) + hypocalcemia.

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Differential Diagnostics

• Celiac Disease:
  • Diff by positive serology (anti-tTG IgA, anti-DGP), villous atrophy on duodenal biopsy.
• Irritable Bowel Syndrome (IBS):
  • Diff by lack of objective malabsorption (normal B12/folate, no weight loss) and negative breath tests.
• Exocrine Pancreatic Insufficiency (EPI):
  • Diff by low fecal elastase-1 levels; managed with pancreatic enzyme replacement therapy.
• Lactose Intolerance:
  • Diff by symptom correlation strictly with dairy ingestion; late-rising hydrogen breath test (fermentation occurs only in colon).

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Management

1. Antibiotic Therapy:
   • First-line: Rifaximin (poorly absorbed oral antibiotic, targets small bowel).
   • Alternatives: Neomycin (often added for methane-predominant SIBO), Metronidazole, Ciprofloxacin.
2. Dietary Modification:
   • Low-FODMAP diet to reduce substrate for bacterial fermentation.
3. Correct Underlying Cause:
   • Discontinue unnecessary PPIs and antimotility agents.
   • Prokinetic agents (e.g., Erythromycin, Metoclopramide) for motility disorders.
   • Surgical revision of anatomical blind loops/fistulas.
4. Nutritional Repletion:
   • Supplement Vitamin B12 and fat-soluble vitamins (A, D, E, K).

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Complications

• Severe malnutrition and cachexia.
• Osteoporosis/osteopenia (secondary to Vitamin D and calcium malabsorption).
• Subacute combined degeneration (secondary to severe Vitamin B12 deficiency).