Step 1 — is it real?
Check serum osmolality first:
- Normal osmolality → pseudohyponatraemia (severe hyperlipidaemia, paraproteinaemia). A laboratory artefact; do nothing
- High osmolality → translocational: hyperglycaemia, mannitol. Correct sodium by roughly 2.4 mmol/L for every 5.6 mmol/L of glucose above normal, and treat the glucose
- Low osmolality → true hypotonic hyponatraemia. Continue
Step 2 — assess volume status
Hypovolaemic (urine Na < 20 suggests extrarenal losses, > 20 renal losses):
- Extrarenal: vomiting, diarrhoea, burns, third-spacing
- Renal: diuretics — especially thiazides — mineralocorticoid deficiency, salt-wasting nephropathy, cerebral salt wasting
- Treatment: isotonic saline
Euvolaemic:
- SIADH — the big one
- Hypothyroidism, glucocorticoid deficiency, primary polydipsia, beer potomania, low solute intake
- Treatment: fluid restriction first-line; salt tablets or urea; tolvaptan in selected cases
Hypervolaemic — oedematous states where total body sodium is high but water is higher:
- Heart failure, cirrhosis, nephrotic syndrome, advanced CKD
- Treatment: sodium and fluid restriction plus a loop diuretic; treat the underlying disease
SIADH
Diagnosed on a pattern, with the patient euvolaemic and not on diuretics:
- Urine osmolality > 100 mOsm/kg (inappropriately concentrated)
- Urine sodium > 30 mmol/L
- Normal thyroid and adrenal function
- Low serum uric acid supports it
Causes group into four: CNS disease, pulmonary disease (including small cell lung cancer, which secretes ADH ectopically), drugs (SSRIs, carbamazepine, cyclophosphamide, ecstasy), and pain, nausea and the postoperative state.
Step 3 — correct at the right speed
This is what the exam is really testing.
- Severe symptoms — seizures, coma, respiratory arrest: hypertonic 3% saline in boluses, aiming to raise sodium by 4–6 mmol/L quickly enough to stop the symptoms. Do not wait for the cause
- Otherwise: correct no faster than 8 mmol/L in 24 hours (some use 10; be conservative)
- Overcorrection risks osmotic demyelination syndrome — the delayed, often irreversible quadriparesis, dysarthria and locked-in syndrome that appears days later
- Highest risk: chronic hyponatraemia, alcohol use disorder, malnutrition, hypokalaemia, liver disease
- If you overcorrect, re-lower the sodium with dextrose ± desmopressin
Watch for a water diuresis when you treat hypovolaemia: once volume is restored, ADH switches off and sodium can rise abruptly on its own.
hyponatraemiaelectrolytesnephrologymccqe