Gold standard Investigation for Achalasia cardia is :

In long-standing achalasia, the esophagus may assume a sigmoid configuration. The diagnostic criteria for achalasia with esophageal manometry are impaired LES relaxation and absent peristalsis. High-resolution manometry has somewhat advanced this diagnosis; three subtypes of achalasia are differentiated based on the pattern of pressurization in the non peristaltic esophagus. Because manometry identifies early disease before esophageal dilatation and food retention, it is the most sensitive diagnostic test