Gastrointestinal · Year 2 · from Gastrointestinal
Case 3: Small Intestinal Bacterial Overgrowth (SIBO)
Patient Presentation
Demographics: 67-year-old female
Chief Complaint: Bloating, diarrhea, and weight loss for 8 months
History of Present Illness: The patient reports progressive abdominal bloating and distension, particularly after meals. She has 4-6 loose, foul-smelling, fatty stools daily. She has lost 12 pounds despite adequate oral intake. She also notes fatigue and occasional tingling in her feet.
Past Medical History: Type 2 diabetes (15 years), diabetic autonomic neuropathy with orthostatic hypotension, chronic PPI use for GERD (10 years)
Surgical History: None
Physical Examination
- Vital Signs: BP 128/76 mmHg (sitting), 108/68 mmHg (standing), HR 82 bpm
- General: Thin, fatigued-appearing female
- Abdomen: Distended, tympanitic, mild diffuse tenderness, hyperactive bowel sounds
- Neurologic: Decreased vibration sense at ankles bilaterally
Workup and Results
- CBC: Hemoglobin 10.8 g/dL, MCV 108 fL (macrocytic)
- Chemistry: Albumin 3.2 g/dL
- Vitamin Levels: B12 low (180 pg/mL), folate elevated (>20 ng/mL)
- Glucose Hydrogen Breath Test: Positive (hydrogen rise >20 ppm within 90 minutes)
- Stool Studies: Elevated fecal fat
Hydrogen breath test graph showing an early rise in breath hydrogen concentration (>20 ppm) after lactulose ingestion, consistent with small intestinal bacterial overgrowth.
Image Source: Case adapted from clinical literature
Diagnosis
Small Intestinal Bacterial Overgrowth (SIBO)
Clinical Correlation to Motility and Protective Mechanisms
SIBO occurs when protective mechanisms that maintain near-sterile small bowel conditions are compromised. Normal defenses include: (1) gastric acid killing ingested bacteria - chronic PPI use eliminates this barrier; (2) the migrating motor complex (MMC) sweeping bacteria distally during fasting - diabetic autonomic neuropathy impairs this "housekeeper" function; (3) the ileocecal valve preventing colonic bacterial reflux. This patient has multiple risk factors. The characteristic finding of low B12 with elevated folate reflects bacterial consumption of dietary B12 and bacterial folate synthesis. Deconjugation of bile acids by bacteria impairs fat absorption, causing steatorrhea.
Treatment
- Rifaximin 550 mg three times daily for 14 days
- Address underlying causes: consider reducing PPI dose, prokinetic therapy for dysmotility
- B12 supplementation
- Low-FODMAP diet may reduce symptoms
- Anticipate need for cyclic antibiotic therapy if symptoms recur