Changes in distortion product otoacoustic emissions from ears with Menière's disease

To investigate changes in distortion product otoacoustic emissions (DPOAEs) associated with hearing improvement in ears with Menière's disease, four parameters were studied. These were: DPOAE amplitudes for a moderate (L1/L2 = 65/55 dBSPL) and a high (L1/L2 = 74/64 dBSPL) primary level (65/55-DP and 74/64-DP amplitudes, respectively); maximum level of DPOAE amplitudes in the growth function (maximum DP level); and DPOAE detection threshold (DP threshold) of the growth function. Significant correlations were found between all four parameters and hearing threshold, except for the 65/55-DP amplitude at 1 kHz, and each parameter appeared to reflect the hearing threshold to some extent. However, the changes in the four parameters associated with hearing improvement were somewhat complicated. Changes could not be evaluated due to lack of detection of responses when the hearing thresholds exceeded 50 dBHL. Findings that parameters became detectable, the DP amplitude or the maximum DP level increased significantly, or the DP threshold lowered significantly were classified as "improved" findings. The number of ears with "improved" findings was larger for the maximum DP level or the DP threshold than for the 65/55-DP and 74/64-DP amplitudes in ears with a 10 dB or greater improvement in hearing threshold at the corresponding F2 frequency. Furthermore, a significant decrease in amplitude was found in some ears, although the hearing threshold was not elevated when the 65/55- or 74/64-DP amplitude was used as a parameter. On the other hand, "improved" findings for the maximum DP level or the DP threshold were observed for a relatively large number of ears without a significant change in hearing threshold, and it was thought that small changes in cochlear function which did not lead to changes in hearing threshold were detected by changes in DPOAEs. These findings suggest that parameters determined from the growth function, such as maximum DP level and DP threshold are better indicators for monitoring cochlear function than is DPOAE amplitude for a single primary level, and demonstrate the usefulness of the growth function measurement for monitoring purposes. Furthermore, evaluation by changes in the overall growth function using combined parameters appeared to increase sensitivity in detection of subtle changes in cochlear function. In conclusion, DPOAE measurement can be a useful examination for clinical monitoring of cochlear function in Menière's disease when the growth function is measured.