How to differentiate between pertussis and infantile hypertrophic pyloric stenosis (IHPS) in a vomiting baby?

Differentiating Pertussis from Infantile Hypertrophic Pyloric Stenosis in a Vomiting Baby

The key distinction is that pertussis causes posttussive vomiting (vomiting triggered by severe coughing paroxysms) with respiratory symptoms, while IHPS presents with projectile, non-bilious vomiting independent of coughing, typically between 2-8 weeks of age, often with a palpable "olive" mass in the right upper quadrant. 1, 2, 3

Clinical Presentation Patterns

Pertussis-Related Vomiting

• Vomiting occurs after severe coughing paroxysms (posttussive emesis), not spontaneously 4
• Prominent respiratory symptoms including rhinorrhea, severe paroxysmal cough with inspiratory "whoop," and color change/cyanosis during coughing episodes 4
• Can occur at any age, though most severe in infants <12 months 5
• The vomiting is secondary to the forceful coughing, not a primary gastrointestinal problem 4

IHPS-Related Vomiting

• Projectile, non-bilious vomiting that is NOT preceded by coughing 2, 3, 6
• Classic age of onset: 2-8 weeks of life (though rare neonatal cases exist) 2, 7
• Vomiting is effortless and forceful, occurring shortly after feeds 7
• Palpable "olive" mass in the right upper quadrant is pathognomonic when present 2
• Visible peristaltic waves across the abdomen after feeding 3
• Progressive symptoms with poor weight gain and signs of dehydration 3

Critical Diagnostic Features

Physical Examination Findings

• For IHPS: Palpate specifically for the pyloric "olive" in the right upper quadrant—this finding is diagnostic 2
• For pertussis: Look for respiratory distress, cyanosis during coughing episodes, and absence of abdominal masses 4
• Assess hydration status and weight trajectory in both conditions 2, 3

Timing and Pattern Recognition

• IHPS vomiting is independent of any trigger—occurs after most feeds regardless of coughing 3, 6
• Pertussis vomiting is triggered specifically by coughing fits 4
• IHPS typically presents with escalating frequency and severity over days to weeks 3
• Pertussis has a prodromal catarrhal phase (1-2 weeks) before paroxysmal coughing develops 1

Diagnostic Workup

For Suspected IHPS

• Abdominal ultrasound is the imaging modality of choice to confirm pyloric muscle hypertrophy 2
• Check serum electrolytes for hypochloremic, hypokalemic metabolic alkalosis (though less common with earlier diagnosis) 3
• Upper GI series if ultrasound is equivocal 6

For Suspected Pertussis

• Nasopharyngeal swab for direct fluorescent antibody test or PCR for Bordetella pertussis 4
• Clinical diagnosis can be made based on characteristic paroxysmal cough with posttussive vomiting 1
• Start treatment immediately on clinical suspicion without waiting for confirmation 1

Important Clinical Pitfalls

Atypical Presentations

• IHPS can rarely present with bilious vomiting due to earlier diagnosis before complete obstruction develops 8
• Neonatal IHPS (presenting in first days of life) is extremely rare but possible 7
• Azithromycin treatment for pertussis has been associated with subsequent IHPS development, though this is rare 4

Overlapping Scenarios

• An infant with pertussis treated with azithromycin who later develops projectile vomiting may have developed IHPS as a complication 4
• Monitor infants <1 month receiving macrolides for pertussis for signs of IHPS (projectile vomiting, palpable mass) 5, 1
• If an infant with diagnosed pertussis develops projectile vomiting independent of coughing episodes, consider IHPS as a differential 4

Red Flags Requiring Urgent Evaluation

• Bilious vomiting in any infant is a surgical emergency until malrotation with volvulus is excluded 9, 2
• Abdominal distension, bloody stools, or lethargy warrant immediate surgical consultation 2
• Severe dehydration or metabolic derangement requires IV fluid resuscitation before definitive treatment 2

Management Implications

If Pertussis is Diagnosed

• Azithromycin 10 mg/kg/day for 5 days for infants <6 months 1
• Isolate for 5 days after starting antibiotics 1
• Monitor for development of IHPS if macrolide therapy is initiated, especially in infants <1 month 5, 1

If IHPS is Confirmed

• Correct dehydration and electrolyte abnormalities first 2
• Surgical pyloromyotomy is definitive treatment 3
• Withhold feeds until surgical intervention 2