accompanied by fever in the neonate); increased intracranial pressure (ICP) related to cerebral edema, subdural hematoma, or hydrocephalus; metabolic acidosis or hyperammonemia caused by the inborn errors of amino acid and organic acid metabolism; and renal insufficiency or obstruction Such infants usually appear ill, with associated lethargy and irritability In some cases, findings such as a fever or hypothermia, a full fontanel, a diminished urinary stream, an abdominal mass, or respiratory signs will suggest the underlying cause Neonates and young infants with congenital adrenal hyperplasia may present with vomiting, growth failure, and lethargy They may appear critically ill with a similar appearance to those with sepsis, and may have electrolyte abnormalities including hyponatremia and hyperkalemia Any ill-appearing neonate with vomiting, with or without evidence of intestinal obstruction, requires hospitalization and prompt, broad evaluation for sepsis and neurologic, renal, and metabolic disease Commonly, however, a young infant in the first to weeks of life who appears entirely well is brought to the ED with the complaint of persistent vomiting The birth history and perinatal course are unremarkable The baby is vigorous, has gained weight appropriately (5 to oz per week after the first week of life), and has a normal physical examination Usually, a close description of the “vomiting” (or a trial feeding in the ED) reveals the problem to be physiologic regurgitation or reflux This is a common and usually insignificant problem, representing normal variation in the developmental maturation of the lower esophageal sphincter (LES) GER in infants is discussed further below Older Infants Older infants presenting with vomiting must also be evaluated for signs of obstruction, however the causes of obstruction in this age group differ from those in the neonate Although malrotation with volvulus must continue to be considered, other causes of obstruction in this age group include hypertrophic pyloric stenosis (HPS), intussusception, enteric duplication cysts, incarcerated hernia, and complications of Meckel diverticulum Causes of intestinal obstruction most often diagnosed in the neonatal period, such as Hirschsprung disease, may also present at this age, though less commonly The various causes of intestinal obstruction must be considered in the vomiting infant before nonobstructive causes are considered in order to make a timely diagnosis and initiate treatment HPS most commonly presents between and weeks of age It is caused when thickening of the pyloric muscle leads to gastric outlet obstruction Infants present with nonbilious emesis, during or shortly after feeds, which is often described as