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PEER REVIEWED
Feasibility of screening distortion
product otoacoustic emissions to
monitor cochlear functioning in
noise-exposed mineworkers
Anita Edwards1 and Motselisi Taela2, 1Council for Scientific and Industrial Research (CSIR) – Researcher
2
Anglogold Health Services – Audiologist
Corresponding author: Anita Edwards, Researcher, Occupational Health and Ergonomics, Natural Resources and the
Environment, PO Box 17001, Congella 4013
Tel: +27 (0)31 242 2364; Fax: +27 (0)31 261 2509; E-mail: [email protected]
ABSTRACT
Screening distortion product otoacoustic emissions (DPOAEs) is a potential tool for monitoring the success
of a Hearing Conservation Programme (HCP) but the test is very sensitive to ambient noise in the test area,
which could negatively impact on the validity of the results.
The objective of this pilot study was to evaluate the feasibility of using this test in a non-clinical environment. A group of blacksmith workshop employees were tested before and after their working shift to investigate whether the different aspects of the screening DPOAE test, namely noise floor (NF)/emission level
differences, hearing levels and emission level deterioration, would provide information for future research to
investigate this tool more fully.
The research found that 43.88% could be analysed where the NF/emission level difference was >10 dB
sound pressure level (dBSPL). Of the results, 30.39% had a greater than 3 dBSPL deterioration in emission
strength. The emission frequencies of 2 kHz and 3 kHz provided the most usable measurements and the
hearing levels greatly influenced the usefulness of the results. Results of this preliminary research appear to
indicate that with attention to details such as noise in the environment, training of testers and test protocols,
screening DPOAEs may be a feasible tool to use in noise-exposed populations as an indicator of TTS and the
potential risk of NIHL but further research with larger samples and in depth statistical analysis is required to
confirm these pilot study results.
Key words: Hearing conservation, screening DPOAEs.
INTRODUCTION
Noise-induced hearing loss (NIHL) can
be prevented if a comprehensive and
effective hearing conservation programme (HCP) is implemented. The lack
of deterioration in pure-tone thresholds
of the employees on the annual screening audiogram is currently used as an
indication of the success of the HCP.
Temporary threshold shift (TTS) is a
temporary deterioration in hearing
thresholds that occurs after exposure
to high levels of noise above 85 dBA. TTS is thought to be the
precursor to the permanent threshold shift (PTS) of NIHL.1 This
PTS can take a number of years to develop, by which time the
permanent damage to the cochlea has already occurred. The
sensitivity of pure-tone air conduction to TTS has been found to
be variable.2 A more sensitive tool, which gives an immediate indication of changes in the cochlear functioning of a noise-exposed
employee, could enhance the monitoring of noise-exposed workers and the success of an HCP.
A test that is regarded as a sensitive indicator of cochlear
functioning is otoacoustic emissions (OAEs).3,4 Studies evaluating
the clinical use of OAEs in the monitoring of the small changes in
the biomechanical function of the outer hair cells (OHCs) of the
cochlea have shown that this objective measure can be reliably
used in noise-exposed individuals.4,5,6,7 In a population where NIHL
has already taken its toll, distortion product otoacoustic
emissions (DPOAEs) are the
more sensitive of the OAE tests
as they can be measured in
hearing losses up to 60 dB.5,8,9
Studies of reductions in OAEs
as a result of exposure to high
levels of noise show that
OAEs may also be useful in
tracking TTS in workers exposed to high levels of noise.5
18
JANUARY/FEBRUARY 2008
OCCUPATIONAL HEALTH SOUTHERN AFRICA
“… DPOAEs appear to be an ideal indicator of temporary
cochlear change and early NIHL…
”
The potential of DPOAEs being used as a measure of TTS, as a
warning of potential development of PTS, seems to be a logical
possibility.
However, the measurement of DPOAEs requires control over
the ambient noise in the area where the measurement is made
because the test measures minute emissions from the ear and the
ambient noise level, the so-called noise floor (NF), must be 7–
10 dBSPL lower than the measured emission to confirm the presence of an emission.4 The test is therefore routine in a diagnostic
audiology environment where noise levels are usually well controlled by soundproof booths and sound-treated testing environments. In less perfect and less controlled environments where
noise levels compromise the accuracy and usefulness of the
OAE results, a screening version of DPOAE testing can be used
despite the lack of control in the environment,4 because the equipment is portable and the settings on the equipment have less
stringent filters in place. The screening DPOAE test has been
shown to be useful for newborn screening in a neonatal nursery,
but little research has been performed on the use of the test in
an industrial environment where NIHL is the predominate
health hazard.
