Evoked otoacoustic emissions as a predictive factor of hearing loss -

thesis presented for degree of Doctor of Philosophy at Macquarie University, 1999

Narelle M. Murray

Abstract

A longitudinal investigation was undertaken to examine the feasibility of using click-evoked otoacoustic emissions as a factor in predicting hearing loss. Two cohorts were studied: (1) 33 subjects working in a non-noisy environment and (2) 74 mildly noise-exposed orchestral instrumentalists. Both cohorts were tracked for up to six years with transiently evoked otoacoustic emissions and by pure tone audiometry. Results were analysed for both cohorts on a group basis and on an individual basis based on broadband and filtered (above and below 2000 Hz) otoacoustic emission parameters of Waverepro% and Coherent Emission Strength, and for pure tone audiometric thresholds averaged for frequencies 0.5, 1,2 kHz, 1,2,4 kHz and 6 kHz, deemed to be the most appropriate for comparison purposes. Two notable findings were: 1) both otoacoustic emission strength and pure tone audiometric thresholds for groups are seen to improve over time. This was also observable in some individuals; 2) a "see-saw" (teeter-totter) effect in both otoacoustic emissions and pure tone thresholds is seen in many individuals. Significant declines as well as improvements occurred in all parameters of both otoacoustic emissions and pure tone thresholds, sometimes occurring in only one parameter, sometimes in combination. Tests of sensitivity and specificity did not point to any one parameter of otoacoustic emissions or any frequency combination being a decisively sensitive indicator of hearing loss; high specificity was evident in all cases. However, more ears showed low Waverepro% and/or low Coheren Emission Strength than poor pure tone thresholds. Comparisons of changes over time between otoacoustic emission parameters and pure tone threshold frequency combinations indicate the empirically derived Coherent Emission Strength otoacoustic emission parameter identifies more ears which appear to be at risk of hearing loss (defined as ears which have low or significantly declining emission strength and pure tone thresholds within normal limits or unchanged). From this investigation Coherent Emission Strength appears to be a sound factor in predicting hearing loss.

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