The fundamental pathology in HD is the absence of ganglion cells in the submucosal and intermuscular nerve plexuses and is associated with an increase in the nerve fibers in the affected segment .
That aganglionic segment usually involves the terminal intestine, i.e. the rectum or rectosigmoid. The aganglionic segment may, however, include the entire large bowel and even small bowel.
The gross pathologic feature of HD is a dilated proximal intestine with gradual or abrupt transition to normal calibrated distal intestine . The TZ is typically funnel like or cone shaped .
The colon proximal to the aganglionic segment, in an effort to overcome the partial obstruction, becomes distended and its wall markedly thickened because of muscle hypertrophy
The degree of hypertrophy and dilatation depends upon the duration and degree of obstruction and thus, indirectly to the age of the patient.