Otoacoustic emissions (OAEs) are a sensitive and frequency-specific tool for assessing cochlear damage. Parameters of an OAE signal decrease at the frequency that approximately corresponds with the injured region. However, damage in the basal turn of the cochlea is important for signal processing and OAE generation in the higher cochlear partition.
In workers exposed to noise, the amplitude of OAEs decreases at the frequencies characteristic for acoustic trauma. These changes may occur prior to the audiometric threshold shift, which supports the superiority of OAEs in early detection of noise-induced damage. Therefore, OAEs may be applied as a quantitative test for individual assessment and monitoring of industrial hearing loss.
Otoacoustic emissions are an established screening tool in the examination of newborn and infant hearing. In addition, studies on adult patients demonstrate high sensitivity and specificity if applied in screening sensorineural hearing loss, especially with multivariate analyses engaged. OAEs may be used as a screening test in subjects with an increased risk of exposure to noise and in epidemiological studies on industrial and environmental noise effects.
Although otoacoustic emissions have a remarkable advantage in the evaluation of industrial hearing loss, there are some doubts about their utility in hearing conservation programs. The legislation and financial compensation associated with the diagnosis of occupational illness are based on the quantitative "gold standard", pure-tone audiometry. In addition, as it is not possible to reproduce the audiogram precisely, the OAEs may not be currently applied as a reliable test of hearing loss in malingerers. However, in some countries where tinnitus is eligible for compensation, OAEs may indeed appear helpful in the reliable diagnosis of cochlear damage within the respective frequency range.
Keywords: transient-evoked otoaocustic emission, distortion product otoaocustic emission, occupational noise-induced hearing loss, temporary threshold shift, permanent thresholds shift, tinnitus, hearing monitoring, hearing screening
|How to cite this article:|
Sliwinska-Kowalska M, Kotylo P. Otoacoustic emissions in industrial hearing loss assessment. Noise Health 2001;3:75-84
| Introduction|| |
Otoacoustic emissions (OAEs) were introduced as a clinical diagnostic tool of cochlea assessment in audiology departments in the late 1980s. The generation of otoacoustic emissions is determined by the proper function of the cochlear amplifier, in particular, the outer hair cells. The number of outer hair cells (about 12000) exceeds by nearly three times the number of inner hair cells (about 3500). The outer hair cells are innervated almost exclusively by descending efferent nerve fibres of the olivocochlear system and they display unique motile abilities in response to electrical stimuli. The active properties of outer hair cells increase stimulus-specific vibrations of the basilar membrane, leading to backward acoustic wave emission into ear canal.
Chronic exposure to industrial noise at moderately high levels, commonly found in industrial environments, brings about damage to the cochlear sensory elements. Noteworthy, the outer hair cells are the most susceptible to damage of this kind. Therefore, otoacoustic emissions seem to be the method of choice for the evaluation of noise-induced damage. As there are some data on the changes of otoacoustic emission parameters that appear prior to the impairment of pure tone threshold, otoacoustic emissions might be used to detect and monitor early noise-induced changes at the level of Corti's organ. This study aimed to evaluate the advantages and disadvantages of otoacoustic emission and its role in evaluation of industrial hearing loss. This literature review deals with the application of two kinds of evoked otoacoustic emissions: transient-evoked otoacoustic emission (TEOAE) and distortionproduct otoacoustic emission (DPOAE). The data presented refer exclusively to the results of studies on humans.
