SIADH and Hyponatremia in Cancer

Diagnosis, causes, and stepwise management of the most common electrolyte disorder in cancer patients

Key Points

• Hyponatremia (serum sodium <135 mEq/L) is the most common electrolyte abnormality in cancer patients, occurring in up to 47% of hospitalized oncology patients.
• SIADH (syndrome of inappropriate antidiuretic hormone secretion) is the most frequent cause of euvolemic hyponatremia in cancer — small cell lung cancer (SCLC) is the classic culprit, accounting for 75% of cancer-associated SIADH.
• The diagnostic hallmarks of SIADH are: low serum osmolality (<275 mOsm/kg), inappropriately concentrated urine (urine osmolality >100 mOsm/kg), urine sodium >40 mEq/L, euvolemia, and normal adrenal/thyroid function.
• Chronic severe hyponatremia (Na <120 mEq/L) causes encephalopathy, seizures, and herniation — a medical emergency requiring ICU-level management.
• Correction must never exceed 10–12 mEq/L per 24 hours — overly rapid correction causes osmotic demyelination syndrome (ODS), a devastating and often irreversible neurologic complication.
• Vaptans (tolvaptan, conivaptan) — V2 receptor antagonists that block ADH — are FDA-approved for euvolemic and hypervolemic hyponatremia; tolvaptan carries a black box warning for hepatotoxicity.

Epidemiology and Classification of Hyponatremia in Cancer

Hyponatremia (serum sodium 48 hours); chronic hyponatremia allows cerebral adaptation (idiogenic osmole production)

SIADH in Cancer — Causes and Pathophysiology

SIADH is defined by the inappropriate secretion of ADH (vasopressin) in the absence of physiologic stimuli — hypovolemia or hyperosmolality. ADH acts on V2 receptors in the renal collecting duct, inserting aquaporin-2 water channels that cause water retention and dilutional hyponatremia. **Tumor-related SIADH (ectopic ADH production):** Malignant cells produce and secrete vasopressin autonomously, independent of serum osmolality. This is the most common mechanism in cancer-associated SIADH. - **Small cell lung cancer (SCLC)**: the prototypical cause. SCLC cells are neuroendocrine and produce…

Diagnosis of SIADH

SIADH is a diagnosis of exclusion — other causes of euvolemic hyponatremia must be ruled out first. **Diagnostic criteria for SIADH (Schwartz-Bartter criteria, updated):** 1. Serum sodium 100 mOsm/kg (urine inappropriately concentrated — ideally >300 mOsm/kg in established SIADH) 3. Urine sodium >40 mEq/L (natriuresis despite hyponatremia — kidneys continue excreting sodium because volume sensors perceive euvolemia or slight hypervolemia) 4. Clinical euvolemia — no edema, no signs of dehydration 5. Normal thyroid function (TSH normal) 6. Normal adrenal function (morning cortisol ≥18 mcg/dL,…

Management of SIADH and Hyponatremia

**Treatment goals:** 1. Raise serum sodium safely (avoid ODS) 2. Relieve neurological symptoms 3. Treat the underlying cause (disease control, drug discontinuation) **The non-negotiable rule — rate of correction:** - **Chronic hyponatremia (>48 hours or unknown duration):** correct at ≤10 mEq/L in 24 hours, ≤18 mEq/L in 48 hours - **Acute hyponatremia ( 500 mOsm/kg predicts poor response to fluid restriction alone. - **Oral sodium chloride tablets**: 1–3 g two to three times daily; enhances renal sodium retention and water excretion; effective adjunct to fluid restriction - **Loop…