Hyponatraemia - diagnostic approach and management by volume status
Core concept
- Hyponatraemia is a disorder of WATER, not of sodium. The sodium tells you nothing about total body sodium
- Two things must both be true for hypotonic hyponatraemia: water intake, and inability to excrete it (ADH, or inadequate solute)
- Three-step algorithm
- 1. Serum osmolality - low (true), normal (pseudo - lipids/protein), high (translocational - glucose, mannitol)
- 2. Urine osmolality - <100 mOsm/kg = appropriately dilute urine -> primary polydipsia, beer potomania, low solute intake, reset osmostat
- >100 = ADH is acting (appropriately or not)
- 3. Urine sodium + clinical volume status
- Urine Na <20-30 mmol/L -> hypovolaemia or effective arterial underfilling (cardiac failure, cirrhosis, nephrosis)
- Urine Na >30-40 mmol/L with euvolaemia -> SIADH, adrenal insufficiency, hypothyroidism, diuretics, salt-wasting
3 more sections, plus exam facts
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