What is the workup for a 5-week-old infant presenting with poor feeding and projectile vomiting?

Workup for a 5-Week-Old Infant with Poor Feeding and Projectile Vomiting

The initial workup for a 5-week-old infant with poor feeding and projectile vomiting should include an abdominal ultrasound to evaluate for hypertrophic pyloric stenosis, followed by appropriate laboratory studies and potentially upper GI series if ultrasound is inconclusive. 1

Initial Assessment

History - Key Elements to Obtain

• Timing and nature of vomiting (projectile, non-bilious vs. bilious)
• Feeding pattern and volume
• Weight gain/loss pattern
• Stool pattern and characteristics
• Associated symptoms (irritability, lethargy)

Physical Examination - Critical Findings

• Hydration status assessment
• Abdominal examination for palpable "olive" (hypertrophic pyloric muscle)
• Fontanelle examination (bulging may indicate increased intracranial pressure)
• Weight and growth parameters compared to baseline

Diagnostic Algorithm

Step 1: Initial Imaging

• Abdominal ultrasound is the first-line imaging study for evaluating projectile vomiting in this age group 1
  • Specifically looking for pyloric muscle thickness >3mm and length >15mm
  • Sensitivity of 95-100% for hypertrophic pyloric stenosis

Step 2: Laboratory Studies

• Electrolytes, BUN, creatinine to assess for hypochloremic, hypokalemic metabolic alkalosis (typical in pyloric stenosis)
• CBC to evaluate for infection
• Consider glucose level if symptoms of hypoglycemia present

Step 3: Additional Imaging (if ultrasound inconclusive)

• Upper GI series if:
  • Ultrasound is negative but clinical suspicion remains high
  • Need to evaluate for malrotation with volvulus (if bilious vomiting)
  • Need to assess for gastroesophageal reflux disease

Differential Diagnosis to Consider

1. Hypertrophic pyloric stenosis - Most likely given age (typically presents 3-6 weeks) and projectile vomiting 2, 3
2. Gastroesophageal reflux disease - Common in infants but typically not projectile 4
3. Gastric volvulus - Can present with projectile vomiting 2, 5
4. Formula intolerance/allergy - Usually with associated diarrhea/rash
5. Increased intracranial pressure - Would expect neurological symptoms
6. Metabolic disorders - Would expect additional systemic symptoms

Red Flags Requiring Urgent Evaluation

• Bilious vomiting (suggests intestinal obstruction distal to ampulla of Vater)
• Hematemesis or blood in stool
• Signs of dehydration (decreased urine output, sunken fontanelle)
• Lethargy or altered mental status
• Abdominal distension or tenderness
• Failure to thrive 1, 4

Management Considerations

• NPO status may be necessary if surgical condition suspected
• IV fluid resuscitation if dehydrated
• Surgical consultation for confirmed pyloric stenosis
• Antiemetics generally not recommended in this age group except in specific circumstances 6

Common Pitfalls to Avoid

• Mistaking normal physiologic gastroesophageal reflux for pathologic condition requiring medication 4
• Delaying surgical consultation when pyloric stenosis is suspected
• Failing to recognize bilious vomiting as a surgical emergency
• Overlooking metabolic derangements that may accompany prolonged vomiting
• Using car seats or semi-reclined positions after feeding which can worsen reflux 4

Remember that projectile vomiting in a 5-week-old infant is highly concerning for pyloric stenosis until proven otherwise, and prompt diagnosis is essential to prevent complications from dehydration and metabolic derangements.