|Year : 2013 | Volume
| Issue : 1 | Page : 9-12
Effect of short-duration noise exposure on behavioral threshold and transient evoked otoacoustic emission
Prawin Kumar1, Kaushlendra Kumar2, Animesh Barman1
1 Department of Audiology, AIISH, Mysore, India
2 Department of Audiology and Speech Language Pathology, KMC, Mangalore, India
|Date of Web Publication||6-Mar-2013|
Department of Audiology, AIISH, Mysore-06
Source of Support: None, Conflict of Interest: None
Background: It is well known that the short- or long-duration exposure to loud noise can either cause temporary or permanent threshold shift. Pure tone audiometry is most widely used to predict the individual who is susceptible to such noise-induced hearing loss (NIHL). Transient evoked otoacoustic emission (TEOAE) is a noninvasive, objective technique and required less time to administer to find out such effect. However, one must know the sensitivity of TEOAEs to predict susceptibility of NIHL. Objectives: Thus, the present study was taken up to find out the susceptibility of TEOAE to identify the individual who is more susceptible by comparing behavioral temporary threshold shift (TTS). Materials and Methods: There were 28 ears in the age range of 18 to 30 years participated in the study. All participants were evaluated for pure tone thresholds and TEOAEs amplitude before and after exposure to the short-duration noise. Results: Results indicated that there were statistically significant differences observed for both behavioral thresholds shift and TEOAEs amplitude reduction after exposure to short-duration noise at 0.05 levels at different frequencies. Conclusion: Further, it was observed that though there was agreement between the two, TEOAE amplitude shift was not as much as pure tone TTS. Reason and the importance of the TEOAE to use as a tool to find out susceptibility are being discussed in the article.
Keywords: Behavioral threshold shift, Noise exposure, Otoacoustic emission, Temporary threshold shift
|How to cite this article:|
Kumar P, Kumar K, Barman A. Effect of short-duration noise exposure on behavioral threshold and transient evoked otoacoustic emission. Indian J Otol 2013;19:9-12
|How to cite this URL:|
Kumar P, Kumar K, Barman A. Effect of short-duration noise exposure on behavioral threshold and transient evoked otoacoustic emission. Indian J Otol [serial online] 2013 [cited 2022 Jul 4];19:9-12. Available from: https://www.indianjotol.org/text.asp?2013/19/1/9/108150
| Introduction|| |
It may be assumed that 5-10% of young normally hearing adults have an increased susceptibility to noise, so-called "vulnerable" inner ear. Individuals with a "vulnerable cochlea" develop a hearing loss from noise exposures even below official limits. 
Noise primarily damages the mechano-electrical transduction process located in the hair bundle of outer hair cells.  This disturbance alters the mechanical amplification of the cochlea, causing an elevation of hearing threshold  and a loss of frequency selectivity and temporal resolution.  Otoacoustic emissions (OAEs) provide objective information about the functional integrity of the outer hair cells ,, and may therefore be particularly useful for assessing damage caused by acoustic overexposure. OAEs may not be a good tool to accurately estimate absolute audiometric thresholds,  but may be useful to detect audiometric threshold changes due to noise exposure.
OAE are known as outer hair cells (OHC) reference and to be affected preferentially during the initial stages of noise damage.  Most healthy, normal hearing ears have transient evoked otoacoustic emissions (TEOAEs)  however most noise-damaged ears have fewer, smaller, or no TEOAEs.  Small changes in the biomechanical function of the cochlea can be monitored by measuring TEOAE, which is generated within the cochlea by active nonlinear processes involving the OHCs.  Noise exposure is believed to influence cochlear function and thus alter the amplitude or frequency composition of OAEs. 
Noise begins to stress the auditory system when sound levels exceed 75-85 dBA range; the chances of developing a temporary threshold shift (TTS) are considerable when the noise level is above the recommended limit of 85 dBA. Individual susceptibility to noise exposure makes it difficult to predict temporary or permanent damage to hearing sensitivity. 
