Monitoring with click-evoked otoacoustic emissions

Narelle M. Murray and Eric L. LePage

Presented at Audiological Society of Australia Conference, Adelaide, June, 2000

Abstract

Two Cohorts, one non-noise-exposed, the other moderately noise-exposed, have been followed for several years in an endeavour to establish the changes occurring in outer hair cell functionality over time. This paper focuses on the patterns of changes occurring in the non-noise-exposed population and the suitability of click-evoked otoacoustic emissions for monitoring ear damage. Over a period of seven years 33 subjects were tested on more than one occasion with both pure tone audiometry (PTA) and click evoked otoacoustic emissions (OAEs). Air conduction PTA was carried out using a Madsen OB822 audiometer with standard TDH49 earphones set in MX/41AR cushions; frequencies tested were from 0.5 to 6kHz. An Otodynamics Otoacoustic ILO88 Analyser set at a stimulus screening level of 80 (±1.5) dB SPL with 260 repetitions of the pulsed click train was used for otoacoustic emission testing. All testing was carried out in a sound treated booth. Results of both types of testing will be presented. OAE results are described in terms of the Otodynamics Analyser parameter "WAVEREPRO%" and the empirically derived parameter Coherent Emission Strength (CES) and in turn compared with PTA results of mean hearing levels at 1,2,4 kHz (mean), 05,1,2 kHz (mean) and 6kHz. Any OAE changes over time are considered significant indicators of cochlear ageing if they exceed test/retest variations (Murray, LePage & Tran, 1997; Murray, PhD Thesis, 1999), while PTA variations of $15 dB (Aust/NZ Standard 1269) are considered significant. Results indicate that on a group basis that there are no statistically significant changes over time. However, there is a noticeable upward trend in results and this is discussed in terms of expectations and widely variant observations occurring in a nonlinear system. Consideration is given to the application of CES as the preferable parameter to identify individual changes over time.

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