Achalasia is a disease of the myenteric plexus of the esophagus where the peristalsis is decreased or absent and the lower esophageal sphincter fails to relax.

Primary or idiopathic achalasia usually has its onset in early adulthood and results in dysphagia, regurgitation, and other complications of esophageal stasis including bad breath, aspiration pneumonia, and esophageal candidiasis. In long standing achalasia, there is an increased incidence of squamous cell carcinoma (5-10%), likely secondary to chronic mucosal irritation from retained food.

Secondary achalasia usually results from malignant infiltration of the myenteric plexus of the distal esophagus and can occur from invasive gastric carcinoma, lymphoma or metastatic disease. This usually has a more abrupt onset later in life.
Chagas disease can also mimic achalasia, but is not common in the US. It is frequently seen in Brazil and tropical areas and is caused by a protozoa (Trypanosoma cruzi) which is carried by the reduviid bug.

Stricture of the distal esophagus (e.g. from reflux or tumor) can also produce similar findings, but an important difference is that a stricture is an anatomic narrowing whereas achalasia is a functional narrowing.