Syndrome of Inappropriate Antiduretic Hormone Secretion

Aetiologies

• Small cell lung cancer (secreting a vasopressin-like peptide)
• Damage to the central nervous system (stroke, haemorrhage, surgery) - although this may be a response of appropriate vasopressin release following volume depletion which results from cerebral salt wasting
• Pneumonia
• HIV
• Hereditary SIADH
• Drugs:
  • carbamazepine
  • cyclophosphamide
  • phenothiazines
  • SSRIs
  • nicotine
  • tricyclics
  • vinca alkaloids eg. vincristine
  • interferon
  • cisplatin
  • MDMA
  • amiodarone
  • ciprofloxacin
  • sodium valproate
  • NSAIDs
  • Thiazides

Diagnostic Criteria

• Hypoosmolar hyponatremia
• Urine osmolality greater than plasma osmolality
• Urine sodium excretion greater than 20mmol/L
• Normal renal, hepatic, cardiac, pituitary, adrenal and thyroid function
• Absence of hypotension, hypovolemia, oedema and ADH-influencing drugs
• Hyponatremia corrects with water restriction

Workup

1. Review medication list for medications causing SIADH
2. Review smoking history for occult lung cancer
3. Consider neurological examination
4. Chest X-ray
5. Consider head CT

Management

• Fluid restriction
• Demeclocycline
• Loop diuretics with/without salt supplementation
• Urea
• Hypertonic 3% saline
• Vasopresisn antagonists

Special Cases

• SIADH + Subarachnoid Haemorrhage:
  • Cannot fluid restrict because hypovolaemia causes vasospasm and can’t leave hyponatraemia because it is associated with poorer outcomes
  • Manage with hypertonic saline infusion to increase sodium to >135 mmol/L

Sources

• Deranged physiology: Syndrome of inappropriate ADH secretion (SIADH)