Achalasia is a rare disorder that affects the esophagus, the muscular tube that connects the throat to the stomach. It is characterized by the inability of the lower esophageal sphincter (LES) to relax and allow food to pass into the stomach, leading to difficulty swallowing, regurgitation, and chest pain. The exact cause of achalasia is unknown, but it is believed to be related to a malfunction of the nerves that control the muscles of the esophagus.