Hypernatremia

Hypernatraemia is serum sodium above 145 mmol/L, always reflecting a hyperosmolar state with water deficit relative to sodium.

• Common in hospitalised and elderly patients (1–3% of admissions, up to 9% of ICU patients); mortality 40–60% with severe or acutely developing disease.
• Causes by volume status:
  • Hypovolaemic: sweating, diarrhoea, vomiting, osmotic diuresis (hyperglycaemia, mannitol), diuretics, burns.
  • Euvolaemic: diabetes insipidus (central, nephrogenic), reduced thirst or water access (infants, dementia, intubation), insensible losses (fever, hyperventilation).
  • Hypervolaemic: sodium loading (hypertonic saline, sodium bicarbonate, salt poisoning), mineralocorticoid excess (Conn, Cushing).
• Mechanism: cellular dehydration, brain shrinkage with risk of intracranial haemorrhage, osmotic adaptation (idiogenic osmoles) over 24–48 hours.
• Complications: confusion, seizures, coma, venous thrombosis, rhabdomyolysis, cerebral oedema with rapid correction.

• Thirst (absent in impaired patients), irritability, lethargy, muscle twitching, hyperreflexia, ataxia, confusion, seizures, coma.
• Signs of volume depletion (tachycardia, postural hypotension, dry mucosa) or overload (oedema).
• Polyuria in diabetes insipidus or osmotic diuresis.
• Infants: high-pitched cry, doughy skin, fever.
• Severity and acuity determine neurological features (symptoms usually above 160 mmol/L).

• Serum sodium, osmolality, potassium, glucose, calcium, creatinine, urea, bicarbonate.
• Urine osmolality, sodium, volume: urine osmolality over 800 mOsm/kg indicates extrarenal water loss or inadequate intake with intact ADH; below 300 mOsm/kg with polyuria suggests DI; 300–800 suggests partial DI or osmotic diuresis.
• Assess volume status and fluid balance; measure daily weights and urine output.
• Water deprivation test or desmopressin challenge to differentiate central from nephrogenic DI (only when safe).
• Evaluate cause: infection, diabetes mellitus, hypercalcaemia, hypokalaemia, drug review (lithium, amphotericin), pituitary MRI, plasma copeptin.
• Estimate water deficit: total body water x [(serum Na/140) − 1], with total body water 0.6 x weight (men), 0.5 (women, elderly).

• Treat shock with isotonic saline first (restore perfusion), then correct water deficit.
• Replace free water slowly: oral or enteral water preferred; IV 5% dextrose or 0.45% saline if necessary.
• Rate of correction: chronic hypernatraemia (over 48 hours) not faster than 10 mmol/L per 24 hours (0.5 mmol/L/h); acute (under 24 hours) can be corrected faster (up to 1 mmol/L/h) until 145–150.
• Calculate: infusion rate = (water deficit + ongoing losses) over 48–72 hours; recheck sodium every 4–6 hours initially; adjust. Include insensible losses (about 10 mL/kg/day) and urinary losses.
• Hypovolaemic: isotonic saline then hypotonic fluid; hypervolaemic: stop sodium load, loop diuretic (furosemide 20–40 mg) with free water, dialysis for severe overload/renal failure.
• Central DI: desmopressin 0.1 to 0.2 mg orally 2–3 times daily, or 1–4 micrograms IV/SC twice daily, intranasal 10–20 micrograms; monitor sodium to avoid hyponatraemia.
• Nephrogenic DI: thiazide 25–50 mg, amiloride 5–10 mg, low sodium diet; treat cause.
• Treat seizures and cerebral oedema (rapid overcorrection): 3% saline or dextrose-free fluids and consider hypertonic saline if neurological deterioration during correction.
• Monitor sodium, glucose, urine output, neurological status; cause-specific prevention (adequate water access in elderly/infants).