What is the best diagnostic study for a neonate with forceful, nonbilious emesis and vomiting after feeding, presenting with hypotension, tachycardia, and poor tone?

Ultrasound is the Best Diagnostic Study

For a 20-day-old neonate presenting with forceful, nonbilious, projectile vomiting after feeding, ultrasound is the definitive diagnostic study to evaluate for hypertrophic pyloric stenosis (HPS), which is the most likely diagnosis given this classic presentation. 1

Clinical Reasoning

This neonate's presentation is pathognomonic for HPS:

• Age: 20 days old (HPS typically presents between 2-12 weeks of life) 2
• Vomiting pattern: Forceful, nonbilious, occurring shortly after feeding 2, 3
• Metabolic derangement: The hypotension (BP 72/54), tachycardia (P 182), and poor tone suggest hypovolemia and likely hypochloremic, hypokalemic metabolic alkalosis from persistent vomiting 3
• Abdominal exam: Soft, nontender, nondistended abdomen is typical for HPS (unlike obstructive processes) 3

Why Ultrasound is Superior

Ultrasound directly visualizes the thickened pyloric muscle and is the gold standard for diagnosing HPS. 1 The European Society of Paediatric and Neonatal Intensive Care guidelines acknowledge that POCUS may recognize hypertrophic pyloric stenosis, though they recommend confirmation by a pediatric radiologist for definitive diagnosis. 1

Diagnostic Criteria on Ultrasound:

• Pyloric muscle thickness >3-4 mm
• Pyloric channel length >15-17 mm
• Target sign on transverse view 1

Why Other Modalities Are Inappropriate

X-ray (Option D):

Plain radiographs may show a distended stomach with minimal distal gas, but these findings are nonspecific and do not establish the diagnosis of HPS. 3 X-rays cannot visualize the pyloric muscle thickness required for diagnosis. 1

Fluoroscopy/Upper GI Series (Option A):

While upper GI series can demonstrate the "string sign" (elongated, narrowed pyloric channel) or "shoulder sign" in HPS, it is not the first-line study because: 1

• It exposes the infant to radiation unnecessarily
• Ultrasound is more specific and sensitive
• The ACR Appropriateness Criteria explicitly state that UGI series is reserved for when ultrasound is equivocal or to evaluate for other causes if HPS is ruled out 1

CT Scan (Option B):

CT has no role in evaluating suspected HPS in neonates due to excessive radiation exposure and lack of superiority over ultrasound. 1

Critical Pitfall to Avoid

Do not confuse this presentation with bilious vomiting, which would indicate intestinal obstruction (malrotation with volvulus) and require urgent upper GI series. 1, 4 The nonbilious nature of this infant's emesis excludes obstruction distal to the ampulla of Vater and points directly to gastric outlet obstruction from HPS. 1, 4

Management After Diagnosis

Once HPS is confirmed by ultrasound:

1. Correct fluid and electrolyte abnormalities (hypochloremic, hypokalemic metabolic alkalosis) before surgery 3
2. Surgical consultation for pyloromyotomy 2, 3
3. NPO status and nasogastric decompression if needed 3

The combination of age, nonbilious projectile vomiting, and signs of dehydration makes HPS the diagnosis until proven otherwise, and ultrasound is the single best test to confirm it. 1, 2, 3