The mining industry internationally, and more specifically in South
Africa, is an industry where exposure of employees to very high
noise levels is the cause of NIHL.10
Annual medical surveillance only allows for the counselling of
employees concerning the progress of their NIHL once a year
based on the pure-tone screening results. An alert to the deterioration in hearing levels therefore only occurs once the permanent
damage has taken place.
If a screening test could be done at the workplace without
disrupting the work schedule and in a way that avoided travelling
time to a diagnostic test site, as well as in a quick, easy manner,
the early warning to employees of cochlear over-burden could
improve the success of the HCP. DPOAEs appear to be an ideal
indicator of temporary cochlear change and early NIHL, if the
practical aspects of noise control outside of a clinical environment
and the limitations placed by pre-existing hearing loss could be
overcome.
Purpose of the study
With the above-mentioned discussion in mind, the audiologists in a
mining company investigated the feasibility of the use of screening-DPOAEs in a non-clinical environment to monitor changes in
cochlear functioning in noise-exposed mineworkers. The research
questions asked were:
1. Is it feasible to use screening DPOAEs in a workshop environment?
2. Is it feasible to use screening DPOAEs on a population with preexisting hearing loss and noise exposure?
3. Can the screening DPOAE identify cochlear changes in this
environment?
METHODOLOGY
Study design
A descriptive design was used for the study as this study was
OCCUPATIONAL HEALTH SOUTHERN AFRICA
Table 1. Hearing levels of participants as a percentage
of the sample
Hearing level
Normal
ENIHL
Mild
Moderate
Severe
Total
N
3
3
1
2
2
11
% of sample
27.3%
27.3%
9.2%
18.1%
18.1%
the first stage of evaluating the feasibility of using the tool in an
industrial setting on noise-exposed participants. The descriptive
design would allow the test results to provide indicators of the
type of information that could be expected when using this tool in
this environment.
Sampling strategy
As the current pilot study merely intended to investigate the feasibility of using a screening test in an environment not previously
reported for this test and in a population that was known to have
many confounding factors, the participant variables were not
controlled. The main concern was to obtain an accessible sample
of the population of noise-exposed miners. To this end, a convenience sample of eleven male employees, who worked in the blacksmith workshop at a goldmine in the North West Province in South
Africa, was used. The following information serves to highlight
the variability in the population and some of the factors that will
need to be addressed and controlled in a larger study.
The participants’ years of exposure to noise in the blacksmith
workshop environment ranged from 2 years to 27 years, as determined by informal questioning during the research.
The hearing levels of these participants were obtained from
the annual medical surveillance records to ensure that all degrees
of hearing loss were represented in the sample. See Table 1.
The type of hearing protection devices (HPDs) used by the
participants was not considered. However, all participants wore
the company supplied HPDs during the working period in compliance with legislated and company policies.
Ethics
The pilot study was conducted in-house in a mining company by
the company audiologists. Permission to conduct the pilot study
was therefore obtained from the mine management. Union representatives at a company level and at a departmental level were
consulted to explain the purpose of the pilot study and to obtain
their agreement. The researchers committed to maintain complete
confidentiality of results. Participants were assured of their voluntary participation and their right to withdraw from the study at
any time with no repercussions in the workplace.
Equipment
A GSI Audioscreener was used to collect the data. The stimulus
frequencies used were 2000 Hz, 3000 Hz, 4000 Hz and 6000 Hz
and were set for high noise in the environment. Settings were
‘Quick DPOAE’, f1/f2 ratio preset to 1.2 and the L1 was 65 dBSPL
and L2 equalled 55 dBSPL.
