A patient with small cell carcinoma of the lung develops increasing fatigue but is otherwise alert and oriented. Serum electrolytes show a serum sodium of 118 mg/L. There is no evidence of edema, orthostatic hypotension, or dehydration. Urine is concentrated with an osmolality of 550 mmol/L. Serum BUN, creatinine, and glucose are within normal range. Which of the following is the next appropriate step?
A. Normal saline infusionThe patient described has hyponatremia, normovolemia, and concentrated urine. These features are sufficient to make a diagnosis of inappropriate antidiuretic hormone secretion. If ADH were responding normally to the patient’s hypotonic state, the urine would be dilute and the excess water load would be excreted. Treatment necessitates restriction of fluid (free-water) intake. Insensible and urinary water loss results in a rise in serum Na + and serum osmolality and symptom improvement. If the patient has CNS symptoms such as confusion, obtundation, or seizures, hypertonic saline is cautiously administered to raise the serum sodium out of the danger zone (usually a rise of 4-8 mEq/L). Normal saline would treat volume depletion, but this patient is euvolemic. Isotonic saline would not address the freewater excess. Loop diuretics lead to modest free-water loss in the urine but would be less important than fluid restriction. The tetracycline derivative demeclocycline decreases renal response to ADH and can be used in cases where the hyponatremia does not respond to fluid restriction. SIADH can occur as a side effect of many drugs or from carcinoma (especially small cell carcinoma of the lung), CNS disorders (head trauma, CNS infection) or benign lung diseases (especially lung abscesses or other chronic infections).