Otoacoustic emissions and hearing acuity
The substantial quantitative correlation between otoacoustic emission and pure-tone audiometry was demonstrated in primary studies on OAEs in subjects with sensorineural hearing loss (Harris and Probst, 1991; Lonsbury-Martin et al., 1991; Probst et al., 1987; Sliwinska-Kowalska, 1998). The hearing loss at the impaired frequencies is accompanied by a weakened otoacoustic emission. This relation, however, is more specific in the case of distortion product otoacoustic emissions (DPOAE) due to the application of frequency specific stimuli and broader range of test frequencies. The examples depicting this relation are presented at [Figure - 1]. The first pattern was found in a person at the early stage of noise-induced hearing loss. A typical, high frequency hearing loss with a notch at 4-6 kHz is observed in pure-tone audiometry. The DP-gram closely follows the audiogram and reveals a similar notch around 3 kHz. In addition, the TEOAE spectrum shows a selective elimination of otoacoustic emission signal within this frequency [Figure - 1]a. [Figure - 1]b presents the results of pure-tone audiometry, DPOAE and TEOAE in presbycusis. The audiometry curve gradually descends at high frequencies, similar to that of DP-gram. There is also no response detected at high frequencies (45 kHz) in the TEOAE spectrum. The following patterns [Figure - 1]c present data in a patient with progressive cochlear hearing loss of unknown origin. The damage mainly affects the low frequencies and partially the middle frequencies, whereas hearing thresholds at high frequencies are within normal values. Consistent with the audiometric findings, there is no low frequency DPOAE or TEOAE. Thus, the general characteristics of the DP-gram reflect pure-tone audiometry findings. One month of corticosteroid therapy in this patient did not change the results of pure-tone audiometry. However, examination of OAEs revealed a significant decrease of response signal in the right ear, confirming further disease progress and ineffectiveness of the treatment.
The amplitude of otoacoustic emissions is associated with the state of hearing at high frequencies. Avan studied healthy subjects with normal hearing (threshold < 15 dB HL) and analysed the correlation between TEOAE amplitude and hearing threshold (Avan, 1997). There was no correlation within the standard audiometric frequencies. However, a significant negative correlation was observed between TEOAE level and hearing threshold at the frequency of 16 kHz. This suggests that the condition of hair cells at the basal turn of cochlea may significantly influence otoacoustic emissions at lower frequencies (TEOAE covers the frequency range from 0.75 - 5 kHz).
Although otoacoustic emissions are useful in quantitative evaluation of cochlear damage, there are some basic limitations of their application for the precise reconstruction of pure-tone audiometric thresholds. This is due to the preneural nature of OAEs and in high intersubject variability of otoacoustic emission intensity. On the other hand, only a small range of acoustic deficits (up to 40-60 dB HL) can be assayed with evoked otoacoustic emissions. These limitations were precisely illustrated in the study of Gorga et al. (Gorga et al., 1997). The authors assessed on a population of more than 800 normal hearing subjects and patients with sensorineural hearing loss and evaluated the correlation between audiometric hearing thresholds and DPOAE signal parameters. They found a broad distribution of otoacoustic emission values, particularly at low and middle frequencies. Further, evaluation of DPOAE signal to noise ratios demonstrated that DPOAEs may be of diagnostic value only in subjects with hearing loss below 50 - 60 dB. Suckfull et al. correlated TEOAE and DPOAE with auditory thresholds (Suckfull et al., 1996). In a cross sectional study of 102 ears with sensorineural hearing loss, they showed rather poor correlation between OAEs and audiometric indices, with lower correlation coefficients and less frequency specificity for TEOAE, compared to DPOAE. By fitting a multivariate linear regression model, with TEOAE and DPOAE used simultaneously as predictors of hearing loss, they obtained 95% prediction intervals of 19-39 dB in cases of sensorineural hearing loss. The prediction intervals decreased to 18-26 dB if the maximum hearing loss was limited to 70 dB HL. The authors concluded that the precision range was not narrow enough to accurately determine hearing threshold using otoacoustic emission.
Similar to other authors, our study on the relationship between TEOAE and DPOAE amplitudes to audiometric thresholds, in a population over 100 subjects exposed to noise at the workplace and non-exposed controls, shows only a weak correlation between hearing loss and emission values. The analysis performed separately for noise-exposed and control subjects, both with hearing thresholds below 30 dB HL, revealed a weak, negative correlation between OAE parameters only for control subjects, whereas no correlation could be found in workers exposed-to-noise [Figure - 2]. Based on single subject analysis, it may be hypothesised that, in some individuals with normal pure tone thresholds, otoacoustic emission signals are attenuated because of noiseinduced, subclinical loss of outer hair cells at very high (above 8 kHz) frequencies. Evidence to support this thesis are based on hearing monitoring in persons exposed to noise and are cited in the following part of the paper.
