In a 6-month-old infant weighing 8 kg with two vomiting episodes in 14 hours—first projectile, second non-projectile after feeding—what is the most likely diagnosis?

Differential Diagnosis: Not Simply Viral Gastroenteritis

While viral gastroenteritis is possible in this 6-month-old infant, the initial projectile vomiting episode is a red flag that mandates urgent evaluation for hypertrophic pyloric stenosis (HPS) and other obstructive causes before attributing symptoms to a benign viral illness. 1

Critical Clinical Reasoning

Why This Cannot Be Dismissed as Viral Gastroenteritis

The ACR guidelines explicitly state that "bilious emesis or repeated forceful vomiting should be evaluated for underlying obstruction" 1. Your patient has:

• Projectile vomiting (first episode) - a hallmark of obstruction
• Age of 6 months - within the range for HPS (though typically 3-5 weeks, delayed presentations up to 10 months are documented) 2
• Two episodes within 14 hours - establishing a pattern

The Diagnostic Trap

Viral gastroenteritis typically presents with 1:

• Sudden onset
• Mild fever
• Diarrhea (notably absent in your case)
• Short duration
• Often occurs in epidemics

Your patient's presentation lacks these supporting features, particularly the absence of diarrhea, which makes viral gastroenteritis less likely.

Immediate Diagnostic Approach

Must-Evaluate Conditions in This Infant

Hypertrophic Pyloric Stenosis (HPS):

• Most common surgical cause of non-bilious projectile vomiting 2, 3
• Peak at 3-5 weeks, but can present up to 10 months 2
• Physical exam: palpate for "olive" mass in right upper quadrant 1, 4
• First-line imaging: Pyloric ultrasound (not upper GI series) 4

Other Critical Differentials:

• Malrotation with volvulus (can present at any age, not just neonates) 1
• Intussusception (though unusual before 3 months, your patient is 6 months) 1
• Gastroesophageal reflux with complications 4
• Food protein-induced enterocolitis syndrome (FPIES) 5

Red Flags Present in This Case

The guidelines identify concerning features 4, 6:

• ✓ Consistently forceful vomiting (projectile first episode)
• Need to assess for: bilious content, blood, fever, lethargy, poor weight gain, abdominal distension

Recommended Clinical Pathway

Immediate Actions:

1. Detailed physical examination:

   • Palpate for pyloric "olive" (present in only some HPS cases)
   • Assess hydration status (8 kg at 6 months suggests adequate growth, but evaluate current status)
   • Check for abdominal distension or tenderness
   • Evaluate for signs of increased intracranial pressure (bulging fontanelle, altered mental status)

2. Obtain focused history:

   • Character of vomitus: bilious? bloody? coffee-ground?
   • Relationship to feeding (HPS: hungry after vomiting, feeds well)
   • Presence of diarrhea (absent argues against gastroenteritis)
   • Pattern: cyclical? progressive?
   • Family history of migraine (suggests cyclic vomiting syndrome) 7

3. Imaging if projectile vomiting persists or recurs:

   • Pyloric ultrasound as first-line for suspected HPS 4
   • Consider abdominal radiograph if obstruction suspected 1
   • Upper GI series reserved for equivocal ultrasound or to evaluate anatomy 4

Common Pitfalls to Avoid

• Assuming viral gastroenteritis without diarrhea - gastroenteritis almost always includes diarrhea 1
• Missing delayed HPS presentation - while uncommon after 3 months, cases up to 10 months are documented 2, 3
• Ordering upper GI series before ultrasound for pyloric stenosis - ultrasound is superior and first-line 4
• Dismissing single projectile episode - this warrants investigation, not reassurance

Clinical Decision Point

If vomiting recurs or was truly projectile, obtain pyloric ultrasound urgently. If the infant remains well, is feeding normally, gaining weight appropriately, and the second episode was clearly related to handling/positioning after feeding (suggesting simple regurgitation), close observation with 24-48 hour follow-up is reasonable. However, any recurrence of forceful/projectile vomiting mandates immediate imaging 1.

The diagnosis of viral gastroenteritis should be one of exclusion after ruling out surgical causes in an infant with projectile vomiting, not a presumptive diagnosis.