What could be the cause of a 5-month-old baby's bloating, projectile vomiting, and recent food change, without signs of dehydration, fever, or meningitis, who passes gas and stool regularly?

Projectile Vomiting in a 5-Month-Old with Recent Food Change

The most likely diagnosis is food protein-induced allergic proctocolitis or gastroesophageal reflux disease (GERD) exacerbated by the recent food change, though hypertrophic pyloric stenosis must be urgently excluded given the projectile nature of vomiting at this age. 1

Immediate Diagnostic Priorities

Hypertrophic pyloric stenosis (HPS) is the most critical diagnosis to exclude, as it is the most common surgical cause of non-bilious projectile vomiting, though it typically presents between 2-8 weeks of age—your patient at 5 months is slightly older than typical but not outside the possible range 1. Key examination findings to assess:

• Palpate the right upper quadrant for an "olive" mass, which is pathognomonic for HPS and can clinch the diagnosis clinically 2, 1
• If the olive is not palpable but projectile vomiting persists, ultrasound is the imaging modality of choice to evaluate for pyloric stenosis 1

Most Likely Diagnosis: Food-Related GERD

Given the temporal relationship with food change and the absence of dehydration despite ongoing vomiting, this presentation most likely represents gastroesophageal reflux disease triggered or worsened by the new food 3, 4:

• GERD peaks at approximately 4 months of age and your patient at 5 months falls within this typical timeframe 3
• The fact that the infant passes gas and stool regularly argues strongly against mechanical obstruction 2
• Bloating with any food suggests possible food protein intolerance, particularly cow's milk protein allergy, which commonly presents as worsening reflux 4

Red Flags to Exclude

Ensure the vomiting is non-bilious (not green or yellow-green), as bilious vomiting would indicate obstruction distal to the ampulla of Vater and require urgent surgical evaluation 2, 5, 6:

• Bilious or bloody vomiting, abdominal distension, or lethargy would necessitate immediate imaging and surgical consultation 1, 6
• The absence of fever and meningitis symptoms makes infectious and neurological causes less likely 2

Management Algorithm

Step 1: Conservative Dietary Management (First-Line)

For formula-fed infants with suspected food protein intolerance:

• Switch to extensively hydrolyzed or amino acid-based formula as a therapeutic trial 3
• Consider thickening agents added to feeds to reduce reflux episodes 1, 3
• Implement smaller, more frequent feeds rather than large volume feeds 1

For breastfed infants:

• Maternal elimination of dairy products from the diet for 2-4 weeks to assess response 3

Step 2: Monitor Response (2-4 Weeks)

• If symptoms improve with dietary modification, continue the intervention 3
• If projectile vomiting persists despite dietary changes, proceed to imaging evaluation 1

Step 3: Pharmacologic Trial if Conservative Measures Fail

Empiric acid-suppression therapy for 4-8 weeks may be warranted if dietary modifications are insufficient 3:

• This should only be considered after mechanical causes are excluded 3
• Reassess regularly, as most infantile GERD resolves by 12 months of age 3

Common Pitfalls to Avoid

• Do not dismiss projectile vomiting as "just reflux" without first excluding HPS, especially if the infant shows any signs of dehydration or poor weight gain 1
• Do not start acid suppression empirically before ensuring adequate dietary trial, as food protein intolerance requires formula change, not just acid reduction 3, 4
• Malrotation with volvulus can present at any age, not just in newborns, so maintain vigilance for any change in vomiting pattern to bilious emesis 2, 1
• Regular weight checks are essential—poor weight gain would elevate concern for GERD disease rather than benign reflux and warrant more aggressive intervention 2, 4