Thus, it is evident that the short-duration exposures to loud noise are likely to affect OHCs function and causing elevation of behavioral threshold. Test retest variation in behavioral threshold might be up to 10 dB HL. Thus, TTS due to short-duration exposure of noise is usually obtained using pure-tone audiometry. This is known to be the good predictor of identifying individuals who are susceptible to hearing loss after exposure to noise. However, the major factor, which can influence the test result using this method, is variation of reducing the predictability of such effect. TEOAEs, which are also known to be generated by OHCs motility, thus can be an objective tool to predict such effect.
Several authors tried to observe effect on TEOAEs due to short- or long-duration exposure to noise. ,, However, the sensitivity of TEOAEs to predict susceptibility of noise-induced hearing loss (NIHL) has not been verified using other audiological tests. Individual susceptibility (or vulnerability) to noise along with the degree of hearing loss varies greatly among people, which means that after the same exposure to noise, some persons develop substantial hearing loss, whereas others develop little or no hearing loss at all.  Thus, in the current study, pure tone behavioral threshold shift has been used to predict the sensitivity of TEOAE in predicting susceptibility of NIHL. TEOAEs can be more useful as it is an objective test, which can be administered within very short time.
Aim of the study
The present study aimed to find out the relationship between behavioral TTS and reduction in TEOAE amplitude after the exposure to short-duration broadband noise (BBN). It also aimed to find out the efficiency of TEOAE to predict susceptibility of NIHL based on the correlation obtained between behavioral TTS and TEOAE amplitude shift.
| Materials and Methods|| |
A total of 28 ears from 14 individuals in the age range of 18-30 years were taken for the study. Seven male and seven female were taken to minimize the gender effect and evaluated in random order so that order effects should not contaminate the results. It was ensured that all the subjects were in good health and had no history of otological symptoms. Oral consent was taken from all the participants and they were explained in detail about the procedure.
To evaluate the middle ear function, GSI Tympstar was used. All the ears had "A" type tympanogram with presence of acoustic reflexes at 250-4000 Hz. Behavioral thresholds were estimated using GSI-61 audiometer before and after exposure of noise. Individuals with pure tone threshold <15 dB HL at octave and mid-octave frequencies from 250 to 8000 Hz before exposures to noise were taken for the experiment.
TEOAE were measured using ILO 292 (software version 5) in standard default operational mode. Nonlinear click at 80 dB peak SPL was used to estimate TEOAE responses. A suitable probe tip with appropriate fitting was used for recording TEOAE. Probe was taken out after recording TEOAEs before exposure to noise.
All the measurement was carried out in an acoustically treated double room situation. The ambient noise level was within the permissible level. 
The experiments carried out in three stages for each ear.
Baseline pure tone thresholds were obtained at octave and mid-octave frequencies between 500 and 8000 Hz using modified Hughson Westlake method. Intensity was varied in 2 dB steps to obtain more accurate behavioral threshold. TEOAE were measured after the behavioral thresholds obtained. A good probe fit was ensured prior to TEOAE measurement. A total of 260 click stimuli were presented and the responses were averaged. Reproducibility of more than 50% and a signal-to-noise ratio (SNR) of 3 dB or above was considered as a response. Overall TEOAE response and SNR at 1000, 1500, 2000, 3000, and 4000 Hz were noted.
After obtaining the baseline behavioral threshold and TEOAE response, all the subjects were exposed to 90-dBSPL white noise for 2 min through TDH-39 headphone.
