Excessive growth of bacteria in the small intestine, interfering with absorption of the intestinal contents, due to a range of possible underlying causes.

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Dependent on aetiology.
Elderly and paediatric populations can experience this without an underlying disorder.

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What stops bugs growing in small bowel?
HCl in stomach
Immunoglobulin secretion.
Peristalsis (motility - most crucial).
Structural viability.

Extra bacteria may grow if:
Acid is reduced.
Motility is interrupted (e.g. strictures, partial obstructions).
Gut immune system impaired.
New spaces formed (e.g. diverticulae / fistulae / blind loops)

Seive of Causes
Jejunal diverticulosis.
Neurofibromatosis (pseudo-obstructions -> overgrowth).
Crohn's, UC, peptic ulcer disease (fistulae).
Vasculitis & SLE (decreased motility due to strictures or muscle dysfx).
Hypothyroidism (motility).
Amyloidosis (motility).
Chronic gastritis (decreased acid).
Nodular lymphoid hyperplasia, lymphoma (strictures).
Carcinoma (fistulae).
Diabetic autonomic neuropathy (motility).
Scleroderma, myotonic dystrophy, familial visceral myopathies and neuropathies (all inhibit motility).
Surgical alterations decreasing motility, or allowing backflow of bacteria up ileocaecal valve.
Stricture-causing tx (e.g. radiation).
Vagotomy (motility).
Gastric resection (decreased acid).
Multiple laparotomies with adhesions.
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Normal colony count in small bowel less than 10^4/ml.
With the underlying defect above, and introduction of bacteria into small bowel, the bacteria may multiply rapidly.

In overgrowth numbers, they deconjugate bile salts (fat malabsorption, DEKA vitamins).
Damage the intestinal mucosa and brush border enzymes through their metabolites.
Produce hydrogen or carbon dioxide gas anaerobically.
Use up factors such as vitamin B12.

These interruptions with normal gut function lead to the clinical manifestations.

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Diarrhoea (osmotic, malabsorptive).
Flatulence (gas production).
Abdo pain (general, not severe, and not always).
Pale bulky, offensive tools, float (moderate steatorrhoea).
Others as appropriate to cause.
Weight loss (malabsorption).
Ca2+ malabsorption -> muscle cramps, tetany, tingling, numbness, painful bones and osteoporosis / osteomalacia in later life.
Vit A to night blindness, dryness of conjunctiva, corneae (xerophthalmia, keratomalacia).
Vit D to rickets - painful bones, deformities.
Vit B12 anaemia (common ++) with weakness, fatigue, malaise, headaches etc...

Evidence of malabsorption - weight, pallor, bones, etc...
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FBC, electrolytes, vitamin levels, protein (albumin, clotting time).
Lactulose - hydrogen breath tests (2-h 50-g glucose H2 test reliable).
Confirm with polymicrobial / heavy numbers in aspirates or biopsies.
X-Ray may show excess gas / even gas in peritoneum.
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As appropriate to cause. Generally:

1. nutritional support, vitamins etc.

2. Empiric antibiotics
-  metronidazole, ciprofloxacin.
Single or intermittent courses.
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