Background
The noise-exposed population in the mining industry in South Africa poses unique
problems to the occupational audiologist working in this environment, due to the
broad linguistic and cultural diversity in the audiology and mining environment.
Unfortunately, the problems are also exacerbated by a high incidence of
pseudohypacusis within this population who are incentivised by compensation for
NIHL. A solution to these specific problems would be the reliable and valid use of
an objective test of function such as the DPOAE. The rationale for the study
therefore was to extend the body of knowledge about the use of DPOAEs in the
noise-exposed mining population.
Methodology
The current study was divided into two phases: phase one’s objectives entailed the
investigation of the characteristics of DPOAEs in a noise-exposed mining
population; phase two aimed to develop a multivariate regression model that would
facilitate the prediction of the hearing threshold levels from the DPOAE levels in
this population.
Objectives
The objectives in phase one of the study were to investigate the bivariate
correlations between DPOAE levels and air-conduction hearing threshold levels in
noise-exposed gold miners, for the three stimulus procedures. The study also
aimed to investigate the bivariate correlations between various pure-tone averages
(PTA) and the DPOAE averages of f2 frequencies closest to those pure-tone
frequencies. Similarly, the Speech Recognition Thresholds (SRT) were correlated
with DPOAE averages of f2 frequencies closest to the PTA.
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The study further aimed to investigate the characteristics of DPOAEs in noiseexposed
gold miners by comparing the average DPOAE levels for different age
category groups, different ethnic groups and for different occupation types. Finally,
phase one aimed to describe the characteristics of emission level and noise floor
differences (DP-NF) in a DPOAE database of a noise-exposed gold mining
population.
Phase two of the study had the objective of developing a multivariate prediction
model using stepwise regression analysis to identify which of the DPOAE
frequencies produced the best prediction of the audiogram frequencies when
multivariate inputs were used for each stimulus procedure. The objective was also
to evaluate the use of the predicted audiograms’ calculated percentage loss of
hearing (PLH) with that of the actual PLH.
This retrospective record review used an audiological database from a mine in the
North West province of South Africa that contained 4800 records. The required
sample size to be representative of the population was statistically determined. The
records were randomly selected resulting a sample size for the FB2-S group of
161, for the FB1-S group of 177 and the FB1-S group of 155 respectively. The
hearing loss characteristics in the samples ranged from normal to profound losses
with the majority being mild to moderate hearing losses.
Results
The findings of phase one showed negative correlations ranging from -0.327 to
-0.573 for Frequency Band 1- Replicated (FB1-R) between DPOAE levels and air
conduction hearing threshold levels. Similarly, Frequency Band 1-Single (FB1-S)
and Frequency Band 2-Single (FB2-S) also showed negative correlations (ranging
from -0.203 to -0.609 and -0.274 to -0.738 respectively). These correlation
strengths have been confirmed previously by other published studies.
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Correlations between groups of frequencies on an audiogram and averaged match
groups of DPOAE frequencies by intensity levels, both for PTA and SRT, ranged
between -0.323 and -0.661. No statistically significant differences were found
between the DPOAE measurements and ethnic groups of African and Caucasian
(Sample size of 175 for FB1-S, 137 for FB1-R and 161 for FB2-S). No differences
were found between the DPOAE levels and the occupation types of mining team
members, stopers and drillers. There was, however, a relational finding of a
progressive decrement of DPOAE intensity levels by decade of age increase
(Sample size of 37 for FB1-S, 45 for FB1-R and 155 for FB2-S).
Mean DP levels in this population ranged from 1.5 to -14 dB SPL, and mean NF
levels in the sample ranged from 0.1 to -16.8 dB SPL with the mean DP-NF
difference ranges form 0.4 to 9.3 dBSPL. More than 60% of the data collected
resulted in a DP-NF of less than 10 dB SPL.
The simple correlation relationship between hearing threshold levels and DPOAEs
did not sufficiently explain the variance within the sample and due to the fact that a
number of the independent variables in the sample were highly correlated, there
was a call to use a method that allows for multicolinearity (i.e. stepwise regression
analysis) in order to develop a prediction model. Consequently, phase two of the
study was able to compare actual air-conduction hearing threshold levels with
those calculated with the prediction model, and then calculate predicted
percentage loss of hearing (PLH) with actual PLH found in the noise-exposed gold
miners.
In phase two, with the use of the predictive models, the predicted hearing threshold
levels were found to differ from the actual thresholds by no more than 7dB HL
across all frequencies (average of 5 dB HL for FB1-R, 2 dB HL for FB1-S and 3 dB
HL for FB2-S). The differences for each audiogram frequency between the actual
and the predicted thresholds are represented on scatter plots in phase two of the
thesis. The PLH of the predicted audiograms was calculated using the weighted
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tables prescribed by the Compensation for Occupational Diseases and Injuries Act
(COIDA). A comparison of the predicted PLH with the actual PLH indicated that the
predicted PLH ranged between minus 1.3% PLH and plus 6.7% PLH of the actual
PLH.
Results of the study are discussed with regards to the clinical implications, and the
implications for training occupational audiologists in South Africa. The results of
this study will improve and inform practice in the mining environment and in the
field of compensation for NIHL. By developing a reliable prediction tool which is
implemented on an objective test proven to document the extent of damage
incurred from noise-exposure, a clinician will gain greater confidence in an
accurate diagnosis, thereby further safeguarding a vulnerable population. The
results from this study are highly relevant to the mining industry and will add value
to the industrial development of South Africa by informing the policy on hearing
conservation and compensation, thereby increasing the awareness of the need for
improved occupational health and safety conditions and sustainable development
in the mining industry.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/7571 |
| Date | 26 February 2010 |
| Creators | Edwards, Anita Lynne |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf |
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