A 4-week-old boy presents with his parents to the pediatrician for a well-child visit. His mother reports he was eating well until 1 week ago when he began vomiting after breastfeeding. His mother has tried increasing the frequency of feeds and decreasing the amount of each feed, but the vomiting seems to be getting worse. The patient now vomits after every feed. The vomitus looks like breast milk. The patient’s mother is exclusively breastfeeding and would prefer not to switch to formula but worries that the patient is not getting the nutrition he needs. Two weeks ago, the patient was in the 75th percentile for weight and 70th for height. He is now in the 60th percentile for weight and 68th percentile for height. His temperature is 98.2°F (36.8°C), blood pressure is 58/37 mmHg, pulse is 144/min, and respirations are 34/min. On physical exam, the patient has dry mucous membranes. His abdomen is soft and non-distended. Which of the following is the most appropriate next step in management?
Abdominal ultrasound
Trial of empiric proton pump inhibitor
Abdominal radiograph
Supplement breastfeeding with formula
Abdominal ultrasound
This patient presents with a history of nonbilious, forceful vomiting, poor weight gain, and signs of dehydration, which is consistent with pyloric stenosis. The most appropriate next step in management is abdominal ultrasound to confirm the diagnosis.

Pyloric stenosis presents in 4- to 6-week-old infants with nonbilious, post-prandial projectile vomiting. Although pyloric stenosis classically presents with an “olive-shaped” mass in the epigastrium, the mass cannot be palpated in some patients. Any infant who develops new, persistent vomiting in the first 2 months of life and has associated dehydration or failure to thrive should undergo an abdominal ultrasound to evaluate for pyloric stenosis. Patients should undergo resuscitation prior to the definitive treatment, which is a pyloromyotomy.

Adams and Stanton review the evidence regarding the diagnosis and treatment of neonatal bowel obstruction. They discuss how malrotation, jejunoileal atresia, duodenal atresia, and colonic atresia are common causes of this finding. They recommend surgical management of these conditions.

Incorrect Answers:
Answer A: Abdominal radiograph is not helpful in evaluating for pyloric stenosis. It is more useful in the workup of bilious vomiting, as several etiologies of bilious vomiting (such as duodenal atresia and Hirschsprung disease) can be seen on an abdominal radiograph. Treatment of these conditions is surgical excision of the defective segment of colon.

Answer C: Supplementing breastfeeding with formula would not be appropriate for this patient as his failure to thrive is driven by gastric outlet obstruction. In the case of pyloric stenosis, formula would cause vomiting just as breastmilk does.

Answer D: A trial of a dairy-free diet is used to improve gastroesophageal reflux, but in the setting of this patient’s age and the description of vomiting, he should undergo an abdominal ultrasound to evaluate for pyloric stenosis. Treatment of reflux also involves positional changes during feeding.

Answer E: A trial of empiric proton pump inhibitor is indicated for patients with gastroesophageal reflux disease, but it would not address this patient’s pyloric stenosis. Patients can also undergo conservative treatments such as changing positioning during feeding.

Bullet Summary:
The diagnostic test of choice for pyloric stenosis in infants is an abdominal ultrasound.