Expansion
Absent ganglion cells starting at the rectum and extending proximally, with the narrow segment being the abnormal one
Expansion
Neural crest cells migrate craniocaudally along the gut during weeks 5 to 12. If migration arrests, the distal bowel is left aganglionic, lacking both the myenteric and submucosal plexuses.
Key anatomical points:
- Always involves the rectum and extends proximally - it never skips
- The aganglionic segment is narrow and contracted, because it cannot relax
- The proximal dilated bowel is normal, distended behind the functional obstruction
- The transition zone lies between them
About 75 per cent are confined to the rectosigmoid.
Presentation is failure to pass meconium within 48 hours, abdominal distension and bilious vomiting. Diagnosis requires rectal suction biopsy taken above the dentate line, since a normal hypoganglionic zone exists just above it. Associations include Down syndrome.