JANUARY/FEBRUARY 2008
19
Procedure
Data collection
Testing of DPOAEs was conducted after the pre-shift morning
meeting (before exposure to noise) and again immediately after
the shift. The test site was the workshop tearoom immediately
adjacent to the workshop. Ambient noise was controlled as far as
was practical by:
• asking participants and others in the tearoom to remain quiet
during testing;
• pausing testing procedures when trains passed outside the
building;
• restarting the test when the door was opened during the test;
• switching the kettle off during testing and;
• work in the workshop was not in progress.
The exact timing after the shift could not be completely controlled but the tests were all completed within 47 minutes after the
shift. The handheld screener did not have the facility to calibrate
the instrument before testing and the aspects of diagnostic DPOAE
testing such as detailed case history, otoscopic evaluation and
repeated measures were excluded from the data collection protocol. This deliberate exclusion of what is regarded as standard
practice for this test diagnostically was to enable an evaluation of
the screening test results when non-standard procedures were
being followed and being conducted by potentially minimally trained
testers, to inform further studies.
The researchers had the opportunity of testing on two consecutive days and the test procedure was repeated with the
workers willing to participate.
Data analysis
Test results were manually analysed and then downloaded onto a
personal computer using the GSI Audioscreener software to con-
Hearing
level
Number of
readings that
had >10 dBSPL
difference
Normal
Normal
Normal
ENIHL
ENIHL
ENIHL
Mild
Moderate
Moderate
Severe
Severe
Total
Percentage
14 of 30
23 of 30
5 of 42
24 of 30
24 of 42
27 of 30
20 of 42
2 of 27
5 of 30
3 of 30
5 of 42
46.6
76.6
11.9
80
57.1
90
74.6
7.4
16.6
10
11.9
151 of 375
43.88
Average
45.03
75.7
32.8
10.95
Table 3. Incidence of >3 dBSPL shift in DPOAE
measurements
Hearing
level
Presence/
reading
Normal
Normal
Normal
ENIHL
ENIHL
ENIHL
Mild
Moderate
Moderate
Severe
Severe
5
6
2
6
7
5
4
5
2
2
3
out
out
out
out
out
out
out
out
out
out
out
of
of
of
of
of
of
of
of
of
of
of
12
12
12
18
18
12
12
12
12
12
17
Total
31 out of 84
Percentage
41.6
50
16.6
33.3
38.8
41.6
33.3
41.6
16.6
16.6
17.6
Incidence
36.06
37.9
30.5
17.1
30.39
firm manual calculations.
Results for each ear and each frequency for each participant
that had a pre- and corresponding post-shift measurement were
this finding and implies the need for further investigation into what
the noise levels in the average potential test sites in a mining
used, resulting in 375 readings. The noise-floor result was deducted from the emission level result. If the difference between
the two readings was greater than 10 dBSPL the reading was
environment actually are. A methodological change of testing participants individually in a quieter office might improve the percentage of usable readings.
regarded as a usable result in terms of being indicative of an
emission. This decision accords with the theoretical concepts
found in the literature.4
The implication of less than half of the measurements being
useful as a result of interference from noise is not surprising in an
industrial setting and severely restricts the use of DPOAEs. If this
The pre-test and post-test usable results were then compared
and a >3 dBSPL deterioration in the emission level was regarded
as an indication of TTS. This deterioration was decided on by the
test method were used in a HCP, a number of aspects would need
attention, such as care in training the testers and well chosen test
sites. These are not insurmountable obstacles and, with innovative
researchers in line with animal histological research that has shown
that the DPOAE amplitude decreases at a rate of 3–4 dBSPL for
each 10% loss of OHC.17,18,19
Percentages and averages were calculated for analysing and
management, this test may be suitable.
It is well documented that hearing loss affects the results of a
DPOAE4,16,17,18,19 because of the fact that the NF/emission level dif-
RESULTS AND DISCUSSION
ference is not greater than 10 dBSPL in frequencies where OHC
damage has occurred. Table 2 shows the results of this pilot
study which indicate that the number of readings where the difference between the NF/emission is >10 dBSPL varied and no
The three research questions were aimed at investigating the
feasibility of using the screening DPOAE test in a non-clinical test
environment. The first and second research questions are closely
patterns could be found in the normal and early NIHL (ENIHL)
groups (45% and 75% respectively). This is an unexpected finding, as one would have expected the participants with the normal
linked as the hearing level of the participant will impact on the
difference between the NF/emission as would the ambient noise
level. See Table 2.
hearing to have the highest number of usable measurements. The
result may be due to the small sample size or variables such as
the presence of middle ear conditions, which were not controlled
Of the 375 measurements taken, only 43.88% could be analysed where the NF/emission level difference was >10 dBSPL.
