Approach to Hyponatraemia
| Step | Assessment | Finding → Diagnosis |
|---|---|---|
| 1. Assess plasma osmolality | Normal (275–295 mOsm/kg) | Pseudohyponatraemia (hyperlipidaemia, hyperproteinaemia) |
| 2. Assess volume status | Hypovolaemic (dry) | Diarrhoea, vomiting, diuretics, Addison's, burns |
| 3. Assess volume status | Euvolaemic (normal) | SIADH, hypothyroidism, glucocorticoid deficiency |
| 4. Assess volume status | Hypervolaemic (oedematous) | Heart failure, cirrhosis, nephrotic syndrome |
Most Common Causes
- SIADH (syndrome of inappropriate ADH) — most common in hospital setting (lung cancer, pneumonia, drugs, CNS disease)
- Thiazide diuretics — especially in elderly women
- Heart failure and cirrhosis — dilutional hyponatraemia
- Hypothyroidism — reduces cardiac output and free water excretion
- Polydipsia (excessive water intake) — primary or psychogenic
Rapid Sodium Correction Is DangerousCorrecting hyponatraemia too quickly causes osmotic demyelination syndrome (pontine myelinolysis) — a devastating neurological complication. Maximum correction: 8–10 mmol/L in 24 hours.
Urine Sodium & OsmolalityCheck urine sodium and osmolality alongside plasma. In SIADH: urine osmolality >100 mOsm/kg and urine sodium >30 mmol/L despite low plasma sodium. This pattern confirms inappropriate ADH activity.
What are symptoms of low sodium?
Mild (130–135): nausea, headache, malaise. Moderate (125–130): vomiting, confusion, falls. Severe (<125): seizures, coma, respiratory arrest. Rate of fall matters as much as the level.
What causes high sodium (hypernatraemia)?
Almost always represents water deficit rather than sodium excess. Causes: inadequate water intake (elderly, unconscious patients), excessive sweating, diabetes insipidus, osmotic diuresis (diabetes, mannitol).
What is SIADH?
Syndrome of Inappropriate Antidiuretic Hormone — excess ADH causes the kidneys to retain water, diluting sodium. Caused by lung cancer (especially small cell), pneumonia, CNS disease, medications (SSRIs, carbamazepine, NSAIDs).
How is hyponatraemia treated?
Depends on cause and severity. Mild/chronic: fluid restrict to 1L/day (SIADH); treat cause. Severe/symptomatic: 3% hypertonic saline with maximum 8–10 mmol/L correction per 24 hours. Tolvaptan for SIADH.
Medical Disclaimer: This page is for general education only and does not replace professional medical advice. Always consult a qualified healthcare provider.