Malignancy-Associated Hyponatraemia

Often due to the syndrome of inappropriate antidiuresis (SIAD) — tumour production of an ADH analogue. A broad range of cancers can cause this, most commonly small-cell lung cancer.

Recognition

Both malignant and non-malignant SIAD cause water retention, characterised by low serum osmolality and hyponatraemia in the presence of concentrated urine and normal extracellular fluid volume. SIAD often goes unrecognised because its symptoms mimic other cancer effects — headache, confusion, nausea/vomiting, anorexia, muscle weakness and cramps. Serum sodium <120 mmol/L can cause delirium, seizures and coma.

Hyponatraemia may indicate far-advanced disease.

Management

• If correction is clinically appropriate, SIAD can be managed in the community by restricting fluid intake to less than expected insensible + urinary loss — usually ~500 mL–1 L/24 hours in temperate climates.
• Severe/life-threatening hyponatraemia needs IV hypertonic saline — hospital, specialist care only. Rapid correction risks permanent CNS injury from osmotic demyelination.