Changes in the Physical Characteristics of the Eardrum Induced by Otitis Media with Effusion

Detecting changes in the physical characteristics of the eardrum by ocular inspection is difficult. Herein, we propose an alternative method for the quantitative detection of such changes using an acoustic conductance tympanogram, termed as G tympanograms, which exhibit the real part of acoustic admittance. 　The middle ear, including the eardrum, can be modeled using a mechano-acoustic system with physical parameters comprising mass, spring, and friction, and we have developed a procedure to numerically evaluate these parameters. We report the results obtained thus far and discuss their significance and implications. 　Specifically, we performed pure tone audiometry, 226-Hz tympanometry, and rigid endoscope otoscopy on 175 children aged 5-13 years. Of 350 total ears, we selected 248 ears with normal hearing levels, type A tympanogram, no effusion in the middle ear, non-retracted eardrums, and no eardrum calcification. We measured the distortion product of otoacoustic emissions (DPOAE) and performed four-frequency (i.e., 226, 678, 800 and 1000 Hz) tympanometry. From the latter analysis, we extracted G-tympanograms, which enabled us to calculate the above-mentioned parameters, the spring constant in particular, for each of the 248 selected ears. We classified these ears into three groups: group I (68 ears), which showed no evidence of acute otitis media (OMA) or otitis media with effusion (OME); group II (68 ears), which showed evidence of OMA but not of OME; and group III (112 ears), which showed evidence of OME and OMA. 　We found that ears in group III had reduced spring constants, even after apparently recovering from OME. We also found a strong correlation between the spring constant and OAE value, which suggests that eardrums with low OAE values tend to have reduced spring constants. Because DPOAE-measurement is much easier to perform than four-frequency tympanometry, we suggest that DPOAE is a practical means of detecting reduced spring constants.