Overview
Definition
A syndrome characterized by excessive release of antidiuretic hormone (ADH) despite low plasma osmolality and normal circulating blood volume, leading to dilutional hyponatremia.
Epidemiology
One of the most common causes of hyponatremia in hospitalized patients.
Etiology & Risk Factors
- CNS disorders (stroke, trauma, meningitis)
- Ectopic ADH secretion (Small Cell Lung Cancer)
- Pulmonary diseases (pneumonia, TB)
- Medications (SSRIs, Carbamazepine, Cyclophosphamide)
Clinical Symptoms
- Often asymptomatic if mild
- Headache, nausea, confusion
- Muscle cramps and weakness
- Seizures, coma, and cerebral edema in severe, rapid-onset hyponatremia
Clinical Approach
Diagnosis
- Euvolemic hyponatremia (Serum Na < 135 mEq/L)
- Low serum osmolality (< 275 mOsm/kg)
- Inappropriately concentrated urine (Urine osmolality > 100 mOsm/kg)
- Elevated urine sodium (> 40 mEq/L)
Management
- Fluid restriction (< 800-1000 mL/day)
- Oral salt tablets and Loop diuretics
- Vasopressin receptor antagonists (Vaptans)
- Hypertonic saline (3% NaCl) slowly for severe, symptomatic cases
Complications
- Seizures
- Brain herniation
- Osmotic Demyelination Syndrome (if corrected too rapidly)