Impact of Posture on Integrated Residual Pressure and Subtype Classification in Achalasia

Background The Chicago Classification version 4.0 (CC ver4), updated in 2021, recommends evaluating esophageal motility with both supine and upright water swallows as part of high-resolution manometry (HRM). CC ver4 technical reviews showed suggestions about the diagnosis using either supine or upright positioning as the primary posture. However, while healthy individuals have been reported to show lower IRP in the upright position compared to the supine position, no such reports exist for achalasia patients. This study aimed to investigate differences in IRP and subtype classification between supine and upright positions in patients with achalasia. Methods We retrospectively analyzed 48 consecutive achalasia patients (mean age 53.0 ± 18.4 years, 22 males) diagnosed between 2012 and 2020 through upper gastrointestinal endoscopy, HRM, and barium esophagogram. HRM(ManoScan, Medtronic, Minneapolis, USA) was performed in both the supine position (10 swallows) and the upright position (at least 5 swallows). Based on CC ver4, an IRP median ≥ 15 mmHg in supine and ≥ 12 mmHg in upright was considered indicative of esophagogastric junction (EGJ) outflow disorder. Achalasia subtypes were classified as follows: type I (absent contractility), type II (≥20% pan-esophageal pressurization) and type III (≥20% with premature contractions). Results The median IRP was lower in upright (22.6 mmHg: IQR 18.9–29.7) than in supine (27.3 mmHg: IQR 22.3–33.9) (p < 0.001). IRP exceeded the upper limit in all 48 cases (100%) in supine and 47 cases (98%) in upright. Subtype classification varied by position: supine—type I: 13 (27%), type II: 29 (60%), type III: 6 (12.5%); upright—type I: 32 (67%), type II: 13 (27%), type III: 3 (6.3%), with more type I cases in upright. Subtypes matched in only 24 cases (50%), and 18 of 29 type II cases in supine shifted to type I in upright. Conclusion In patients with achalasia, IRP was lower in the upright position than in the supine position. Subtype classification varied depending on posture, suggesting that the primary posture used during HRM should be carefully considered when referring to subtype classification.