What are the causes of a moderately dilated gas‑filled rectum and colon in a 7‑week‑old infant?

Moderately Dilated Gas-Filled Rectum and Colon in a 7-Week-Old Infant

In a 7-week-old infant with moderately dilated gas-filled rectum and colon, the most likely diagnoses are Hirschsprung disease, functional constipation, or less commonly intussusception, with Hirschsprung disease being the most critical to exclude given the potential for enterocolitis and perforation. 1, 2, 3

Algorithmic Approach to Differential Diagnosis

Primary Considerations by Clinical Context

If the infant has bilious vomiting:

• This represents a surgical emergency until proven otherwise, requiring immediate evaluation for intestinal obstruction 1, 2
• Obtain plain abdominal radiograph immediately as the first imaging study 1
• The dilated gas-filled rectum with proximal colonic dilation suggests a distal obstruction pattern 2, 4

If the infant has constipation without bilious vomiting:

• Hirschsprung disease is the leading concern, as it presents with failure to pass meconium in the first 48 hours of life, progressive abdominal distension, and constipation 2, 3
• Rectal biopsy is required for definitive diagnosis of Hirschsprung disease 2
• Contrast enema may show a transition zone between aganglionic distal bowel and dilated proximal colon 2, 4

Age-Specific Diagnostic Considerations

At 7 weeks of age, this infant falls into a critical window where several conditions overlap:

Hirschsprung Disease:

• Most common cause of lower intestinal obstruction presenting beyond the immediate neonatal period 2, 3
• Characterized by absence of ganglion cells in the distal colon, leading to functional obstruction 2, 4
• The dilated rectum filled with gas is somewhat atypical, as classic Hirschsprung typically shows a narrow rectum with proximal dilation, but short-segment disease can present variably 4, 5

Intussusception:

• Peak incidence is 5-9 months, but can occur as early as 3 months 6
• At 7 weeks, this is less common but not impossible 1, 6
• Classic presentation includes crampy intermittent abdominal pain (manifested as inconsolable crying or leg drawing), progression to bilious vomiting, and "currant jelly" bloody stools 1, 6
• Ultrasound is the diagnostic modality of choice if suspected 1, 6

Functional Constipation:

• Less likely at this age but possible 3
• Diagnosis of exclusion after ruling out anatomic and pathologic causes 3

Meconium Plug Syndrome:

• Typically presents in the first days of life, so less likely at 7 weeks 2, 4
• Usually resolves with observation or therapeutic enema 2

Critical Imaging Findings to Assess

On plain abdominal radiograph, evaluate for:

• Pattern of gas distribution throughout the bowel 1, 4
• Presence of air-fluid levels suggesting obstruction 1
• Caliber transition points between dilated and normal bowel 4, 5
• Free air indicating perforation (absolute contraindication to enema reduction if intussusception suspected) 6

The presence of gas throughout the rectum and colon suggests:

• Patent lumen without complete mechanical obstruction 4, 5
• Functional obstruction (Hirschsprung) or partial obstruction more likely than complete atresia 2, 4

Common Pitfalls and Caveats

Critical errors to avoid:

• Assuming functional constipation without excluding Hirschsprung disease, which can lead to life-threatening enterocolitis 2, 3
• Missing signs of perforation or peritonitis before attempting enema reduction for suspected intussusception 6
• Delaying surgical consultation when bilious vomiting is present, as conditions like midgut volvulus can cause bowel necrosis within hours 1, 2
• Confusing this presentation with pyloric stenosis, which presents with non-bilious projectile vomiting at 2 weeks to 3 months and would not cause colonic dilation 1

Key distinguishing features:

• Hirschsprung: failure to pass meconium in first 48 hours, chronic constipation, explosive stool passage after rectal examination 2, 3
• Intussusception: acute onset of intermittent colicky pain, lethargy, bloody stools 1, 6
• Meconium ileus (cystic fibrosis): microcolon with abnormal paucity of rectal meconium on imaging 7

Immediate Management Steps

Regardless of suspected etiology:

• Obtain plain abdominal radiograph immediately 1, 4
• Assess for signs of obstruction, perforation, or peritonitis 1, 6
• If bilious vomiting is present, initiate IV crystalloids, insert nasogastric tube, and obtain immediate surgical consultation 1, 6
• If Hirschsprung disease is suspected, arrange for contrast enema and rectal biopsy 2
• If intussusception is suspected based on clinical presentation, obtain ultrasound and prepare for possible enema reduction 1, 6

The specific next diagnostic step depends on the clinical presentation: if there is bilious vomiting, proceed urgently with upper GI series after initial radiograph to exclude malrotation; if there is constipation without bilious vomiting, proceed with contrast enema to evaluate for Hirschsprung disease 1, 2