What could be the cause of a 1-month-old infant's inability to defecate (pass stool) for 3 days, despite passing flatus (gas), and what are the appropriate next steps?

Inability to Defecate for 3 Days in a 1-Month-Old Infant

In a 1-month-old infant who has not passed stool for 3 days but continues to pass flatus, the most likely diagnosis is functional constipation, but you must urgently rule out Hirschsprung disease and other anatomical obstructions before initiating treatment. 1

Immediate Assessment: Red Flags to Exclude

The presence of flatus is reassuring as it suggests the bowel is not completely obstructed, but several life-threatening conditions must be excluded immediately:

• Bilious vomiting is the most critical red flag—this indicates obstruction distal to the ampulla of Vater and constitutes a surgical emergency requiring immediate evaluation for malrotation with volvulus, which can cause intestinal necrosis within hours 2, 3
• Abdominal distension suggests possible mechanical obstruction or Hirschsprung disease 1, 4
• Failure to pass meconium within the first 48 hours of life is highly suggestive of Hirschsprung disease, occurring in 90% of cases 1
• Vomiting (even if non-bilious), poor feeding, or lethargy warrant urgent evaluation 5, 2
• Fever or signs of systemic illness could indicate enterocolitis, particularly in undiagnosed Hirschsprung disease 1

Differential Diagnosis

Most Likely: Functional Constipation

• Functional constipation can occur even in young infants and is characterized by infrequent, painful bowel movements with stool withholding behaviors 1, 6
• The passage of flatus without stool suggests the rectum may be full but the infant is retaining stool, which is consistent with functional constipation 1
• In infants under 6 months, constipation may be related to formula type or recent dietary changes 1, 7

Must Exclude: Hirschsprung Disease

• Hirschsprung disease (congenital aganglionic megacolon) is the most serious organic cause that must be ruled out in any infant with severe constipation 1
• Classic presentation includes delayed passage of meconium (>48 hours after birth), progressive abdominal distension, and bilious vomiting 1, 4
• Rectal examination may reveal an empty rectum despite palpable stool in the abdomen, and explosive passage of stool/gas after digital examination is suggestive 1
• Definitive diagnosis requires rectal biopsy showing absence of ganglion cells 2

Other Organic Causes to Consider

• Anorectal malformations are the most common congenital cause of intestinal obstruction (41% of cases) and should have been identified at birth, but subtle variants may be missed 4
• Hypothyroidism can present with constipation and should be screened if newborn screening was not performed 1
• Cystic fibrosis may present with meconium ileus or constipation in infancy 1
• Spinal cord abnormalities should be considered if there are associated neurological findings 1

Diagnostic Approach

History

• Timing of first meconium passage: Delayed beyond 48 hours strongly suggests Hirschsprung disease 1
• Stool frequency and consistency before this episode: Establish baseline pattern 1, 6
• Feeding history: Type of feeding (breast vs. formula), recent changes, and adequacy of intake 1, 7
• Presence of painful defecation or stool withholding behaviors: Screaming, arching back, or straining without producing stool suggests functional constipation 6
• Associated symptoms: Vomiting (bilious vs. non-bilious), abdominal distension, poor weight gain, or irritability 5, 1

Physical Examination

• Abdominal examination: Assess for distension, palpable stool in the left lower quadrant, and masses 1, 4
• Perianal inspection: Look for anal position, patency, and any anatomical abnormalities 1
• Digital rectal examination: Assess anal tone, rectal vault size, presence of stool in rectum, and explosive passage of stool/gas after examination (suggests Hirschsprung) 1
• Growth parameters: Poor weight gain is a red flag for organic disease 5, 1
• Neurological examination: Assess lower extremity tone and reflexes to exclude spinal cord pathology 1

Imaging and Testing

• Abdominal radiograph: Obtain if there is abdominal distension, vomiting, or concern for obstruction to assess bowel gas pattern and identify dilated loops 2, 3, 4
• Rectal biopsy: Required for definitive diagnosis of Hirschsprung disease if clinical suspicion is high 2, 1
• Contrast enema: May show transition zone in Hirschsprung disease but is not appropriate as initial evaluation for this presentation 3

Management

If Red Flags Present

• Immediate surgical consultation is required for any infant with bilious vomiting, significant abdominal distension, or high suspicion for Hirschsprung disease 2, 3
• NPO status and IV hydration should be initiated if obstruction is suspected 3

If Functional Constipation is Diagnosed (No Red Flags)

For Infants Under 6 Months

• Glycerin suppositories can provide immediate relief and are safe for young infants 1, 7
• Lactulose or lactitol are authorized and effective for infants under 6 months of age at appropriate dosing 7
• Polyethylene glycol (PEG) is authorized for infants over 6 months and is highly effective and well-tolerated 1, 7

Dietary Modifications

• Trial of withholding cow's milk: Cow's milk protein may promote constipation in some infants, so consider switching formula or maternal dietary modification if breastfeeding 1
• Adequate hydration: Ensure the infant is receiving sufficient fluids 1
• Note: Adding fiber is not appropriate for a 1-month-old infant who is exclusively milk-fed 1

Education and Follow-Up

• Parent education is crucial: Explain that functional constipation is common and treatable, but may require prolonged treatment (months to years) as relapse is common 1, 6
• Close follow-up: Monitor stool frequency, consistency, and growth parameters 1, 6
• Recovery rates: Infants under 2 years have significantly higher recovery rates (63% overall) compared to older children, but 37% may have persistent symptoms requiring continued treatment 6

Common Pitfalls

• Assuming all infant constipation is benign: Failure to exclude Hirschsprung disease can lead to life-threatening enterocolitis 1
• Delaying treatment: Chronic constipation can lead to psychosocial and digestive consequences, and early intervention improves outcomes 7, 6
• Insufficient dosing or duration: The rule for treatment is "sufficient dose for a long time"—underdosing or premature discontinuation leads to relapse 7, 6
• Overlooking dietary factors: Cow's milk protein intolerance may be contributing and warrants a trial of elimination 1
• Missing the transition to bilious vomiting: Any change in vomiting pattern to bilious emesis requires immediate re-evaluation for malrotation with volvulus, which can occur at any age 3, 8