Hirschsprung disease is a developmental anomaly affecting the intestinal tract and is due to a developmental anomaly of the enteric nervous system. It is characterized by the absence of ganglial cells that are responsible for the intrinsic innervation of muscle layers in the terminal part of the intestine. It leads to occlusion in the lower part of the intestine or severe contispation. There is abnormally large or dilated colon (Congenital MEGACOLON) due to congenital absence of myenteric ganglion cells (aganglionosis) in a distal segment of the large bowel; resultant loss of motor function in this segment causes massive hypertrophic dilatation of the normal proximal colon; condition appears soon after birth; called also Hirschsprung's disease, aganglionic megacolon and pelvirectal achalasia. In most cases, the aganglionic segment is within the RECTUM and SIGMOID COLON.