As the name implies, Megaesophagus is the enlargement of the esophagus. While I cannot speak to how common this disease is per se, I can attest to the fact that I have encountered enough people who have had dogs, or known dogs, affected by this condition.
"Peristalsis" is the term used to describe the motion in our digestive track that moves food along. In megaesophagus patients, this motion isn't there. The esophagus becomes enlarged, and food cannot effectively navigate its way into the stomach. As a result, (and thusly a symptom), the dog regurgitates his food. Regurgitation and vomiting are not the same. A dog regurgitates when the food hasn't reached the stomach, and vomit implies it has.
There is a congenital form, as well as an acquired form. If congenital, you will often realize the condition to be present when your pup starts to wean. That's when they have moved on to trying to consume more solid foods. Great Danes, German Shepherds, Irish Setters, Newfoundlands, Shar-Peis, and Greyhounds are on the list of potentially being predisposed to the congenital form.
Aquired Megaesophagus can occur at any age. I have read that potential diseases linked to this onset can include distemper, tick paralysis, lead poisoning, and other neuromuscular issues. Like many diseases, the onset may not have any apparent causes.
When our dogs are afflicted, we tend to try to feed them in an elevated position, to allow the food an easier passage into the stomach. I have heard of an invention called The Bailey Chair, which was developed by a pet owner whose dog suffered from this condition. The elevated position asks gravity to help the food we give our pups move "south" easier. I have not owned a dog with this condition, so I cannot speak to the effectiveness of this device, but here is a link:
I book I have (Common Diseases of Companion Animals, Alleice Summers) also sites that sometimes liquid diets may be effective, as the liquid is more easily passed through the throat. The author also sites that other studies find small, "meatballs" of canned food can simulate a small degree of natural peristalsis. Ultimately, the owner wants to decrease regurgitation and prevent any further enlargement of the esophagus. Smaller food portions, more frequently through the day, is the bottom line protocol.
The main symptom is regurgitation, but because we cannot always "catch" our dogs doing some things, we may notice lack of growth or weight loss. Due to the regurgitation, we fear our pet can breathe in some of those fluids (aspiration) and thusly contract pneumonia. Other symptoms are coughing, difficulty breathing (dyspnea) and drooling.
Your veterinarian can diagnose this condition with radiographs. The picture below shows a megaesophagus, made visible by the use of a contrast agent given orally to the dog before the x-ray was taken. While there is no way to cure this condition, it is very treatable! Elevated feeding, high caloric food given in a soft form, and feeding several small meals throughout the day makes this disease manageable.
Remember that while many of us get our dogs while young and seemingly impervious to health issues, everyone grows older one day. If it's not a condition like this, it's potentially another condition where we as owners are called upon to give our dogs the extra care they so deserve. If considering adopting, never let a potential special need deter you. Speak to your vet. Every dog is different, and I can't express this enough! Before adopting a dog of any age, always remind yourself that the time our dogs need from us may change throughout their lives, and we need to always be ready to give them the extra care and time they have earned in order to maintain their quality of living! We see owners surrender dogs once their health isn't perfect, yet this needs to be a consideration made before you adopt. Pets are forever!