📊 Definitions
• Hyponatremia: serum Na <135 mEq/L
• Usually indicates excess water relative to sodium
• Symptoms: nausea, vomiting, headache, confusion, seizures, coma (severe: <120-125)
• Risk of cerebral edema highest with acute hyponatremia
💧 Hypovolemic Hyponatremia (Most Common)
• Sodium and water loss, but water loss less than sodium loss
• Causes: GI losses (diarrhea, vomiting), diuretics, adrenal insufficiency, salt-wasting nephropathy, third-spacing (burns, pancreatitis)
• Treatment: IV normal saline (isotonic) → restore volume → correct Na slowly
🩸 Euvolemic Hyponatremia
• SIADH (syndrome of inappropriate antidiuretic hormone): urine osmolality >100, urine Na >40, normal volume status
• Causes: CNS disorders (meningitis, tumor, trauma), pulmonary disease (pneumonia, asthma), medications (SSRIs, carbamazepine, vincristine), postoperative, pain
• Treatment: fluid restriction (if asymptomatic), hypertonic saline (3%) if severe symptoms
⚠️ Hypervolemic Hyponatremia
• Heart failure, cirrhosis, nephrotic syndrome, renal failure
• Increased total body water and sodium, but water excess greater
• Treatment: fluid restriction, loop diuretics, treat underlying disease
📌 Decision strategy: Assess volume status first. Hypovolemic → NS bolus. Euvolemic → consider SIADH (urine Na, osmolality). Hypervolemic → fluid restriction. Severe symptoms (seizures, coma) → hypertonic saline (3%) even if euvolemic.