Estimation of Minor Conductive Hearing Loss in Humans Using Distortion Product Otoacoustic Emissions

Conductive hearing loss (CHL) systematically alters distortion product otoacoustic emission (DPOAE) levels through attenuation of both the primary tones and the evoked response by the middle ear, as well as through modification of the effective L1-L2 relationship within the cochlea. It has been postulated that, if optimal primary tone level relationships for an ear without CHL are known or can be estimated accurately and a CHL can be presumed to attenuate both primary tones to a similar extent, the adjustment to L1 required to restore an optimal L1-L2 separation following CHL induction can be utilized to estimate CHL magnitude objectively. The primary aim of this study was to assess the feasibility of objectively estimating experimentally produced CHL in humans by comparing CHL estimates resulting from DPOAE- and pure-tone audiometry-based methods. A secondary aim was to compare the accuracy of DPOAE-based CHL estimates when obtained using generic, as opposed to ear-specific, optimal primary tone level formula parameters.

For a single ear of 30 adults with normal hearing, auditory threshold for a 1 kHz tone was obtained using automated Békésy audiometry at an ear-canal pressure of 0 daPa, as well as at a negative pressure sufficient for increasing threshold by 3 to 10 dB. The difference in threshold for the ear-canal pressure conditions was defined as the pure-tone audiometry-based estimate of CHL (CHLPT). For the same two ear-canal pressures, optimal DPOAE primary tone level relationships were identified for f2 = 1 kHz. Specifically, for 20 ≤ L2 ≤ 70 dB SPL, L1 was varied 15 dB above and below the recommendation of L1 = 0.49 L2 + 41 (dB SPL). The difference between the optimal L1-L2 relationships for the two pressure conditions was defined as ΔL1OPT. A DPOAE-based estimate of CHL (CHLDP) was obtained using the formula CHLDP = ΔL1OPT/(1 - a), where a represents the slope of the optimal L1-L2 relationship observed in the absence of CHL.

A highly significant linear dependence was identified between pure-tone audiometry- and DPOAE-based estimates of CHL, r(19) = 0.71, p < 0.001. However, the correlation was only significant when ear-specific optimization formula parameters were known. Use of generic, frequency-nonspecific parameters resulted in significantly less accurate estimates than did either ear-specific (p < 0.001) or generic, frequency-specific parameters (p = 0.007).

This study provides empirical support for a theory of how CHL, through a combination of middle ear filtering and alteration of effective primary tone level relationships within the cochlea, systematically affects DPOAE amplitude. Although CHLDP was shown to be significantly predictive of CHLPT when optimization formula parameters for a given ear, both with and without mild CHL, were known, the lack of a meaningful relationship when using generic primary tone level formula parameters significantly limits the method's potential for clinical utility.