The type and the level of stimulation is a significant element of otoacoustic emission assessment. Sutton et al. studied the influence of primary tone level and the effect of f2 level decrease on DPOAE sensitivity to acoustic overstimulation (Sutton et al., 1994). The subjects with normal hearing were exposed to a pure-tone at the frequency of 2.8 kHz at 105 dB SPL for 3 min. Otoacoustic emissions were evaluated before and after the exposure. An f1 level of 55 dB SPL and an f2 level 25 dB below f1 significantly enhanced DPOAE sensitivity to acoustic overstimulation. The time course of DPOAE recovery was very close to the behaviourally measured temporary threshold shifts. This indicates that with appropriate test parameters, DPOAEs may be as sensitive as pure-tone audiometry for evaluation of temporal changes elicited by the exposure to short-lasting moderately intense tones.
Otoacoustic emissions in monitoring of noiseinduced hearing loss
The essential aim of hearing monitoring is to compare consecutive results in a given person over the period of ototoxic agent action. This not only allows the sensitivity of otoacoustic emission to be assessed directly, but also overcomes errors associated with intersubject variability, and technical differences associated with measurements in different persons.
Otoacoustic emissions in monitoring the effects of acute exposure to noise
Short exposure to noise altered the frequency spectrum of otoacoustic emission and increased their threshold (Rossi et al., 1991, Avan et al., 1993). Several recent reports have compared temporary changes in pure-tone audiometry (TTS) and otoacoustic emissions after acute exposure to noise; the results suggest that otoacoustic emissions are a sensitive measure of TTS. Attias and Bresloff studied the changes in pure-tone audiometry and TEOAE in soldiers exposed for 10 minutes to white noise at 90 dB SL (Attias and Bresloff, 1996). They demonstrated a positive correlation between TEOAE and hearing threshold shifts. There was a significant decrease of otoacoustic emissions at high frequencies, confirming the noise effects to the inner ear. Moreover, in some of the subjects studied, TEOAE changes were not accompanied by audiometric hearing threshold shifts; this might support the hypothesis that otoacoustic emissions are a sensitive measure of minor changes in cochlea function. Two other reports (Kvaerner et al., 1995; Sliwinska-Kowalska et al., 1999) considered the influence of industrial noise exposure on TEOAE pattern. Kvaerner et al. studied TEOAE and pure-tone audiometry patterns in employees after three consecutive days of 7-hour industrial noise exposure at 85-90 dB (A). They found a significant reduction of the mean TEOAE amplitude and elevation of audiometric thresholds after the exposure. However, they were not able to demonstrate any correlation between TEOAE changes and TTS. Also, there was no decrease of otoacoustic emissions at high frequencies, typical for noisetrauma. TEOAE and pure-tone audiometry prior to and after 6-hour industrial noise exposure at 85-97 dB (A) were investigated in our studies. We were able to demonstrate a decrease of overall TEOAE response and worsening pure tone thresholds; however, there was no correlation between these parameters [Figure - 3]. Similar to the results of Attias and Bresloff (1996), decreased otoacoustic emissions with no alternation of pure tone thresholds were found in some subjects. However, temporary threshold shift without changes in otoacoustic emissions was also found in another persons. The comparative analysis of three studies dealing with occupational noise, performed by Kvaerner (1995), Sliwinska-Kowalska (1999) and Attias and Bresloff (1996), indicates that greater average threshold shifts in pure-tone audiometry are accompanied by greater average decrease of TEOAE responses. On the basis of the cited studies, it is not possible to state whether otoacoustic emissions are better in the early detection of noise damage than pure-tone audiometry as the noise exposures were rather high and resulted in TTS and recovery measures were not taken.
