What is the best next step in diagnosis for a 2-day-old neonate with delayed passage of meconium, abdominal distension, and bilious vomiting, who has a gush of stool on rectal examination?

Best Next Step in Diagnosis

The best next step is a contrast enema (barium enema), which serves as both the diagnostic procedure of choice for suspected distal bowel obstruction and can differentiate between multiple conditions including Hirschsprung disease, meconium plug, ileal atresia, and meconium ileus. 1

Clinical Presentation Analysis

This neonate presents with the classic triad highly suggestive of distal bowel obstruction:

• Delayed passage of meconium beyond 48 hours of life 2, 3
• Abdominal distension 2
• Bilious vomiting 2

The gush of stool on rectal examination is a critical finding that strongly suggests Hirschsprung disease, but this clinical sign alone is not diagnostic and occurs in only a subset of cases. 2

Why Contrast Enema is the Correct Answer

The American College of Radiology explicitly recommends contrast enema as the diagnostic imaging procedure of choice for suspected distal bowel obstruction because it can differentiate between conditions that require different management approaches. 1

Diagnostic Capabilities of Contrast Enema:

• Demonstrates the transition zone between narrow aganglionic segment and dilated proximal colon in Hirschsprung disease 1
• Shows microcolon in cases of structural atresia (ileal or colonic) 1
• Identifies meconium plugs that may be therapeutically evacuated 1
• Differentiates between meconium plug, ileal atresia, meconium ileus, and Hirschsprung disease requiring rectal biopsy 1

Dual Diagnostic and Therapeutic Role:

The contrast enema is both diagnostic and potentially therapeutic, as it may relieve functional obstructions like meconium plug syndrome while simultaneously identifying the underlying pathology. 1, 4

Why NOT Rectal Biopsy First

The American College of Radiology specifically advises against assuming Hirschsprung disease and proceeding directly to rectal biopsy, as the clinical presentation is identical for multiple causes of distal obstruction. 1

While rectal biopsy is the definitive diagnostic test for Hirschsprung disease, it should be performed after contrast enema confirms the diagnosis by demonstrating a transition zone or when contrast enema findings are suggestive but not definitive. 1

Why NOT Ultrasound

The American College of Radiology explicitly states that ultrasound has no role in evaluating neonates with suspected distal obstruction, as there is no relevant literature supporting its use in this clinical scenario. 1

Critical Pitfall to Avoid

While this presentation is highly suspicious for Hirschsprung disease (98% of HD patients have one or more of these classic symptoms 2), do not skip the contrast enema step. The contrast enema will:

• Confirm the diagnosis by showing the transition zone 1
• Rule out other treatable causes like meconium plug 1
• Potentially provide therapeutic relief if the obstruction is functional 1, 4
• Guide the location for rectal biopsy if Hirschsprung disease is confirmed 1