A patient with SIADH has serum Na+ of 118 mEq/L and urine osmolality of 600 mOsm/kg. Plasma osmolality is 240 mOsm/kg. The MOST appropriate initial management is:
- A 3% hypertonic saline at 1–2 mL/kg/hr to correct sodium rapidly by 12 mEq/L over 24 hours
- B Isotonic (0.9%) saline infusion to restore sodium levels gradually
- C Free water restriction to 800–1000 mL/day as initial therapy for chronic, asymptomatic hyponatremia ✓
- D Immediate furosemide to excrete free water and raise sodium
Explanation
In chronic, asymptomatic SIADH (as implied by the clinical picture without seizures or severe neurological symptoms), water restriction to 800–1000 mL/day is the first-line treatment. Correction must not exceed 10–12 mEq/L in 24 hours (or 18 mEq/L in 48 hours) to prevent osmotic demyelination syndrome (central pontine myelinolysis). Hypertonic saline (3%) is reserved for acute symptomatic hyponatremia (seizures, obtundation) with correction of only 1–2 mEq/L/hr until symptoms resolve. Isotonic saline in SIADH can paradoxically worsen hyponatremia because the kidney excretes the Na+ but retains the water.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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