A 7-week-old boy presents to the pediatrician for vomiting. Three weeks ago, the patient began vomiting after meals. The vomitus appears to be normal stomach contents without streaks of red or green. His parents have already tried repositioning him during mealtimes and switching his formula to eliminate cow’s milk and soy. Despite these adjustments, the vomiting has become more frequent and forceful. The patient is voiding about 4 times per day and his urine appears dark yellow. He has fallen 1 standard deviation off of the growth curve. The patient's mother reports that the pregnancy was uncomplicated other than an episode of sinusitis in the 3rd trimester, for which she was treated with azithromycin. The patient's temperature is 98.7°F (37.1°C), blood pressure is 58/41 mmHg, pulse is 166/min, and respirations are 16/min. On physical exam, the patient looks small for his age. His abdomen is soft, non-tender, and non-distended. Which of the following is the most appropriate next step in management?
Abdominal ultrasound
Thickening feeds
MRI of the head
Intravenous hydration
Intravenous hydration
This patient presents with nonbilious, forceful vomiting, poor weight gain, signs of dehydration, and a history of exposure to azithromycin, which suggests a diagnosis of pyloric stenosis. The most appropriate next step in management is intravenous hydration.

Pyloric stenosis classically presents in a male infant between 4 to 6 weeks of age with nonbilious, projectile vomiting after meals. Risk factors for pyloric stenosis include bottle feeding and exposure to macrolide antibiotics in the patient or mother. Protracted vomiting frequently leads to hypochloremic, hypokalemic metabolic alkalosis. Patients should be resuscitated with intravenous hydration and evaluated for electrolyte abnormalities that require repletion. After a patient is adequately resuscitated, patients can undergo an ultrasound to confirm the diagnosis and then be treated surgically for definitive therapy.

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 ultrasound is the test of choice to diagnose pyloric stenosis, but evaluation for electrolyte derangements and subsequent intravenous hydration and electrolyte repletion should be initiated first in this patient with signs of dehydration and malnutrition. Further evaluation is reserved after a patient is adequately resuscitated.

Answer B: MRI of the head would be appropriate if this patient had additional signs of a mass in the head, such as neurological changes or worsening of the vomiting in the morning. In the absence of these signs and the presence of a mass in the epigastrium, this patient is more likely to have pyloric stenosis.

Answer C: Pyloromyotomy is the definitive management of pyloric stenosis, but this patient should undergo fluid replacement and have any electrolyte derangements detected and managed before undergoing surgery. Surgical intervention can carry significant risks if patients are not adequately resuscitated.

Answer E: Thickening feeds are one of the techniques used to manage gastroesophageal reflux. In the setting of this patient’s epigastric mass and clinical picture, pyloric stenosis is the more likely etiology of his vomiting. Reflex presents with non-projectile vomiting that is dependent on position.

Bullet Summary:
Patients with pyloric stenosis should first undergo intravenous rehydration and electrolyte repletion, as protracted vomiting may lead to severe hypochloremic, hypokalemic metabolic alkalosis.