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):
- Small cell lung cancer (SCLC) → most common.
- Head & neck cancers, pancreatic, prostate, lymphoma.
🔹 Drug-induced (chemotherapy & supportive meds):
- Vinca alkaloids: Vincristine, Vinblastine.
- Platinum compounds: Cisplatin (also causes renal salt wasting).
- Alkylating agents: Cyclophosphamide (esp. high-dose IV).
- Others: Ifosfamide, Melphalan, certain antidepressants, opioids.
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).