This is a condition in which the esophageal muscles lose their strength
and the esophagus dilates to a much larger size. Usually dogs with this
condition regurgitate food as well as water but I have seen at least one
other dog that started out just regurgitating when it drank water. Sometimes
the enlarged esophagus is visible on plain X-rays of the chest but often
it is necessary to use some kind of contrast medium like barium to make
the enlargement visible.
There are other possible problems like esophageal diverticuli, hiatal hernia, esophagitis, pylorospasm, esophageal tumors and probably others. In addition, there are about thirty disorders that can cause a secondary megaesophagus problem.