Approach to hypernatremia (pediatrics) Quiz: Ace Your Exams

An 11-day-old boy is being evaluated in the neonatal intensive care unit (NICU) and is found to have hyperextension of the neck and irritability. The patient was born by vaginal delivery at 30 weeks' gestation. Immediately after birth, the patient developed respiratory distress syndrome. He was placed on mechanical ventilation and in a radiant warmer. He is receiving parenteral nutrition and caffeine. There is no vomiting, diarrhea, or temperature instability. Temperature is 36.8°C (98.2°F), pulse is 200/min, respirations are 30/min, blood pressure is 70/38 mm Hg, and oxygen saturation is 94% on FiO₂ 60% on mechanical ventilator. Birth weight was 1.4 kg and current weight is 1.2 kg. Urine output is 0.6 ml/kg/hr. Physical examination reveals a lethargic infant with capillary refill time > 3 seconds who is arching his neck and back backwards consistent with opisthotonus posturing from muscle spasms, and he has brisk reflexes. Cardiopulmonary exam is unremarkable. Lab results are shown below. Which of the following is the most likely cause of hypernatremia in this patient?

Laboratory value	Results
Hemoglobin	21 g/dL
Leukocytes	19,600 /μL
Platelets	160,000/μL
Random blood glucose	70 mg/dL
Serum sodium	151 mEq/L
Serum potassium	3.7 mEq/L
Blood urea nitrogen	15 mg/dL
Creatinine	0.7 mg/dL
Serum osmolality	300 mOsm/kg
Urine osmolality	870 mOsm/kg
Fractional excretion of sodium (FENa)	0.8%