Objective: Otoacoustic emissions (OAEs) are useful clinical tools that can be used as a test to reflect hearing, especially cochlear function. When OAEs are recorded, the sound energy is conducted inwards and outwards through the middle ear system, but the degree to which the middle ear condition affects the OAEs level remains obscure. The objectives of the present study were to estimate the effect of the condition of the middle ear expressed by multifrequency tympanometry on the distortion product otoacoustic emissions (DPOAEs) adjusting for age and the corresponding pure-tone thresholds.
Subjects and methods: The evaluation was conducted using 1,043 subjects without a history of ear disease or occupational noise exposure out of a population-based sample of 2,259 adults aged 40-82 years. Multifrequency tympanometry, DPOAEs, and pure-tone audiometry were administered. The resonance frequency (RF), static admittance (SA) and tympanometric peak pressure (PP) were taken as variables representing middle ear function. Subjects were categorized into three groups of variables < or = 5th percentile, between 5th and 95th percentile, and > or = 95th percentile. The signal-to-noise ratio in DPOAE levels were compared among three groups adjusting for age and the pure-tone threshold level.
Results: Both abnormally high and low RF exerted a negative effect on DPOAEs. Smaller RF values were related to smaller DPOAEs for the frequency around 1000 Hz, and greater RF values were related to smaller DPOAEs for the frequency around 4000 Hz. The tendency was similar between genders. Abnormally high SA had a negative influence on DPOAEs, while abnormally low SA had no significant effect on either gender. Smaller PP values were significantly related to smaller DPOAEs.
Conclusions: We interpreted the findings of the present study as verification of the effect of the condition of the middle ear on DPOAEs after adjustment for age and the pure-tone threshold level, although ears out of normal range in tympanometric variables might conceivably include not only middle ear dysfunction but also inner ear abnormality. Tympanometric assessment must always be taken into account when OAEs are analyzed for an estimation of the cochlea function.