What is the most appropriate initial investigation for a child presenting with greenish vomiting and abdominal pain?

Abdominal X-ray (Option C)

For a child presenting with greenish (bilious) vomiting and abdominal pain, an abdominal X-ray is the most appropriate initial investigation. This approach follows the American College of Radiology guidelines and allows rapid identification of bowel obstruction patterns that guide subsequent management 1, 2.

Why Abdominal X-ray First

Bilious vomiting in a child is a surgical emergency until proven otherwise, requiring immediate evaluation for intestinal obstruction, malrotation with volvulus, or other life-threatening conditions 2, 3. The abdominal X-ray serves as the critical first step because it:

• Rapidly identifies signs of obstruction including dilated bowel loops, air-fluid levels, and abnormal gas distribution patterns that indicate the level and severity of obstruction 2, 4
• Guides the next diagnostic step by determining whether upper GI contrast study, lower GI contrast study, or immediate surgical consultation is needed 1, 2
• Takes only minutes to obtain without requiring sedation, contrast administration, or specialized equipment, making it ideal for urgent evaluation 4

The Sequential Diagnostic Algorithm

Step 1: Abdominal X-ray (Initial Study)

The plain radiograph provides immediate information about:

• Proximal vs. distal obstruction patterns (double bubble suggests duodenal obstruction; multiple dilated loops suggest more distal pathology) 1
• Presence of free air indicating perforation requiring immediate surgery 4
• Bowel gas distribution helping differentiate mechanical obstruction from ileus 2

Step 2: Upper GI Contrast Study (If X-ray Shows Obstruction)

After the initial radiograph confirms obstruction, upper GI series is usually appropriate for children older than 2 days with bilious vomiting to evaluate for malrotation and midgut volvulus, with 96% sensitivity for detecting malrotation 1, 2. The upper GI series directly visualizes:

• Abnormal position of the duodenojejunal junction (ligament of Treitz), the critical finding indicating malrotation 2
• Corkscrew appearance of twisted bowel in volvulus 1

Step 3: Lower GI Contrast Study (Only for Specific Patterns)

Contrast enema is reserved for radiographs showing distal bowel obstruction patterns, such as suspected Hirschsprung disease or meconium ileus 1. It is NOT appropriate as the initial study for bilious vomiting 2.

Critical Clinical Context

The ACR Appropriateness Criteria specifically address this scenario in Variant 5, stating that for infants older than 2 days with bilious vomiting (suspected malrotation), fluoroscopy UGI series is usually appropriate for initial imaging, but notably the panel acknowledged controversy about whether radiography abdomen should precede it 1. However, in real-world practice:

• Most pediatric surgeons and emergency physicians obtain the X-ray first because it takes 2-3 minutes, costs less, and provides essential information about obstruction severity 4, 5
• Going directly to upper GI contrast without an X-ray risks missing free air or other findings that would change management to immediate surgery rather than fluoroscopy 2
• The X-ray helps triage urgency: a child with massive bowel dilation and air-fluid levels needs more urgent surgical consultation than one with minimal findings 5

Common Pitfalls to Avoid

Do not confuse this presentation with pyloric stenosis, which causes non-bilious projectile vomiting in infants 2 weeks to 3 months old and would warrant ultrasound as the initial study 2, 6. The greenish (bilious) nature of the vomiting indicates obstruction distal to the ampulla of Vater, making this a completely different clinical scenario 2.

Do not delay imaging for extensive laboratory workup when bilious vomiting is present—this is a time-sensitive surgical emergency where volvulus can cause bowel necrosis within hours 2, 3.

Do not order lower GI contrast study first—it evaluates the wrong anatomic region for bilious vomiting and delays appropriate diagnosis 2.

Time-Critical Nature

Limiting abdominal radiographs to patients with abnormal bowel sounds, abdominal distention, peritoneal signs, or bilious vomiting identifies all patients with diagnostic radiographs for major diseases while eliminating unnecessary studies 4. Your patient with greenish vomiting and abdominal pain clearly meets these criteria, making the X-ray both appropriate and essential 4, 5.