Recently, Vinck et al. evaluated the sensitivity and applicability of TEOAEs and DPOAEs as quantitative indices for the functional integrity of OHC during TTS (Vinck et al., 1999). In the first set of experiments, a small group of volunteers were exposed to a broadband noise at 90 dB SPL for 1 h. The complex parameters of TEOAE responses, including repeatability, amplitude, signal to noise ratio and DP-gram responses were registered before, during and 6 hours after the exposure. Although the exposure did not cause a threshold shift in pure-tone audiometry, the authors demonstrated a significant decrease of otoacoustic emission amplitude at 4 kHz and higher along with a decreased repeatability of TEOAE. After exposure gradual recovery of otoacoustic emission was observed.
Those results show that a decrease of otoacoustic emissions due to noise appears prior to hearing impairment measured with pure-tone audiometry. Thus, OAEs seem to be more sensitive to temporal changes in cochlear function after acute noise exposure than conventional methods. In the second set of experiments, eight young healthy adults were exposed to discotheque music for five consecutive hours. A significant TTS was observed with pure-tone audiometry and a greater decrease in OAEs was seen as after the first exposure. The time course of recovery for DPOAE and TEOAE was very similar to behaviourally measured temporary threshold shift.
Otoacoustic emissions in monitoring the effects of prolonged exposure to noise
The human evidence on monitoring early changes caused by prolonged exposure to industrial noise is scarce. Hotz et al. indicated that a 17-week exposure to impulse noise led to a significant reduction of TEOAE amplitude in soldiers within the frequency range of 2-4 kHz (Hotz et al., 1993). Compared to the initial level of OAEs, amplitude was reduced by 84% in the right ear and 90% in the left. Unfortunately, pure-tone audiometry changes were not monitored in this particular study group. The comparison of extrapolated data with pure-tone audiometry results of another soldier group, exposed to comparable noise level, suggested higher sensitivity of TEOAE than conventional methods.
Our own preliminary studies of hearing loss monitoring have been performed for 2 years using TEOAE, DPOAE and pure-tone audiometry and included metal-factory and weaving-mill employees. The initial employment period significantly varied between the groups. The metal enterprise employees had been exposed to noise of 85-97 dB (A) (depending on specific site of work) for 0.5 to 6 years at the time of study enrolment. Weavingmill employees, however, had been exposed to 88-92 dB (A) and their employment was longer than 6 years at the time of study enrolment (6 up to 24 years). Although there was no hearing loss using pure-tone audiometry during 2 years of observation, a constant, gradual decrease of transient-evoked otoacoustic emission was observed. The decrease of TEOAE was significantly higher in the workers exposed to noise as compared to the controls. This suggests that TEOAE can detect progressive subclinical noise-induced cochlear dysfunction. However, no consistent changes in DPOAE patterns were identified during the 2 years of observation.
Industrial hearing loss screening with otoacoustic emissions
In comparison with the voluminous literature on otoacoustic emission in newborn and infant screening, there are limited data on screening in adults, particularly if hearing conservation programs are considered (Attias et al., 1998; Lucertini et al., 1996; Engdahl et al., 1996). The studies performed so far indicate that DPOAEs are effective in adults, especially if complex multivariate analysis is applied. DPOAE sensitivity in assessing hearing loss exceeding 20-30 dB (variable in different studies) was within 78 to 92% and specificity within a similar range (79 to 90%) (Attias et al., 1998; Gorga et al., 1999, Gorga et al., 1997; Kim et al., 1996; Sun et al., 1996). Receiving operating curve analysis confirmed the clinical usefulness of this test as a rapid and reliable method of differentiating between damaged and normal hearing. The specificity and sensitivity of otoacoustic emissions is improved with multivariate analysis.
