How to diagnose pyloric stenosis in an infant presenting with symptoms such as projectile vomiting?

How to Rule Out Pyloric Stenosis in an Infant

Ultrasound of the abdomen is the definitive imaging study to rule out hypertrophic pyloric stenosis (HPS) in infants presenting with projectile vomiting, and should be performed when clinical examination is equivocal or when the classic "olive" mass cannot be palpated. 1, 2

Clinical Assessment First

Begin by attempting to palpate the pathognomonic "olive" mass in the right upper quadrant during or immediately after feeding, as this firm, mobile, olive-shaped pyloric tumor is diagnostic of HPS and requires no further imaging confirmation. 1, 3 However, clinicians fail to detect this olive in 11-51% of proven HPS cases, making imaging often necessary. 4

Key Clinical Features to Assess:

• Age of presentation: HPS typically presents between 2-8 weeks of age (though can occur up to 3 months). 1, 5
• Vomiting characteristics: Non-bilious, projectile vomiting occurring shortly after feeds. 1, 6
• Hydration status: Look for signs of dehydration including decreased urine output, sunken fontanelle, and poor skin turgor. 1
• Weight pattern: Poor weight gain or weight loss despite vigorous appetite. 1
• Visible gastric peristaltic waves: May be seen moving left to right across the upper abdomen after feeding. 2, 6

Critical Red Flag: Rule Out Bilious Vomiting

If vomiting is bilious (green), this is NOT pyloric stenosis—it indicates obstruction distal to the ampulla of Vater and requires urgent evaluation for malrotation with midgut volvulus. 7, 1, 8 This is a surgical emergency requiring immediate upper GI series, not ultrasound for HPS. 7, 8

Definitive Diagnostic Imaging

Ultrasound Abdomen (First-Line Study)

Ultrasound is the imaging modality of choice and should be the initial study when HPS is suspected but the olive is not palpable. 1, 2, 4 Real-time ultrasound is simple, reliable, and avoids radiation exposure. 2

Diagnostic ultrasound criteria for HPS:

• Pyloric muscle thickness ≥3-4 mm (most accurate single measurement). 2
• Pyloric channel length ≥15-17 mm. 2
• Pyloric diameter measurements. 2

The thickened hypertrophic muscle has a characteristic sonographic appearance that makes HPS readily identifiable. 4 Recent advances in ultrasound technology allow earlier diagnosis before classic clinical features fully develop. 5

When Ultrasound is NOT Needed

Do not order imaging if you palpate a definite olive mass in an infant with typical symptoms—this is diagnostic and proceeding directly to surgical consultation avoids unnecessary delays and costs. 3 Studies show that performing redundant imaging studies after palpating an olive is associated with delayed diagnosis and potentially adverse clinical outcomes. 3

Common Pitfalls to Avoid

Pitfall #1: Ordering upper GI series instead of ultrasound. Upper GI series is NOT the appropriate study for suspected HPS—it is used for evaluating malrotation, not pyloric stenosis. 7, 1

Pitfall #2: Delaying imaging when the olive cannot be palpated. The absence of a palpable olive does NOT rule out HPS, as physical examination sensitivity is only 49-89%. 4, 5 The frequency of palpable olives has decreased significantly in recent years due to earlier diagnosis. 5

Pitfall #3: Assuming normal labs rule out HPS. Early-stage HPS may not yet have the classic metabolic alkalosis (hypochloremic, hypokalemic) seen in advanced cases. 5 Earlier diagnosis means you may encounter HPS before severe dehydration and electrolyte abnormalities develop. 5

Pitfall #4: Missing the diagnosis in older infants. While typical age is 2-8 weeks, HPS can present later, and the age at diagnosis has been increasing in recent studies. 5

Algorithm Summary

1. Assess vomiting type: If bilious → urgent evaluation for malrotation (NOT HPS). 1, 8
2. Attempt to palpate olive: If present with typical history → proceed to surgery, no imaging needed. 3
3. If olive not palpable but HPS suspected: Order abdominal ultrasound. 1, 2, 4
4. If ultrasound negative but high clinical suspicion persists: Consider repeat ultrasound or alternative diagnoses (gastroesophageal reflux, overfeeding, pylorospasm). 7, 1