Evaluation of cochlear hearing disorders: normative distortion product otoacoustic emission measurements

The overall objective was to begin the investigation, in humans, of distortion product otoacoustic emission measurements, which are intended to be part of a diagnostic protocol being developed. This protocol, designed to distinguish among different cochlear hearing disorders, has been tested to date only through lesion studies in the gerbil (Mills, Ear and Hearing, 27, 508-525, 2006). To be applied successfully to human subjects, it was required that parameters and procedures for emission measurements be found, which resulted in sufficiently small intersubject variability in normal subjects, among other requirements. To attain these objectives, measurements of particular otoacoustic emission responses were made in a reference group of young adults having excellent hearing.

Twenty young adults (age 18 to 24 yr; 40 ears) comprised the subject group, with equal numbers of men and women. Inclusion criteria included hearing thresholds of 10 dB HL or better in both ears at all frequencies (0.5, 1, 2, 3, 4, 6, and 8 kHz), plus a tympanometric peak response located within +/-30 daPa of ambient pressure in both ears. The otoacoustic emission stimulus consisted of two tones (frequencies f1 and f2) varied in level using 5-dB steps with the lower-frequency stimulus level always 10 dB greater than that of the higher-frequency stimulus. The emission isoresponse threshold was defined to be the stimulus level required to obtain an emission amplitude of -10 dB SPL. Another potential measure was defined to be the emission amplitude at the highest stimulus levels routinely tested. Mean emission amplitudes and thresholds were determined for f2 frequencies equal to audiometric frequencies from 1 to 8 kHz, using two different stimulus frequency ratios, f2/f1=1.21 and 1.28.

One result of the study was the derivation of reference standards for the emission threshold level, similar to the HL scale for pure-tone thresholds. For use in diagnosis, the optimal measure was found to be the otoacoustic emission threshold for the stimulus frequency ratio f2/f1=1.21 and for f2 frequencies from 1 to 6 kHz. The f2 frequency of 8 kHz seemed less useful because the emission had a relatively high mean threshold. For frequencies 1 to 6 kHz, the variances were adequately small: 95% of the emission thresholds fell within +/-13 dB of the mean at each frequency, a variability only slightly larger than that for the gerbil. Finally, even within the 10-dB HL limit, responses showed a slight trend for increased emission thresholds with increased auditory threshold. Only at 8 kHz was the amount of covariance important, however, with the relationship between emission and auditory thresholds strongest for men.

Emission reference standards can be developed by testing a group of young adults with excellent hearing. The diagnostic procedure previously proposed on the basis of gerbil lesion studies may be adapted with relatively little modification for use in human subjects. However, validity of the test and specific numerical results for human subjects remain to be firmly established for the purpose of distinguishing among different cochlear disorders.