Small bowel bacterial overgrowth syndrome

Introduction

Small Intestinal Bacterial Overgrowth (SIBO) is a condition characterised by excessive growth of colonic-type bacteria in the small intestine, leading to gastrointestinal symptoms, malabsorption, and nutritional deficiencies.

Peak Incidence

• Most commonly affects adults over 60 years of age.
• Risk increases with ageing, structural abnormalities, or motility disorders of the small bowel.

Pathophysiology

• In normal physiology, the small intestine contains a relatively low bacterial load, maintained by:
  • Gastric acid.
  • Intestinal motility.
  • An intact ileocaecal valve.
  • Immunological defence.
• When these protective mechanisms are disrupted, excessive bacterial colonisation occurs in the small intestine. This leads to:
  • Fermentation of carbohydrates → gas, bloating, diarrhoea.
  • Deconjugation of bile salts → fat malabsorption.
  • Competition for nutrients → deficiencies, especially vitamin B12.

Symptoms

• Bloating and excess flatulence.
• Abdominal discomfort or cramping.
• Persistent diarrhoea or loose stools.
• Steatorrhoea – Oily, pale, foul-smelling stools.
• Unintentional weight loss.
• Symptoms of micronutrient deficiency (e.g. fatigue, weakness).

Signs

• Pallor – Due to iron or B12 deficiency anaemia.
• Glossitis and angular cheilitis – Linked to vitamin B12 deficiency.
• Peripheral neuropathy – Also from B12 deficiency.
• Oedema – Due to hypoalbuminaemia from protein malabsorption.

Diagnosis

Preferred Investigations

• Hydrogen Breath Test – First-line and non-invasive:
  • An increase in hydrogen ≥20 ppm above baseline within 90 minutes after carbohydrate ingestion is diagnostic.
• Small Intestinal Aspirate and Culture – Rarely performed due to invasiveness:
  • A bacterial count of ≥10³ CFU/mL confirms SIBO.

Supportive Blood Tests

• FBC – May show:
  • Microcytic anaemia (iron deficiency).
  • Macrocytic anaemia (vitamin B12 deficiency).
• Serum vitamin B12 – Often reduced.
• Fat-soluble vitamin levels (A, D, E, K) – May be low in chronic cases.
• Serum iron and ferritin – Often reduced in iron deficiency.
• Serum albumin – May be low in advanced malnutrition.

Complications

• Nutrient malabsorption:
  • Deficiencies in vitamin B12, fat-soluble vitamins, iron, and zinc.
• Weight loss – Due to prolonged malabsorption.
• Osteoporosis – Secondary to calcium and vitamin D deficiency.

Management

Antibiotic Therapy

• First-line: Rifaximin (typically for 10–14 days).
• Alternatives: Co-amoxiclav or metronidazole.
• An empirical trial of antibiotics may be both diagnostic and therapeutic in high clinical suspicion.

Nutritional Support

• Vitamin B12 – IM injections if deficient.
• Fat-soluble vitamin supplementation (A, D, E, K).
• Iron and zinc – Oral or IV supplementation depending on severity.

Address Underlying Causes

• Structural abnormalities – May require surgical correction.

• Motility disorders – Consider prokinetic agents:

  • e.g. Erythromycin, metoclopramide.

• Dietary modifications – A low-FODMAP diet may help reduce symptoms in selected patients, especially those with overlapping IBS.