The Cause of Severe Metabolic Acidosis With Vomiting in a Neonate.

We present a 22-day-old male born full term who presented with worsening non-projectile, non-bilious vomiting and failure to thrive (FTT) and was admitted to the pediatric intensive care unit (PICU) for severe metabolic acidosis with an elevated anion gap. Despite changing the formula, the patient continued to have spit-ups after feeds since birth. Before this admission, his vomiting worsened with every feed, which was now forceful along with two days of loose stools. Obstructive causes of emesis were ruled out with an upper gastrointestinal series, and a decision was made to evaluate for organic causes of FTT. Transient resolution of symptoms was noticed when the patient was placed NPO (nothing by os/mouth) briefly. His symptoms returned on resuming cow milk-based formula feeds. At this time, a presumptive diagnosis of cow milk protein allergy (CMPA) was made. Positive fecal occult blood supported the diagnosis, and his formula was changed to an extensively hydrolyzed formula (eHF). This is a case of severe CMPA with prolonged vomiting and FTT presenting with severe metabolic acidosis with an elevated anion gap. This case report highlights how CMPA can lead to severe dehydration with metabolic acidosis and increased anion gap.