In an infant with non‑bilious, forceful vomiting, persistent hunger after episodes, and visible abdominal peristalsis, what is the most appropriate next diagnostic test?

Abdominal Ultrasound is the Most Appropriate Next Step

In an infant presenting with non-bilious, forceful vomiting, persistent hunger after episodes, and visible abdominal peristalsis, abdominal ultrasound should be performed immediately to diagnose hypertrophic pyloric stenosis (HPS). 1, 2

Clinical Reasoning

This clinical presentation is classic for hypertrophic pyloric stenosis:

• Non-bilious projectile vomiting with persistent hunger afterward ("hungry vomiter") is pathognomonic for HPS 2, 3
• Visible peristaltic waves on abdominal examination represent the stomach attempting to force contents through the narrowed pylorus 3
• HPS typically presents between 3-6 weeks of age (though the range is 2-8 weeks) 1, 2

Why Ultrasound is the Correct Answer (Option B)

• Abdominal ultrasound is the imaging modality of choice for suspected HPS, with high sensitivity and specificity for detecting thickened pyloric muscle 1, 2
• Diagnostic ultrasound criteria include:
  • Pyloric muscle thickness >3-4 mm 2, 4
  • Pyloric channel length >15-17 mm 2, 4
  • Target sign on transverse view 2
• The American College of Radiology specifically recommends initiating abdominal ultrasound to evaluate non-bilious projectile vomiting for HPS in infants 2 weeks to 3 months of age 1

Why NOT the Other Options

Abdominal X-ray (Option C) is Insufficient

• Plain radiographs may show a distended stomach with minimal distal bowel gas, but this is nonspecific 2
• X-ray cannot definitively diagnose HPS—it only suggests gastric outlet obstruction 2
• The American College of Radiology states that radiographs are useful for bilious vomiting or suspected obstruction patterns (duodenal atresia, jejunal atresia), not for confirming HPS 1

Abdominal CT (Option A) is Not Indicated

• CT exposes the infant to unnecessary radiation 1
• Ultrasound provides superior diagnostic accuracy for HPS without radiation risk 1, 2
• CT has no role in the initial evaluation of suspected pyloric stenosis in infants

Critical Pitfalls to Avoid

• Do not dismiss non-bilious vomiting as benign reflux when it is forceful/projectile and associated with visible peristalsis 2, 4
• The classic palpable "olive" mass in the right upper quadrant is present in only 60-80% of cases, so absence does not rule out HPS 2, 4
• If vomiting becomes bilious at any point, this represents a surgical emergency (malrotation with volvulus) requiring immediate upper GI series 1, 2
• Recent advances in ultrasound technology allow earlier diagnosis of HPS, so classical late-stage findings (severe dehydration, metabolic alkalosis, palpable olive) are now less common 5

Immediate Management After Diagnosis

• Assess and correct dehydration with IV fluid resuscitation before surgery, as HPS causes hypochloremic, hypokalemic metabolic alkalosis from persistent vomiting 2
• Immediate pediatric surgery consultation is mandatory once HPS is confirmed, as pyloromyotomy is the definitive treatment 2