After the exposure of noise, all the subjects were given a gap of 2 min before the hearing assessment, as Hirsch and Ward  reported that TTS is more stable 2 min after the exposure of noise. For 50% of the ear, pure tone threshold were obtained first from 1000 to 4000 Hz including mid-octaves, followed by TEOAEs measurement and 50% of the ear TEOAEs were measured first prior to behavioral threshold measurement to avoid the effect of duration of TEOAE amplitude and behavioral threshold after the exposure.
| Results|| |
Data obtained from all the ears were subjected to paired 't' test to find out significant difference between pre- and post-exposure behavioral threshold at 1000, 1500, 2000, 3000, 4000, and 8000 Hz [Table 1] and [Table 2], and TEOAE amplitude to global response and responses at 1000, 1500, 2000, 3000, and 4000 Hz [Table 3] and [Table 4]. Karl Pearson product correlation test was done to see the correlation between behavioral threshold shift and TEOAE amplitude threshold shift at 1000, 1500, 2000, 3000, and 4000 Hz [Table 5].
|Table 1: The mean (dB HL) and standard deviation (SD) of pre-and post-exposure behavioral threshold|
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|Table 2: The t values and significant levels between the pre-and post-exposure behavioral threshold|
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|Table 4: The t values and significant levels between the pre-and post-exposure TEOAE amplitude|
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|Table 5: Karl Pearson correlation test between the behavioral threshold shift and TEOAE amplitude|
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The result obtained in the present study showed that there are significant differences between pre- and post-exposure behavioral threshold at all the frequencies. It can be noticed that the threshold shifts is more at higher frequencies than at mid frequencies.
It is evident from the above [Table 3] and [Table 4] that there is significant reduction in TEOAE amplitude at all the frequencies, which is statistically significant above 1000 Hz. This different is highly significant for global TEOAE amplitude as well as at 3000 and 4000 Hz TEOAE amplitude.
It can be seen from the [Table 5] that there is weak positive correlation between the behavioral threshold shift and the TEOAE amplitude reduction at all the frequencies except at 1000 Hz.
| Discussion|| |
In present study, there is a significant difference between pre- and post-exposure behavioral threshold and reduction in TEOAE amplitude. There were more shifts in behavioral threshold and reduction in TEOAE amplitude at high frequencies, i.e., above 2 kHz compare to mid frequencies. It is well established that the basal region of the cochlea is more susceptible to noise exposure and that leads to more shift in both behavioral threshold shift as well as reduction in TEOAE amplitude. The shift in TEOAE suggests that OHCs are more prone to such effect as OAEs are generated by electromotility of OHCs. Researcher reported that in chinchilla due to moderate levels of noise exposure, buckling of the supporting cells results in an uncoupling of the OHC stereocilia from the tectorial membrane, which results in a TTS,  whereas moderate level (105-110 dB SPL) noise tends to induce more damage to the stereocilia of the inner hair cells (IHCs) than that of OHCs.
Current study has failed to observe any significant correlation between the behavioral threshold shift and TEOAE amplitude reduction. This could be attributed to the fact that one can expect behavioral threshold variation of an individual if it is assessed at different time. The main drawback of this study could be that after every recording of TEOAE, probe was taken out, which would have led to variation of the TEOAE amplitude and resulted in poor correlation subject to expose to noise. Every time the probe was inserted, there was a variation of ear canal volume. However, it can be still observed that TEOAE amplitude reduction is statistically significant at 3000 and 4000 Hz at 0.01 levels, which does justifies the sensitivity of the test to predict susceptibility of NIHL as noise-induced damage of the cochlea starts predominately in the high-frequency regions. , Further, TEOAEs appear to be sensitive to the global cochlear state on the high-frequency region rather than low-frequency hearing threshold shift.  However, they have also failed to observe significant correlation of TTS compared to a reduction of TEOAE amplitude.
| Conclusion|| |
The present study showed that there are significant differences between the pre- and the post-noise exposure behavioral threshold as well as reduction in the TEOAE amplitude. However, it has failed to reach significant correlation between the threshold shift and the reduction in TEOAE amplitude. Thus, it is difficult to say which one among the two is sensitive enough to predict the susceptibility of NIHL. However, one must notice that there is greater behavioral threshold shift and TEOAE amplitude reduction at high frequencies thus suggesting its efficiency. It can be suggested to use TEOAE to assess the susceptibility rather than use of behavioral threshold shift, as it is an objective test and can also be assessed within very short duration.
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[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]