Learn with Med Nuggets: SIADH

What is SIADH?

• Syndrome of Inappropriate Antidiuretic Hormone Secretion.
• ADH (Vasopressin):
  • Hormone made in the hypothalamus.
  • Stored and released by the posterior pituitary gland.
  • Regulates water balance by causing reabsorption in kidney's distal convoluted tubule and collecting duct cells.
• SIADH:
  • Condition where body makes too much ADH.
  • Leads to increased water retention, diluting solutes (e.g., sodium).
  • Inhibits aldosterone release and stimulates ANP & BNP release.

Pathophysiology

• Aldosterone: Normally causes sodium retention and hydrogen ion excretion; inhibited in SIADH.
• ANP & BNP: Released by atria and ventricles when stretched; promote sodium and water excretion.
• Results in euvolemic hyponatremia.

Etiology

• Causes:
  • Ectopic ADH production outside pituitary, e.g., in lungs.
  • CNS disorders, trauma.
  • Medications (e.g., SSRIs, Carbamazepine, Cyclophosphamide).
• Mnemonic: Lung or brain issues can cause SIADH.

Clinical Importance

• Dangers: Low sodium levels can cause cerebral edema.
• Symptoms: Headaches, confusion, hallucinations, coma, death.

Diagnosis

• Laboratory findings diagnostic of SIADH:
  • Low serum sodium level.
  • Serum osmolality < 275.
  • Urine osmolality > 100.
  • Urine sodium > 40.
  • Normal renal, adrenal, and thyroid function.

Treatment

• Initial steps:
  • Treat underlying cause (e.g., stop causative drug).
  • Restrict fluid intake.
• Medications:
  • Loop diuretics for volume overload.
  • Demeclocycline (ADH inhibitor).
  • Vaptans (e.g., Tolvaptan) - ADH receptor antagonists.
• Severe cases:
  • Acute symptomatic hyponatremia: Use hypertonic saline cautiously.
  • Chronic hyponatremia: Correct sodium slowly (8 mEq/L/day) to avoid osmotic demyelination syndrome.

Conclusion

• Understanding and managing SIADH is crucial to prevent serious complications.
• Always consider the underlying causes and treat accordingly.