Hirschsprung's disease is the congenital absence of enteric ganglion in the bowel wall. As these ganglion migrate from proximal to distal developmentally, the affected segment will usually extend contiguously from the area of onset to the dentate line.

A lower intestinal obstruction in the newborn suggests the differential diagnosis of small left colon syndrome (meconium plug syndrome), Hirschsprung disease, meconium ileus, and ileal atresia (or rarely colonic atresia). After the newborn period, particularly in a patient who has been passing stool, meconium plug, meconium ileus, and atresia become much less likely.

In the newborn, a water-soluble contrast enema should be performed in this setting, as a hypertonic contrast medium will be theraputic, as well as diagnostic, in the case of small left colon syndrome or meconium ileus. In the investigation of chronic constipation, including suspected Hirschsprung's disease, cathartics and cleansing enemas are not given prior to the barium enema, and contrast media are allowed to fill only the rectum and descending portion of the colon. Once a transition zone is identified fluoroscopically, the examination is discontinued in order to avoid overfilling the colon.

A contrast enema in patients with meconium ileus and ileal atresia will show a microcolon. With meconium ileus, contrast may eventually outline the dilated, meconium-packed distal ileum. In small left colon syndrome, contrast will usually outline meconium packed in the dilated colon, proximal to the obstruction,usually at the level of the transverse colon. The usual form of Hirschsprung's disease is manifested by a transitional zone in the recto-sigmoid area, with dilated colon proximal to this. Newborn infants may not show the characteristic transition zone. In such cases, failure of the infants to evacuate the enema after 24 hours is an indication for rectal biopsy. Long segment Hirschsprung's disease is very uncommon, but is similar in appearance to the small left colon syndrome, although Hirschsprung's disease should be suspected if one observes segmental areas of contraction in the long aganglionic segment.