A 5‑month‑old infant presents with bilious vomiting and signs of intestinal obstruction; what is the appropriate initial investigation?

Initial Management of Bilious Vomiting in a 5-Month-Old Infant

The correct answer is D - obtain an abdominal X-ray immediately as the first imaging study, followed by an upper GI contrast series if obstruction is confirmed. 1, 2, 3

Immediate Clinical Actions

• Place a nasogastric or orogastric tube immediately to decompress the stomach before any imaging is performed 4
• Bilious (greenish) vomiting in any infant is a surgical emergency until proven otherwise, as it indicates obstruction distal to the ampulla of Vater 2, 4
• The most critical diagnosis to exclude is midgut malrotation with volvulus, which can cause intestinal necrosis within hours due to vascular compromise around the superior mesenteric artery 2

Algorithmic Diagnostic Approach

Step 1: Plain Abdominal Radiograph (Answer D)

• Abdominal X-ray is the mandatory first imaging study for any infant with bilious vomiting and signs of obstruction 1, 2, 3
• Look for specific patterns:
  • "Double bubble" sign suggesting duodenal obstruction 2
  • Multiple dilated bowel loops with absent or decreased distal gas 2
  • Air-fluid levels indicating mechanical obstruction 3, 4
• Critical caveat: Normal abdominal radiographs do NOT exclude malrotation or volvulus - clinical suspicion based on bilious vomiting alone mandates proceeding to upper GI study regardless of radiograph findings 2

Step 2: Upper GI Contrast Series (Not Answer A Initially)

• After the initial radiograph, an upper GI series is the definitive study and should be performed urgently 2, 3
• This study has 96% sensitivity for detecting malrotation by identifying abnormal position of the duodenojejunal junction (ligament of Treitz) 2, 3
• Upper GI series directly visualizes the stomach and small bowel to confirm or exclude volvulus requiring immediate surgery 2, 3
• Do not delay upper GI series if clinical suspicion is high, even with normal plain films 2

Why Other Options Are Incorrect

Lower GI Contrast (Answer B)

• Contrast enema is NOT appropriate for initial evaluation of bilious vomiting in this age group 3
• It is reserved only for suspected distal bowel obstruction and has approximately 20% false-negative rate for malrotation, making it inferior to upper GI series 2, 3

Endoscopy (Answer C)

• Endoscopy has no role in the acute evaluation of intestinal obstruction in infants 3
• It cannot assess the position of the duodenojejunal junction or identify volvulus 3

Upper GI Contrast First (Answer A)

• While upper GI series is essential, there is no literature supporting performing it as the initial study prior to an abdominal radiograph 1
• The plain film provides crucial initial information about bowel gas patterns and helps guide subsequent management 1, 2

Age-Specific Differential Diagnoses for 5-Month-Old

At 5 months, consider:

• Malrotation with volvulus (can present at any age with decreasing frequency) 1, 2
• Intussusception (unusual before 3 months but possible at 5 months, presents with crampy intermittent pain, bloody stools, lethargy) 1, 3
• Less likely: intestinal atresia, Hirschsprung disease (typically present earlier) 1, 2

Time-Critical Management Points

• Immediate pediatric surgical consultation is necessary upon confirmation of bilious vomiting, given the potential for volvulus to compromise intestinal vascularization within hours 3
• If upper GI series confirms malrotation/volvulus, proceed directly to surgery 2
• The combination of abnormal plain radiograph and abdominal distention increases odds of malrotation 10-fold; adding abdominal tenderness increases odds of midgut volvulus 25-fold 5

Common Pitfall to Avoid

Never assume normal plain films exclude surgical pathology - up to 15% of upper GI studies may be inconclusive requiring repeat studies, and physical examination plus plain radiograph cannot substitute for upper GI series when malrotation is suspected 2, 5