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|Year : 1999 | Volume
| Issue : 2 | Page : 50--57
Comparing changes in transient-evoked otoacoustic emission and pure-tone audiometry following short exposure to industrial noise
Mariola Sliwinska-Kowalska, Piotr Kotylo, Beata Hendler
Department of Physical Hazards, Institute of Occupational Medicine, Lódz, Poland
Department of Physical Hazards, Institute of Occupational Medicine, ul. Sw. Teresy 8 90-950 Lódz
It has been suggested that otoacoustic emissions, particularly transient-evoked otoacoustic emissions (TEOAE), might be more sensitive in assessment of changes to the cochlea caused by noise than pure-tone audiometry (PTA). The aim of the study was to compare temporary threshold shifts with the changes in TEOAE following a six-hour exposure to industrial noise at the intensity of 85-97 dB (A). Thirty two male employees of a metal factory were screened. TEOAE, PTA and tympanometry were included in the hearing test battery. Both, PTA and TEOAE showed significant reduction due to noise exposure, but no correlation between temporary threshold shifts and decreases in either the overall TOAE level or the level of otoacoustic emission in the frequency bands was found. Our results confirm the high sensitivity of TEOAE to short exposure to industrial noise. This study may recommend this measurement as a method of evaluation for TTS conditions for hearing conservation programme purposes, in addition to pure-tone audiometry.
|How to cite this article:|
Sliwinska-Kowalska M, Kotylo P, Hendler B. Comparing changes in transient-evoked otoacoustic emission and pure-tone audiometry following short exposure to industrial noise.Noise Health 1999;1:50-57
|How to cite this URL:|
Sliwinska-Kowalska M, Kotylo P, Hendler B. Comparing changes in transient-evoked otoacoustic emission and pure-tone audiometry following short exposure to industrial noise. Noise Health [serial online] 1999 [cited 2020 Sep 24 ];1:50-57
Available from: http://www.noiseandhealth.org/text.asp?1999/1/2/50/31705
Although the damaging effects of noise to the inner ear are well understood, the hearing conservation programmes still need to be improved. The hearing loss caused by prolonged exposures to a high level of industrial noise is irreversible, so the most important method of prevention is early diagnosis of changes to the cochlea due to such exposures. The most suitable tool for this purpose seems to be otoacoustic emissions (OAEs), which are strictly related to the function of outer hair cells, known to be the most vulnerable to acoustic trauma. It has been shown that short exposures to noise result in the decrease of OAE amplitude, in the alteration of the frequency composition of otoacoustic emissions and in the increase in their threshold (Rossi et al., 1991, Avan et al., 1993). Recently, it has also been suggested that, as compared to pure-tone audiometry, transientevoked otoacoustic emissions (TEOAEs) may be more sensitive in assessment of changes to the cochlea, either temporary or permanent, caused by noise or other ototoxic agents (Hotz et al., 1993, Kvaerner et al., 1995, Attias and Bresloff, 1996, Liebel et al., 1996, Engdahl et al., 1996, Ozturan et al., 1996). Attias and Bresloff (1996) showed that after ten minutes of exposure to white noise at the level of 90 dBSL, a positive significant correlation was obtained between the TEOAE shifts and temporary threshold shifts (TTSs). A significant decrease in TEOAE response was observed particularly in the higher frequencies, confirming the effects of noise to the cochlea. Moreover, TEOAE shifts in some subjects were not accompanied by TTS, suggesting that otoacoustic emissions are more sensitive in evaluation of temporary changes to the cochlea after exposure to noise. Kvaerner et al., studying the effects of seven hour exposure to industrial noise at the level of 85-90 dB, confirmed a significant reduction in mean TEOAEs amplitude after exposure. However, they were not able to show any correlation between the changes in TEOAEs and TTSs. Also, no significant post-exposure reduction in otoacoustic emissions in the high frequency band (3-6 kHz), typical for exposure to noise, was shown. This raises the question of whether TEOAEs are really a more sensitive method of TTS evaluation. The weak side of the latter study was to measure TEOAEs after the onset of exposure (within the first 40 minutes of the work shift), which may have influenced pre-exposure recordings, and also to include a small number of subjects. In the present study, the TEOAEs and pure-tone audiometry were performed before and immediately after the work-shift, in a larger group of employees. The aim of the study was to assess the value of TEOAEs in evaluating temporary changes to the cochlea, after such exposures, for hearing conservation programme purposes.
