Megaesophagus is a condition in humans and dogs where peristalsis fails to occur properly and the esophagus is enlarged. Normally, when the dog's esophagus is functioning properly, it acts as a muscle and pushes the food down the esophagus into the stomach. However, when a dog has megaesophagus, the esophagus stays enlarged and does not push the food down to the stomach. Therefore, the food fails to enter the stomach and often stays in the esophagus, and is eventually regurgitated.
In human pathology a condition known as achalasia may predispose a person to slowly develop megaesophagus. Achalasia is due to the loss of ganglion cells of the myenteric plexus. It occurs mostly in middle aged adults. There is a marked lack of contraction in the muscle contractions involving peristalsis with a constant contraction of the lower esophageal sphincter. Dilation of the esophagus results in difficulty swallowing. Retention of food bolus is also noted. Achalasia also predisposes to esophageal carcinoma.
Megaesophagus can also be a symptom of another disease in dogs called myasthenia gravis. Myasthenia gravis is a neuromuscular disease where the primary symptom is weakness in various body parts of the dog. However, when myasthenia gravis occurs in older dogs it is thought of as an immune-mediated disease. Often when myasthenia gravis is diagnosed in older dogs the first symptom the dog may manifest is megaesophagus.
Myasthenia gravis occurs when acetylcholine receptors (nicotinic acetylcholine receptors or AChRs) fail to function properly. Due to the inability of the nerve receptors to function appropriately, the muscles fail to have a stimulus that causes them to contract.
An important distinction in recognizing megaesophagus is the difference between when a dog regurgitates or vomits. Regurgitating is a common symptom of megaesophagus while vomiting is not necessarily associated with megaesophagus. When a dog regurgitates there is usually not as much effort involved as when a dog vomits. Often when regurgitating, the dog will tip its head down and the liquid and/or food will almost appear to "spill out" of its throat.
One of the primary dangers to a dog with megaesophagus is aspiration pneumonia. Because the food stays lodged in the throat, it can often be inhaled into the lungs causing aspiration pneumonia. One way to avoid this is to make sure that every time the dog eats or drinks anything, that the dog sits for at least 10 minutes afterwards or is held in a sitting up or begging position. This requires that all food and liquid intake be closely monitored and specifically administered to the dog in regular intervals (sometimes as often as 2-3 hours) in smaller quantities. Obviously, if smaller quantities of food and water are administered, one needs to take the necessary precaution to ensure the dog has a sufficient caloric and water intake. The prognosis for this condition is guarded.
An extensive list of breeds affected via known familial predisposition may be found at Genetic Predisposition