The aganglionic segment is encountered in which part of the colon in case of Hirschsprung disease -
Congenital aganglionic megacolon (Hirschsprung disease) 

Hirschsprung disease is a congenital disorder characterized by aganglionosis of a portion of the intestinal tract.
An intestinal segment lacks both Meissner submucosal and Auerbach myenteric plexus. This leads to func­tional obstruction and intestinal dilation Proximal to the affected segment.
Histological findings are:-


The absence of ganglion cells and ganglia in the muscle wall and submucosa of the affected segment.
Thickening and hypertrophy of nerve trunk.


The rectum is always affected with the involvement of more proximal colon to the variable extent most cases involve the rectum and sigmoid only.
Proximal to the aganglionic segment, the colon undergoes progressive dilation and hypertrophy.
With time, the proximal innervated colon may become massively distended megacolon.
Distal to dilated segment
In Whole colon
Proximal to dilated segment
In the dilated segment