Material and Methods
The study included 32 paid, highly-cooperative male, metal-factory workers, ages 19 to 32 (mean 26.7±4.3 1 s.d.), with the length of employment from 2 months to 2 years. They were in good health, had no histories of ear disease and their ENT examinations were normal. All ears had type A tympanogram and present stapedial reflexes. In all ears, except two, transient-evoked otoacoustic emissions were also present. Bilateral pure-tone thresholds were below 40 dB HL, at octave levels from 250 through 8000 Hz. In total 62 ears were tested. Two ears were excluded from the study due to absence of TEOAE.
The subjects were exposed to continuous, broadband, steady-state industrial noise at the workplace. The intensity of noise ranged with location within the factory from 85-97 dB(A). The duration of exposure was equal to 6 hours. No ear protection was used on the day of experiment. Each person was tested two times, before the work-shift and immediately after the exposure. The pure-tone audiometry (PTA) and transient-evoked otoacoustic emissions were included. Audiometric thresholds were measured in 1 dB steps through the frequencies of 250 to 8000 Hz (OB 822 Clinical Audiometer, Madsen).
After exposure, the TEOAE was performed first (before pure-tone audiometry) and the right ear was tested first (prior to the left ear).
TEOAE were recorded and analysed using the ILO 88 Otodynamics Analyzer hardware and software (Otodynamics, Ltd.). Prior to recording the checkfit procedure was used to ensure a flat broad-band click stimulus (Kemp et al., 1990). The stimuli were unfiltered clicks of 80 µs duration, presented at the rate of fifty per second in nonlinear mode (Kemp et al., 1990). The mean stimulus levels in the external ear canal before and after exposure were 81.1 ± 2.7 (1 s.d.) dB SPL and 80.6 ± 2.6 (1 s.d.) dB SPL, respectively. The responses were windowed from 2.5 to 20 ms post-stimulus and stored in two buffer after completion of 260 averages. The following requirements were used for selecting TEOAE results for further analysis (according to Hotz et al., 1993): 1. the difference in the peak stimulus levels between two test sessions less than 10 dB; 2. the sum of the pre- and postexposure test reproducibility values above 100% and the difference between the two less than 10%. Such requirements eliminated records with differences that might have been due to variation in test-retest stimulus level and also excluded responses with amplitudes too small for assessing test-retest differences. Results from 9 ears failed to meet these criteria and were excluded.
The following measures were extracted and counted after the response: TOAE level, response levels in the band frequencies of 1, 2, 3, 4 and 5 kHz, and reductions in the amplitude of the responses.
Statistical procedures included linear regression and parametric t-test on paired cases, with the significance level set at p<0.05. In total 53 ears were used for making pre- and post-exposure comparisons.
In the pure-tone audiometry a significant temporary threshold shift after exposure to noise was found throughout all frequencies tested, using parametric t-test on paired cases [Table 1]. The greatest mean difference between pre- and post-exposure thresholds was noted at 6 kHz (9.0 dB ± 9.0).
Also, by measuring TEOAE, a significant reduction in TOAE level after exposure to noise was measured, with the mean value of 1.2 ± 1.1 (p<0.01) and range of - 1.2 to 3.7 dB [Table 2]. In the frequency bands a significant postexposure reduction was observed predominantly at the frequency 4 kHz with the mean value of 2.3 ± 3.2 (p <0.01) and also at the frequency 2 kHz, with the mean value of 1.3 ± 1.4 (p<0.01). Weaker correlation was found at frequencies 1 kHz and 3 kHz (p<0.05).
