Syndrome of Inappropriate ADH Secretion

Definition:

  • Disorder of impaired water excretion due to excessive ADH release or enhanced renal sensitivity to ADH.
  • Results in water retention → dilutional hyponatremia with low serum osmolality but inappropriately concentrated urine.

Causes in Oncology:

🔹 Malignancy-related (ectopic ADH production):

🔹 Drug-induced (chemotherapy & supportive meds):

Clinical Features:

  • Mild hyponatremia (Na 125–134 mmol/L): Asymptomatic or mild fatigue, nausea.
  • Moderate (Na 115–124): Headache, confusion, muscle cramps.
  • Severe (<115): Seizures, coma, respiratory arrest (medical emergency).

Diagnosis (labs):

  • Hyponatremia: Serum Na <135 mmol/L.
  • Serum osmolality: Low (<275 mOsm/kg).
  • Urine osmolality: Inappropriately high (>100 mOsm/kg).
  • Urine sodium: Elevated (>30 mmol/L).
  • Volume status: Clinically euvolemic (no edema, no dehydration).

Management (oncology practice):

  • Mild/asymptomatic: Fluid restriction (800–1000 mL/day).
  • Moderate/severe or symptomatic:
    • Hypertonic saline (3%) with close monitoring (correct Na ≤8–10 mmol/L per 24h to avoid osmotic demyelination).
    • Loop diuretics + salt tablets in chronic cases.
    • Demeclocycline (rare, nephrotoxic) or Vaptans (tolvaptan, conivaptan) for refractory cases.
  • Oncology-specific: If drug-induced, hold or discontinue culprit agent (e.g., vincristine). Treat underlying cancer if SIADH is paraneoplastic (esp. SCLC).

Pharmacist Pearls:

  • Always check Na+ before each cycle if patient is on cisplatin, vincristine, or cyclophosphamide.
  • SIADH can mimic chemotherapy fatigue or confusion → check sodium early.
  • If using vincristine, dose should never exceed 2 mg (neurotoxicity + SIADH risk).
  • Cisplatin-induced hyponatremia can also be due to renal salt wasting → differentiate from SIADH (urine sodium/water balance helps).
Synonyms
SIADH
Links