Measurement of conductive hearing loss in mice

In order to discriminate conductive hearing loss from sensorineural impairment, quantitative measurements were used to evaluate the effect of artificial conductive pathology on distortion-product otoacoustic emissions (DPOAEs), auditory brainstem responses (ABRs) and laser-Doppler vibrometry (LDV) in mice. The conductive manipulations were created by perforating the pars flaccida of the tympanic membrane, filling or partially filling the middle-ear cavity with saline, fixing the ossicular chain, and interrupting the incudo-stapedial joint. In the saline-filled and ossicular-fixation groups, averaged DPOAE thresholds increased relative to the control state by 20-36 and 25-39 dB, respectively with the largest threshold shifts occurring at frequencies less than 20 kHz, while averaged ABR thresholds increased 12-19 and 12-25 dB, respectively without the predominant low-frequency effect. Both DPOAE and ABR thresholds were elevated by less than 10 dB in the half-filled saline condition; no significant change was observed after pars flaccida perforation. Conductive pathology generally produced a change in DPOAE threshold in dB that was 1.5-2.5 times larger than the ABR threshold change at frequencies less than 30 kHz; the changes in the two thresholds were nearly equal at the highest frequencies. While mild conductive pathology (ABR threshold shifts of <10 dB) produced parallel shifts in DPOAE growth with level functions, manipulations that produced larger conductive hearing losses (ABR threshold shifts >10 dB) were associated with significant deceases in DPOAE growth rate. Our LDV measurements are consistent with others and suggest that measurements of umbo velocity are not an accurate indicator of conductive hearing loss produced by ossicular lesions in mice.