Hyponatraemia
Hyponatraemia is a common electrolyte disorder in the emergency department. Management must balance two priorities: treat life‑threatening cerebral oedema promptly, and avoid overly rapid correction that causes osmotic demyelination syndrome (ODS). This section provides a practical ED approach to recognition, initial investigation and emergency management, with key UK guideline points highlighted.
Definitions and classification
- Hyponatraemia: serum sodium < 135 mmol/L.
- Practical ED severity:
- Mild: 130-134 mmol/L.
- Moderate: 125-129 mmol/L.
- Severe: < 125 mmol/L (many guidelines use < 120 mmol/L for imminently dangerous presentations).
- Time course:
- Acute: develops < 48 hours - poor brain adaptation, high risk of cerebral oedema.
- Chronic: ≥ 48 hours or unknown - brain adaptation reduces immediate oedema risk but increases ODS risk with rapid correction.
- Osmolar categories to consider:
- Hypotonic (most common).
- Isotonic (pseudohyponatraemia).
- Hypertonic (translocational, e.g. hyperglycaemia).
Why it matters (pathophysiology)
Acute hyponatraemia causes water influx into brain cells → cerebral swelling, raised intracranial pressure, seizures and coma. In chronic hyponatraemia the brain reduces intracellular osmoles; rapid correction then risks osmotic demyelination of the pons and other regions. Clinical decisions therefore depend on both sodium value and the acuity/neurological state.