GastroenterologyTier 2Disease (DEADMAN)

Small intestinal bacterial overgrowth

Description

Excessive bacteria in the small bowel - colonic-type flora where there should be few organisms
Symptoms: bloating, flatulence, abdominal distension and discomfort, diarrhoea; in severe cases steatorrhoea, weight loss, B12 deficiency
*Enormously over-diagnosed - symptoms are identical to IBS, and the breath test is poor*
Small intestinal fungal overgrowth and methanogen overgrowth (IMO - intestinal methanogen overgrowth) are related entities; IMO causes CONSTIPATION

Prevalence uncertain (test-dependent): ~5-15% of healthy controls test positive on breath testing
Up to 30-40% of "IBS-D" meets breath-test criteria - whether this is causal remains contested
Much higher after gastric or bariatric surgery, in scleroderma (~30-60%), and in chronic pancreatitis

Mechanism of symptoms

Bacterial fermentation of unabsorbed carbohydrate in the small bowel -> excess HYDROGEN and METHANE -> bloating and flatulence
Deconjugation of bile salts -> impaired micelle formation -> FAT malabsorption -> steatorrhoea + fat-soluble vitamin (ADEK) deficiency
Bacterial consumption of B12 -> B12 DEFICIENCY
*Bacteria SYNTHESISE folate and vitamin K -> folate is often HIGH - the classic discriminating pattern: low B12, high folate*
Mucosal injury from bacterial products -> patchy villous blunting -> protein-losing enteropathy

Causes - anything that defeats the normal defences

Defence lost	Causes
Gastric acid	PPI, atrophic gastritis, post-gastrectomy, vagotomy
Motility (migrating motor complex)	Scleroderma, diabetic autonomic neuropathy, opioids, amyloid, hypothyroidism, chronic intestinal pseudo-obstruction, post-vagotomy
Anatomy	Blind loops, strictures (Crohn), diverticula, fistulae, surgical afferent loop, Roux-en-Y, short bowel
Ileocaecal valve	Ileocaecal resection -> colonic reflux
Immunity	IgA deficiency, CVID, HIV
Other	Chronic pancreatitis, cirrhosis, end-stage renal disease

A. Breath testing - the practical test

Glucose (75 g) or lactulose (10 g) hydrogen/methane breath test
Positive: rise in H2 >=20 ppm above baseline within 90 minutes (North American consensus); methane >=10 ppm at any point = IMO
Preparation: no antibiotics for 4 weeks, no prokinetics/laxatives for 1 week, fermentable-carbohydrate-free diet the day before, overnight fast, no smoking or exercise on the day
*Poor test characteristics*
- Glucose: more specific, but absorbed proximally -> misses distal overgrowth
- Lactulose: reaches the colon -> an early "peak" is often just rapid oro-caecal transit, not SIBO - high false-positive rate
Non-hydrogen producers (~15-20% of people) -> false negative unless methane is also measured

B. Jejunal aspirate and culture - the reference standard

C. Supporting findings - look for the consequence, not just the test

Low B12 with HIGH or normal folate
Macrocytosis, low fat-soluble vitamins, faecal fat, hypoalbuminaemia
Always look for the underlying cause: small bowel imaging (CT/MR enterography), coeliac serology, TFTs, drug and surgical history
*Do not diagnose SIBO without a predisposing cause* - if there is none, reconsider IBS, carbohydrate intolerance or coeliac disease

1. Treat the underlying cause first

Stop or reduce PPIs and opioids where possible
Correct hypothyroidism, optimise glycaemic control, surgical correction of a blind loop or stricture
Anatomical causes recur indefinitely without correction

2. Antibiotics - a course, not a lifestyle

Rifaximin 400-550 mg tds for 10-14 days - best evidence, minimally absorbed, not PBS-subsidised for this indication in Australia (expensive)
Alternatives: amoxicillin-clavulanate, metronidazole, norfloxacin, doxycycline, co-trimoxazole, 7-14 days
Methane-predominant (IMO): rifaximin PLUS neomycin works better than either alone
Recurrence is the rule (~40-50% within 9 months) if the cause persists
- Cyclical antibiotics (e.g. 1 week in 4), rotating agents - avoid continuous therapy: resistance and C. difficile**

3. Nutrition

Replace B12 (parenterally), fat-soluble vitamins, iron, calcium
*Do NOT give folate* - usually already high
Low-FODMAP or low-fermentable-carbohydrate diet for symptoms; MCT if steatorrhoea
Dietitian involvement, especially in short bowel

4. Prokinetics

Low-dose erythromycin or prucalopride between antibiotic courses to restore the migrating motor complex - particularly in scleroderma and diabetic gastroparesis

What NOT to do

Repeated breath testing to "confirm eradication" - it does not change management
Long-term empirical antibiotics for bloating without a predisposing cause
Probiotics - evidence is inconsistent and they may worsen symptoms

Scleroderma / systemic sclerosis - the classic association (hypomotility + dilated bowel)
Diabetic autonomic neuropathy, hypothyroidism, amyloidosis
Crohn disease (strictures, fistulae, ileocaecal resection)
Post-surgical: Roux-en-Y gastric bypass, Billroth II afferent loop, short bowel syndrome, ileocaecal resection
Chronic pancreatitis, cirrhosis, coeliac disease (non-responsive), IBS
PPI and opioid use
IgA deficiency, CVID, HIV
Chronic intestinal pseudo-obstruction, small bowel diverticulosis

Symptoms respond to antibiotics in ~60-70% - but relapse is the rule unless the predisposing cause is corrected
Long-term complications where overgrowth is chronic and severe:
- B12 deficiency -> macrocytic anaemia and subacute combined degeneration
- Fat-soluble vitamin deficiency -> osteomalacia, night blindness, coagulopathy
- Weight loss, protein-losing enteropathy, D-lactic acidosis (short bowel - confusion and ataxia with a normal lactate on the standard assay)
In practice the harm is more often from over-diagnosis - repeated antibiotic courses, restrictive diets, and a missed alternative diagnosis

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