Achalasia affects only about 6,000 people in Britain who are at various stages of managing the condition, so it is rare. For some, it can be a long time before the condition is diagnosed, and can become confused with eating disorders, for instance. Achalasia prevents people from swallowing properly, in that the food does not pass from the oesophagus into the stomach, usually because the lower oesophageal sphincter (located by the diaphragm) is clamped tight shut rather than relaxing to allow food through.
The diagnostic tests are typically a) an endoscopy - a miniature camera passed down your throat - to check on whether there are any physical obstructions that account for the swallowing problems; b) a barium swallow test - where the patient swallows a white liquid whilst being x-rayed; and c) a manometry test that measures the pressure of the muscles at intervals down the length of the oesophagus.
The image is from a barium swallow test that shows how liquid builds up because it cannot progress through into the stomach. In time the extra pressure can make the oesophagus enlarged and baggy.