Although both methods, pure-tone audiometry and TEOAEs, showed significant changes due to noise exposure, there was no correlation between TTS assessed by pure-tone audiometry and a reduction in TEOAE amplitude, neither assessed as an TOAE level nor as a response in the bandpass frequency of 4 kHz [Figure 1],[Figure 2]. Also, no correlation between pre-exposure values (hearing thresholds or TEOAE levels) and the changes induced by noise was stated [Figure 3],[Figure 4]. Considerable variations in TTS in subjects with the same pre-exposure audiometric thresholds were seen [Figure 3].
Despite considerable variations in TTS significant correlation between pre- and postexposure audiometric thresholds was observed (p<0.01)[Figure 5]. A stronger correlation was found between pre- and post- exposure amplitudes of TEOAE (p<0.001)[Figure 6].
Otoacoustic emissions, particularly transientevoked otoacoustic emissions, are becoming an important tool in assessing hearing in subjects exposed to noise in industry and the military (Hotz et al., 1993, Kvaerner et al., 1995, Lucertini et al., 1996, Engdahl et al., 1996). The high sensitivity and specificity of TEOAE as a screening method for the diagnosis of cochlear damage has been shown in military recruits (Lucertini et al., 1996, Attias et al., 1998). Otoacoustic emissions could be also applied to hearing conservation programmes for the purpose of early detection of hearing damage caused by occupational exposures (Hotz et al., 1993, Engdahl et al., 1996). If a greater sensitivity of OAEs is proven, this measurement may be of great value in monitoring the status of the cochlea particularly in subjects exposed to very high level of noise, who have an increased risk of damage (Kvaerner et al., 1995, Attias et al., 1996). As compared to pure-tone audiometry, otoacoustic emissions have several advantages: the measurement is objective, simple and fast, may be conducted in moderately noisy environments and gives repeatable measurements over time. The most important advantage seems to be the objectivity of the test, which excludes the errors of hearing assessment due to subject uncooperativeness.
In our present study we assessed the effects of exposure to genuine industrial noise, known to cause temporary threshold shift on TEOAEs. As compared to a precedent, similar experiment (Kvaerner et al., 1995) mean temporary hearing loss was deeper. Since the largest TTS was determined at 6 kHz, this particular frequency was used for statistics. TTSs were accompanied by a decrease in the TOAE level of otoacoustic emissions, which was also greater than in previous study. A good correlation between TEOAE level before and after exposure to noise confirmed a good test-retest condition.
Spectral analysis revealed a significant decrease in the level of TEOAEs mainly in the high frequency bands. This is in contrast to earlier studies of Kvaerner et al. who were not able to find such changes after smaller exposure to industrial noise. The lack of decrease in otoacoustic emissions shown by these authors might be due to a smaller amount of hearing loss and also to changes in TEOAEs caused by preexposure to noise. With the increased level and/or time of exposure to noise, TEOAE amplitudes showed a significant reduction in higher frequencies (Hotz et al., 1993, Liebel et al., 1996). The reduction in otoacoustic emissions, mainly in the high frequency bands, confirms a great sensitivity of TEOAE in assessing the effects of noise to the cochlea.
Although both methods, pure-tone audiometry and TEOAEs, showed significant changes following exposure to noise, similar to earlier findings, no correlation between TTSs and the decrease in the level of TEOAEs was found. It means that TEOAE selects an entirely different group from that selected by monitoring PTA. From the previous study it is known that all attempts to find a close correlation between auditory thresholds and amplitude of OAEs failed (Avan et al., 1993, Sliwinska-Kowalska et al., 1998). All these data indicate that there is no linear relationship between pre- and postsynaptic events related to hearing.
Our results confirm the high sensitivity of TEOAEs in assessing the changes to the cochlea after short exposure to industrial noise, thus recommending this measurement as a method of evaluation of TTS condition in addition to puretone audiometry.
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