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

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.

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.

A significant negative correlation was found between mdMEP and mastoid cavity volume (R = .467, p < .001). Ears with mdMEP >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 < .001). Patients with mastoid cavity volumes higher than the 75th percentile were significantly younger (p < .001). Multivariate regression analysis for mdMEP showed a good fit (R = .854) using factors including middle ear pressure, admittance and, most importantly, the reciprocal of mastoid volume (Beta = 0.752, p < .001).

The mdMEP, the main parameter of the modified nine-step test, was negatively correlated with the mastoid cavity volume. Therefore, the results of the modified nine-step test should be interpreted with consideration of mastoid cavity volume.