Interpretation of the nine‐step test for Eustachian tube function should consider mastoid cavity volume

Introduction The modified nine‐step test is a classical method for evaluating Eustachian tube function. However, clinical interpretation of the increased maximal difference in middle ear pressure (mdMEP) in the modified nine‐step test is unknown. We hypothesised that the different reservoir effects of the mastoid cavity can bias the results of the modified nine‐step test. Methods A total of 108 consecutive participants (216 ears) were retrospectively screened. Of these, 55 participants (82 ears) who met the inclusion/exclusion criteria were enrolled. The volumetric results of the mastoid cavity, parameters of the modified nine‐step test (mdMEP, middle ear pressure, tympanic membrane compliance), and demographic data were analysed. Results A significant negative correlation was found between mdMEP and mastoid cavity volume (R = .467, p 70 daPa showed poor pneumatization in the mastoid cavity, with volumes less than 3000 mm3 (10th percentile of all ears analysed). Ears with mastoid cavity volumes lower than the 25th percentile showed a significantly higher mdMEP (p