Megacolon occurs for a number of reasons -- anything that can make it hard for the cat to pass stool long enough to lead to serious constipation. It also occurs for no apparent reason in cats. For whatever reason it occurs, the colon loses its muscular tone and enlarges. Stool size can increase to the diameter of the colon, making it hard for the cat to pass the resulting stools through the bony pelvic canal which does not increase in size. When this happens, the colon becomes impacted and serious secondary problems can occur. Cats are often unwilling to eat, may be depressed, might have a paradoxical liquid diarrhea which is passed in small quantities, vomiting may occur and sometimes there is pain associated with this. Unless the impaction is relieved the cat will eventually die. Many cats have chronic constipation that borders on impaction but never quite gets there. Many of them still have megacolon.
In our practice, we have had the best success in long term medical maintenance of affected cats using lactulose (Cephulax Rx, Chronulac Rx). Some cats will respond to increasing the fiber in their diet using psyllium products like Metamucil or Vetasyl. It is also possible to increase fiber in the diet using pumpkin pie filling. Surprisingly, many cats will eat pumpkin pie filling pretty readily. Cisapride (Propulsid Rx) has been recommended for this problem but we haven't had to use it much and don't have real feel for how well it works. Ducosate products have been the standard of treatment for a long time but we never had a lot of success with them. When medical treatment fails, or if it is too difficult to medicate a cat chronically, there is a surgical procedure for this condition known as a "sub-total colectomy". Nearly the entire colon is removed in this procedure. Despite the drastic sounding nature of that, the surgery is reported to work pretty well and not to cause chronic diarrhea or other problems that you might anticipate. We have one cat in our practice who has had this surgery and the owner is happy with the results.