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What is Megaesophagus and explain in detail?
Megaesophagus, also known as esophageal dilatation, is a disorder of the esophagus in humans and other mammals, whereby the esophagus becomes abnormally enlarged. Megaesophagus may be caused by any disease which causes the muscles of the esophagus to fail to properly propel food and liquid from the mouth into the stomach (that is, a failure of peristalsis). Food can become lodged in the flaccid esophagus, where it may decay, be regurgitated, or maybe inhaled into the lungs (leading to aspiration pneumonia).

Humans
Megaesophagus may occur secondary to diseases such as achalasia or Chagas disease. Achalasia is caused by a loss of ganglion cells in the myenteric plexus. There is a marked lack of contraction within the muscles involved in 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.

Other animals
Dogs
Megaesophagus can also be a symptom of the disease 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) fail to function properly, so that the muscle is not stimulated to contract. There is an invention known as the "bailey chair" that uses the force of gravity to push down liquids and food into the dogs stomach. Usually dogs are known to understand when its time to eat in their bailey chair, and this helps prevent issues