Nutcracker esophagus is characterized by a number of criteria described in the literature. The most commonly used criteria are the Castell criteria, named after American gastroenterologist D.O. Castell. The Castell criteria include one major criterion: a mean peristaltic amplitude in the distal esophagus of more than 180 mm Hg. The minor criterion is the presence of repetitive contractions (meaning two or more) that are greater than six seconds in duration. Castell also noted that the lower esophageal sphincter relaxes normally in nutcracker esophagus, but has an elevated pressure of greater than 40 mm Hg at baseline.