gms | German Medical Science

Distortion product otoacoustic emissions (DPOAEs) allow for the objective assessment of peripheral auditory system function to the level of the outer hair cells and represent an indispensable part of the modern audiological test battery. Low-cost, DPOAE-based newborn hearing screening programs, for example, have been implemented in countries around the world to great effect. Unfortunately, the detectability, and therefore clinical utility, of DPOAEs has been reported to be adversely affected by even mild conductive hearing loss (CHL). The primary aims of this study were 1) to simulate the impact on DPOAE detectability of various degrees of CHL and 2) to assess the feasibility of objectively estimating CHL magnitude based upon observed DPOAE levels.

To evaluate the impact of CHL on DPOAE detectability, CHL was applied numerically to DPOAE results obtained within 57 normal-hearing ears (f₂=1–6 kHz, L₂=55 dB SPL, f₂/f₁=1.22). Specifically, CHL of 5, 10, and 15 dB were simulated through identical level reductions of both the stimulating primary tones, as well as the evoked DPOAEs. To assess the feasibility of objectively estimating CHL using DPOAEs, CHL ranging from 3–10 dB was experimentally-induced within 30 normal hearing ears through variation of ear canal air pressure. Accuracy of DPOAE-based CHL estimates (CHL_DP) was assessed via comparison with audiometry-based estimates (CHL_PT).

The influence of CHL on DPOAE level (L_DP) is presented in Figure 1 [Fig. 1]. Across frequency, the proportion of DPOAEs with SNR >6 dB in the presence of 5 dB CHL was very high (mean=99%). The presence of 10 dB CHL, however, was sufficient to statistically significantly reduce the proportion of valid DPOAE measurements. Specifically, a reduction in valid measurements of over 62% was identified when comparing the 0 and 10 dB CHL conditions, while a reduction of 91% was identified when comparing the 0 and 15 dB CHL conditions.

In Figure 2 [Fig. 2], parameter values from three optimization formulas are compared in terms of their utility in estimating CHL. The difference between the given optimization formula and the optimal L₁-L₂ relationship observed for the CHL condition was defined as ΔL_1OPT. A DPOAE-based estimate of CHL (CHL_DP) was obtained using the formula CHL_DP = ΔL_1OPT/(1–a), where a represents the slope of the optimal L₁-L₂ relationship observed in the absence of CHL. In terms of objective CHL estimation, a highly significant relationship was identified between pure tone audiometry- and DPOAE-based estimates of CHL, r(19)=0.71, p<0.001, but only when when optimization formula parameters for a given ear, both with and without mild CHL, were known.

The present results illustrate the significant manner in which even relatively mild CHL effects DPOAE levels. Indeed, CHL traditionally considered not to be of clinical significance (<10 dB) can meaningfully reduce the detectability, and therefore clinical utility, of DPOAEs. Results of the present work also provide support for the feasibility of exploiting the systematic relationship between CHL and DPOAE level to objectively estimate CHL in humans. Taken together, these findings suggest further research efforts investigating the interplay between otoacoustic emissions and middle ear sound transmission could further increase the diagnostic utility of DPOAEs.

CHL significantly affects the level and subsequent detectability of DPOAEs. The systematic nature of this relationship suggests potential for a DPOAE-based quantification of conductive hearing loss.