In a neonate with delayed meconium or distal obstruction, the contrast enema finds the transition zone and the rectosigmoid ratio — the diagnosis is made on rectal biopsy, but the enema tells the surgeon where the aganglionic segment ends.
Orient first
- Aganglionosis starts at the anus and extends proximally; the aganglionic segment is narrow and the normal bowel above it dilates — the transition zone is between them.
- The rectum is normally the widest part of the distal colon (rectosigmoid ratio above 1); a ratio below 1 supports Hirschsprung disease.
- Differential in a neonate with distal obstruction: meconium plug / small left colon, meconium ileus (cystic fibrosis), ileal or colonic atresia, anorectal malformation.
Acquire the study
- Water-soluble contrast enema, early lateral projections of the rectum, then frontal views; a 24-hour delayed radiograph if the diagnosis is uncertain.
The manoeuvre
- Early lateral projection during filling: rectal calibre against the sigmoid (rectosigmoid ratio).
- Transition zone: a cone or abrupt change from narrow distal to dilated proximal bowel — note its level.
- Irregular, saw-tooth contractions of the aganglionic segment.
- Delayed radiograph at 24 hours: retained contrast supports the diagnosis.
- Total colonic disease: a short, featureless colon (microcolon or normal-calibre colon) with no transition.
What confirms it
- A transition zone with a reversed rectosigmoid ratio — to be confirmed by rectal suction biopsy.
What licenses you to exclude it
- A normal enema does not exclude Hirschsprung disease (ultrashort or total colonic forms) — biopsy decides.
The classic misread
- A rectal examination or washout before the enema can hide the transition zone.