The interaction of the hearing levels of the participants and the
for in this study. They may also be as a result of what is noted in
the literature as a pre-clinical indication of cochlear pathology.6 In
the groups where hearing loss was greater, there appears to be
ambient noise levels in the environment is the logical reason for
a decrease in usable measurements since the percentage
interpreting the results.
20
Table 2. Usable readings with >10 dBSPL between
noise floor and emission strength
JANUARY/FEBRUARY 2008
OCCUPATIONAL HEALTH SOUTHERN AFRICA
decreased to 32% in moderate hearing losses and as low as 10%
in severe hearing losses. This confirms the findings in the literature,12,13,14,15 and indicates that the greater the hearing loss the less
likely it is that the method will provide data indicative of emissions.
This in turn will reduce the chances of identifying TTS.
The third research question was aimed at determining the sensitivity of a screening DPOAE test in identifying the presence of
Table 4. Percentage of useful readings for emission
frequencies
Hearing loss
2 kHz
Normal
63%
Mild
100%
Moderate
Not tested
Severe
Not tested
3 kHz
86%
86%
32%
23%
4 kHz
52%
21%
5%
8%
6 kHz
36%
7%
0%
0%
TTS in employees, following exposure to high levels of noise for a
working day, whilst wearing HPDs. See Table 3.
Table 3 shows that of the 84 usable measurements in the
established by this pilot study but the results can be used to
inform future research.
study, 30.39% had a greater than 3 dBSPL deterioration in emission strength. When the presence of TTS is related to hearing
levels, more incidences of TTS occurred in the group with normal
ACKNOWLEDGEMENTS
hearing (36.06%) and ENIHL (37.9%) than in those with more
advanced NIHL. A deterioration in emission levels may indicate
that the cochlear was over-exposed to high noise levels and
could increase the risk of NIHL. The possibility of insufficient protection from the HPDs is a possible reason for the presence of
TTS and in a larger study, if it could be shown that the effectiveness of HPDs can be measured using this tool, it would have the
potential of improving the HCP success. See Table 4.
In addition to the information obtained thus far the results were
analysed further to inform future studies using this tool. The results set out in Table 4 indicate that in normal to mild hearing losses
2 kHz and 3 kHz are the most useful frequencies for producing
readings that have a greater than 10 dBSPL NF/emission level
differences. In the groups where participants had moderate to
severe hearing losses none of the test frequencies had significant numbers of usable measurements. The implication for practice is that when testing, time should not be wasted on frequencies other than 2 and 3 kHz, as very few measurements will be
usable in a population with NIHL and even in employees with
normal hearing. In view of the need for testing that will not interfere with productivity or be an inconvenience after a long day in
difficult working conditions, planners of testing protocols require
a change of mindset from the traditional emphasis on 4 kHz, the
most affected by NIHL, to a paradigm that gives an indication of
TTS in the largest number of employees.
An unexpected phenomenon that presented in this pilot study
and that warrants attention in future, was the improvements in the
DPOAE measurements noted after a working period. This study
only interpreted deteriorated DPOAE readings which were ascribed to the deterioration in the ability of the OHCs to emit a
response to acoustic stimuli. However, for all participants there
were seemingly random improvements in some of the DPOAE
measurements. This may be as a result of a factor identified in the
recent developments in DPOAE research wherein the source of
the response is attributed to more than one site in the cochlea.20
Another explanation could be artefacts due to probe placement.4
If future research identified probe placement as the cause of
improvements in DPOAE measures it would have implications for
training of testers using this tool.
CONCLUSION
The study appears to indicate that with attention to details such as
noise in the environment, training of testers and test protocols
screening DPOAE may be a feasible tool to use in noise-exposed
population as an indicator of TTS and the potential risk of NIHL.
