massive enlargement and dilation of the large intestine (colon). The two main types of the syndrome are congenital megacolon, or Hirschsprung disease, and acquired megacolon. In congenital megacolon, the lowermost portion of the large intestine is congenitally lacking in normal nerve fibres; thus, peristalsis, or involuntary contractions, of the muscles of this part of the intestine cannot occur, and the bowel's contents are not pushed onward. The area of normal intestine above the abnormal part works harder to push on the fecal contents, with the result that the muscular walls of that part of the intestine become enlarged and thickened. The entire colon eventually becomes thick and distended. The symptoms of congenital megacolon include a distended abdomen and severe constipation in infants born with the disorder.

Acquired megacolon in children (childhood disease and disorder) characteristically results from a combination of faulty toilet training and mental or emotional disturbances that result in a child's refusing to try to defecate. The administration of increasingly larger doses of laxatives fails to solve the problem permanently, and over time the child's rectum becomes filled with impacted feces and the colon becomes progressively enlarged. Once the distended bowel has been emptied, the treatment for this disorder is primarily psychiatric and involves persuading the child to accept bowel training.

Megacolon in adults can result from taking certain medications, abnormal thyroid (thyroid gland) function, diabetes mellitus, scleroderma, or amyloidosis. Various surgical procedures are used to correct the condition.