Differential diagnosis of hyponatremia

1. Big picture

Hyponatremia = serum sodium <135 mmol/L, but clinically it usually means too much water relative to sodium, not simply “too little sodium.”

The exam-safe approach:

Confirm true hyponatremia
→ assess symptoms and severity
→ check serum osmolality
→ assess volume status
→ check urine osmolality and urine sodium
→ treat emergency brain edema first
→ correct slowly to avoid osmotic demyelination

The dangerous point is not the number alone, but how fast sodium fell and whether the patient has brain symptoms.