The limitations of a small sample and therefore a lack of in-depth
statistical analysis must be overcome by further research. The
feasibility of using screening DPOAE as a tool has not been clearly
OCCUPATIONAL HEALTH SOUTHERN AFRICA
HASS Medical, Pretoria, and Christene Fourie for the use of the GSI
Audioscreener; Drs D. Barnes and D.B. de Villiers for their support
of the project. The manager and employees of the blacksmith shop
of the host mine are thanked for their participation and co-operation.
REFERENCES
1. Katz J. Handbook of Clinical Audiology (4th Edition). Baltimore USA:
Williams & Wilkens; 1994.
2. Kvaerner KJ, Engdahl B, Arnesen AR & Mair IWS. Temporary threshold
shift and oto-acoustic emissions after industrial noise exposure.
Scandinavian Audiology. 1995; 24:137.
3. Judi A, Lapsley M & Marshal L. Monitoring effects of noise with otoacoustic
emissions. Seminars in Hearing. 2001; 22:393.
4. Hall JW. Handbook of otoacoustic emissions. San Diego: Singular
Publishing; 2000.
5. Vinck BM, van Cauwenberge PB, Leroy L & Corthals P. Sensitivity of
transient-evoked and distortion-product oto-acoustic emissions to the direct effects of noise on the human cochlea. Audiology. 1999; 38:44.
6. Lucertini M, Moleti A & Sisto R. On the detection of early cochlear
damage by otoacoustic emission analysis. Journal of Acoustical Society
of America. 2002; 111(2):972.
7. Hall AJ & Lutman ME. Methods for early identification of noise-induced
hearing loss. Audiology. 1999; 38:277.
8. De Koker E, Clark A, Franz RM & Mackay JG.. Feasibility of using otoacoustic emission methods for screening early hearing impairment in
South African mineworkers. SIMRAC Project Health 802. 2003.
9. Hermanus M. OHS Milestones: Achieving the Milestones-time for Change.
Presentation to SA Mine Ventilation Society, Colliery Branch Workshop,
Secunda, South Africa, 25 October 2006.
10. Delb W, Hoppe U, Liebel J & Iro H. Determination of acute noise
exposure effects using distortion product otoacoustic emissions.
Scandinavian Audiology. 1999;28:67.
11.Attias J, Bresloff I, Reshef I, Horowitz G & Furman V. Evaluating noise
induced hearing loss with distortion product otoacoustic emissions. British
Journal of Audiology. 1998; 32:39.
12. Eddins AC, Zuskov M & Salvi RJ. Changes in distortion-product otoacoustic emissions during prolonged noise exposure. Hearing Research.
1999; 127:119.
13. Engdahl B & Kemp DT. The effect of noise exposure on the details of
distortion-product oto-acoustic emissions in humans. Journal of Audiological Medicine. 1998; 7:15.
14. Gorga MP, Neely ST, Ohlrich B, Hoover B, Redner J & Peter J. From
laboratory to clinic. A large scale study of distortion product otoacoustic
emissions in ears with normal hearing and ears with hearing loss. Ear &
Hearing. 1997; 18(60):440.
15. Kimberley BP, Hernandi I, Lee AM & Brown DK. Predicting pure-tone
thresholds in normal and hearing-impaired ears with distortion-product
emission and age. Ear and Hearing. 1994; 15:199.
16. Sliwinska-Kowalska M & Kotylo P. Is otoacoustic emission useful in
differential diagnosis of occupational noise-induced hearing loss? Medical
Practice. 1997; 48(6):613.
17. Vaizquez A, Luebke A, Martin G & Lonsbury-Martin B. Temporary and
permanent noise-induced changes in distortion product otoacosutic emissions in CBA/CaJ mice. Hearing Research. 2001; 156(1-2):31-43.
18. Chan VS, Wong, EC & McPherson B. Occupational hearing loss:
screening with distortion-product otoacoustic emissions. International Journal of Audiology. 2004; 43(6):323-9.
19. Marshall L & Heller L. Transient-evoked otoacoustic emissions as a
measure of noise-induced threshold shift. Journal of Speech, Language
and Hearing Research. 1998; 41:1319-34.
20. Shera CA & Guinana JJ. Evoked otoacoustic emissions arise by two
fundamentally different mechanisms: A taxonomy for mammalian OAEs.
Journal of Acoustical Society of America. 1999; 105(2)782.
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