Electrolyte Disturbance in an Infant with Failure to Thrive

Presentation

A 6-week-old girl was born to a healthy mother via uncomplicated vaginal delivery at full term and subsequently had a normal newborn screen. She had a birthweight of 4.48 kg (large for gestational age, 97% percentile). At her 1-month health supervision visit, she weighed 3.7 kg. Although offered 2 to 3 oz of pumped human milk each feed, she was taking approximately 1 oz every 2 hours without cyanosis, tachypnea, or sweating. She had no trouble latching onto the bottle, a normal suck and swallow, and no choking with feeds. After feeding, she had frequent nonbloody/nonbilious vomiting episodes that most often occurred soon after a feed, but occasionally a few hours later. Her primary care provider recommended fortifying the human milk with formula to 22 kcal/ounce, but it was not well-tolerated due to vomiting. On a repeat weight check 2 weeks before admission, she demonstrated continued poor weight gain and was started on a histamine 2-blocker for possible gastrointestinal reflux. Two days before admission, this was switched to a proton-pump inhibitor with no improvement in symptoms. The primary care provider obtains bloodwork, urine studies, and an abdominal ultrasound as part of an evaluation for failure to thrive. The bloodwork shows the following: sodium 115 mmol/L, potassium 8.7 mmol/L, chloride 71 mmol/L, carbon dioxide 15 mmol/L, urea nitrogen 53 mg/dL, creatinine 0.8 mg/dL, and glucose 50 mg/dL. The mother is asked to …