Response thresholds in electrocochleography and their relation to the pure tone audiogram.

OBJECTIVE: The relations between response thresholds obtained by 1) transtympanic (TT) and 2) extratympanic (ET) electrocochleography, and by 3) pure tone audiometry, were investigated in a population of patients with cochlear hearing loss of various types and degrees, with the prediction of audiometric from electrocochleographic thresholds as the primary focus.
DESIGN: Analyses were made of the relation between TT response thresholds for tone bursts with octave frequencies from 500 to 8000 Hz and audiometric thresholds in 148 ears. Similar analyses of ET thresholds were given for a subset of 30 ears in which TT and ET physiological responses were simultaneously recorded.
RESULTS: TT electrocochleographic thresholds are highly correlated with audiometric thresholds. Linear regression analysis shows that audiometric thresholds can be predicted from physiological thresholds with an error in the estimate of 11 dB. ET electrocochleography permits similar predictions but with a larger uncertainty of 16 dB. Both methods demonstrate a clear frequency specificity. As a remarkable secondary observation, it appears that electrocochleographic thresholds increase slightly less with increasing cochlear dysfunction than do pure tone thresholds. The hypothesis is elaborated that this is due to the different stimulus durations on which the two threshold measurements are based in interaction with the difference in temporal integration between normal and pathological ears.
CONCLUSION: Electrocochleography is a powerful method for the objective and frequency-specific determination of cochlear response thresholds. TT recording is the preferred method, but ET recording is suitable with a larger uncertainty in the estimate. Electrocochleographic thresholds directly measure the cochlear sensitivity as such, whereas pure tone thresholds measure the combination of loss of cochlear sensitivity and of reduced temporal integration.