Lucertini et al. focused on TEOAE test utility in noise-exposed pilots (Lucertini et al., 1996). The authors concluded that there was restricted usefulness of TEOAE as a method of hearing screening in this population. Attias et al. evaluated DPOAE efficiency as a screening tool in army employees (Attias et al., 1998). They demonstrated a high sensitivity and specificity of the applied methods. However, otoacoustic emissions were present in few subjects with hearing losses of up to 70 dB HL; this may be an artefact related to the high primary tone levels used in the study (70 dB SPL).
Summarizing, otoacoustic emission might be a useful screening tool in industrial audiology, occupational medicine and hearing conservation programs.
Otoacoustic emissions and malingering
Otoacoustic emissions are used in practice to identify malingerers, which are common in people searching for compensation for industrial disease. Normal values of otoacoustic emissions provide evidence for preserved cochlear and outer hair cell integrity. Although many authors describe a reconstruction of actual hearing threshold based on OAE and other electrophysiological procedures (Robinette, 1992, Musiek and Baran, 1996), the evidence discussed earlier in this paper clearly indicates that current methods of OAE measurement do not allow for a precise reconstruction of an audiogram. The presence of evoked otoacoustic emissions suggests that hearing threshold do not exceed 2530 dB HL at a given frequency, but it does not allow for the objective evaluation of hearing loss severity. It should be emphasised that otoacoustic emissions, applied in hearing loss evaluation in malingerers, may be preserved in patients with retrocochlear hearing damage. Therefore, careful differential diagnostics of hearing loss is necessary based on other audiometric tests.
Otoacoustic emissions and tinnitus
Tinnitus incidence among subjects with industrial hearing loss is estimated to be 10.7 - 20.7% (Coles et al., 1990). Evoked otoacoustic emissions, especially fine structure OAEs, allow for detection of cochlear dysfunction at the frequency related to the tinnitus frequency. Abnormal otoacoustic emissions at the tinnitus frequency were detected even in subjects with normal pure-tone audiograms (Cerenic et al., 1995; Shiomi et al., 1997). The decrease of DPOAE amplitude was commonly found at high frequencies, between 4-7 kHz, frequencies associated with noise and ototoxic-induced damage. Thus, in countries where compensation for occupational disease includes tinnitus, fine structure otoacoustic emissions may provide objective confirmation of organic etiology of a subjective disease symptom, as it is in cases of tinnitus.
Perspectives on otoacoustic emission application in hearing conservation programs
Regarding the high, selective sensitivity of otoacoustic emissions in relation to damage of outer hair cells, this test is with no doubt valuable, but not fully taken advantage of as a diagnostic tool for monitoring the early, noiseinduced changes in the inner ear.
However, it is unlikely, that OAEs could replace pure-tone audiometry in hearing conservation programs or legal cases. OAEs are a preneural test that evaluates the integrity of outer hair cells, but it gives no insight, into the function of the entire auditory pathway, as does audiometry. Therefore, OAEs cannot be regarded as an actual hearing test. Although pure-tone audiometry and otoacoustic emission reveal parallel, mean changes in groups of subjects exposed to noise, the analysis of individual patterns indicates that in some cases, the audiogram can show abnormal hearing threshold levels when OAEs are entirely normal; in other cases, the audiogram may be entirely within clinically normal limits, but OAEs are unequivocally abnormal and consistent with cochlear dysfunction (Hall, 1999). Also, the guidelines of hearing conservation programs and medico-legal procedures associated with financial compensation of occupational hearing loss, which were established for conventional puretone audiometry, are based on well defined terms (like temporary threshold shift, permanent threshold shift, etc.) verified by long-term studies. Such definitions cannot be applied to OAEs, so far. In conclusion, otoacoustic emission cannot replace pure-tone audiometry in the evaluation of industrial hearing loss, but it may be used to validate conventional hearing test results, with great potential for monitoring and early diagnosis of subclinical cochlear damage caused by noise.
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Head of the Department of Physical Hazards, Institute of Occupational Medicine, 8 St Teresa Str, 90-950 Lodz
Source of Support: None, Conflict of Interest: None
[Figure - 1], [Figure - 2], [Figure - 3]