Expansion
Hyponatraemia with concentrated urine in a euvolaemic patient with normal endocrine and renal function
Expansion
Criteria:
- Hypotonic hyponatraemia: plasma sodium under 135 with osmolality under 275 mOsm/kg
- Inappropriately concentrated urine: urine osmolality above 100 mOsm/kg
- Urine sodium above 30 mmol/l on a normal salt intake
- Clinical euvolaemia: no oedema, no postural drop
- Normal thyroid, adrenal and renal function
- No diuretics
Causes group into four:
- Central nervous system: stroke, haemorrhage, meningitis, trauma
- Pulmonary: pneumonia, tuberculosis, positive pressure ventilation
- Malignancy: classically small cell lung carcinoma, which secretes ectopically
- Drugs: SSRIs, carbamazepine, cyclophosphamide, opioids, ecstasy
Treatment is fluid restriction first. Hypertonic saline is reserved for severe symptomatic cases. Vaptans (V2 receptor antagonists) produce a pure water diuresis.
A useful trap: giving normal saline can worsen SIADH, because the salt is excreted while the water is retained.