Clinical Scenario:
A 3-year-old boy with Down syndrome is brought to the pediatric clinic with a history of severe constipation since birth. His parents report that he passed meconium more than 48 hours after delivery. He has required repeated enemas and laxatives since infancy. Over the past 2 days, he has developed progressive abdominal distension and bilious vomiting.
On examination:
- Weight is below the 5th percentile.
- Abdomen is markedly distended.
- The rectum is empty on digital rectal examination, followed by explosive passage of stool and flatus after withdrawal of the examining finger.
What is the most likely diagnosis?
A. Celiac disease
B. Chronic intestinal pseudo-obstruction
C. Functional constipation
D. Hirschsprung disease
E. Hypothyroidism
Correct answer & Explanation:
Correct Answer: D. Hirschsprung disease
Explanation:
This 3-year-old boy with Down syndrome has classic features suggestive of Hirschsprung disease (congenital aganglionic megacolon):
- Delayed passage of meconium (>48 hours after birth) → a major warning sign for Hirschsprung disease.
- Severe constipation since infancy requiring enemas and laxatives → suggests an underlying structural motility disorder rather than functional constipation.
- Abdominal distension and bilious vomiting → indicate possible intestinal obstruction due to fecal loading and proximal bowel dilatation.
- Empty rectum on digital examination followed by explosive stool and flatus after withdrawal of finger → the classic “blast sign” or explosive stool passage, caused by sudden release of accumulated feces and gas from the dilated proximal colon.
Association with Down syndrome:
Children with Down syndrome have a significantly increased risk of Hirschsprung disease due to abnormal migration of neural crest cells leading to absence of ganglion cells in the distal colon.
Why other options are incorrect:
A. Celiac disease
- Can cause abdominal distension, poor growth, and constipation in some children, but does not cause delayed meconium passage or explosive stool after rectal examination.
B. Chronic intestinal pseudo-obstruction
- Causes symptoms of bowel obstruction without a mechanical blockage, but typically presents with vomiting, distension, and feeding intolerance rather than delayed meconium with classic rectal findings.
C. Functional constipation
- Usually begins after infancy, often associated with stool withholding behavior, and children typically have a normal rectal examination with fecal impaction. Delayed meconium passage suggests an organic cause.
E. Hypothyroidism
- Can cause constipation and poor growth but does not explain delayed meconium passage and the characteristic explosive stool after rectal examination.
Exam Pearl:
Down syndrome + delayed meconium passage + chronic constipation + empty rectum with explosive stool on examination = Hirschsprung disease.
