What is the best initial diagnostic step for a patient with a 5-month history of greenish vomiting and signs of obstruction?

Initial Diagnostic Approach: Abdominal X-ray First

For a 5-month-old infant presenting with greenish (bilious) vomiting and signs of obstruction, obtain an abdominal X-ray immediately as the first diagnostic step (Answer D), followed urgently by an upper GI contrast study if obstruction is confirmed. 1, 2

Why Abdominal X-ray is the Correct First Step

Plain abdominal radiography is the recommended initial imaging study because it can rapidly identify critical signs of intestinal obstruction including dilated bowel loops, air-fluid levels, and gas distribution patterns that guide subsequent management. 3, 1, 2

Key Diagnostic Findings on Initial X-ray:

• "Double bubble" sign suggests duodenal obstruction 1
• "Triple bubble" sign may indicate jejunal atresia 1, 4
• Multiple dilated loops with absent/decreased distal gas suggests small bowel obstruction 1
• Sensitivity of 96% for detecting malrotation, though false-positives (10-15%) and false-negatives (up to 7%) can occur 1

Critical Clinical Context: Bilious Vomiting is a Surgical Emergency

Greenish vomiting in an infant indicates bile presence, meaning obstruction is distal to the ampulla of Vater, and represents a surgical emergency until proven otherwise. 1, 2 The most life-threatening concern is midgut volvulus, which can cause complete intestinal necrosis within hours due to twisting around the superior mesenteric artery. 1

Life-Threatening Differential Diagnoses at 5 Months:

• Malrotation with midgut volvulus (most urgent) 1, 5
• Intussusception (common at this age) 1, 2
• Intestinal atresia (jejunal, ileal) 1, 4
• Internal hernia 1

Why Other Options Are Incorrect

Upper GI Contrast (Option A):

• This is the SECOND step, not first 1, 2
• Should be performed urgently AFTER the initial X-ray if obstruction is confirmed 1, 2
• Critical caveat: Normal abdominal radiographs do NOT exclude malrotation or volvulus - if clinical suspicion remains high based on bilious vomiting alone, proceed directly to upper GI contrast study regardless of X-ray findings 1

Lower GI Contrast (Option B):

• NOT appropriate for initial evaluation of bilious vomiting 2
• Reserved for suspected distal bowel obstruction scenarios 1, 2
• Has approximately 20% false-negative rate for malrotation, making it inferior to upper GI series 1

Endoscopy (Option C):

• Has NO role in acute evaluation of intestinal obstruction in infants 2
• Cannot visualize the critical anatomic landmarks needed (duodenojejunal junction position) 1
• Delays definitive diagnosis and surgical intervention 2

Immediate Management Priorities

While obtaining imaging, simultaneously initiate:

• Intravenous crystalloid resuscitation with isotonic fluids 3
• Nasogastric tube decompression to prevent aspiration pneumonia and decompress proximal bowel 3, 5
• NPO status (bowel rest) 3
• Urgent pediatric surgical consultation given the potential for volvulus requiring immediate operative intervention 2

Laboratory Testing:

• Complete blood count, electrolytes, renal function 3
• Low bicarbonate, elevated lactate, and marked leukocytosis may indicate intestinal ischemia 3

Algorithm Summary

1. Obtain abdominal X-ray immediately (Answer D) 3, 1, 2
2. If obstruction confirmed or high clinical suspicion persists despite normal X-ray: Proceed urgently to upper GI contrast series 1, 2
3. If upper GI confirms malrotation/volvulus: Immediate surgery 1
4. If intussusception suspected: Ultrasound may be added, but should not delay definitive imaging 2

Common pitfall to avoid: Do not be falsely reassured by normal plain radiographs - bilious vomiting alone mandates proceeding to upper GI contrast study if malrotation remains a clinical concern. 1