Measurement of distortion product otoacoustic emissions in South African gold miners at risk for noise-induced hearing loss.

[摘要] BackgroundThe noise-exposed population in the mining industry in South Africa poses uniqueproblems to the occupational audiologist working in this environment, due to thebroad linguistic and cultural diversity in the audiology and mining environment.Unfortunately, the problems are also exacerbated by a high incidence ofpseudohypacusis within this population who are incentivised by compensation forNIHL. A solution to these specific problems would be the reliable and valid use ofan objective test of function such as the DPOAE. The rationale for the studytherefore was to extend the body of knowledge about the use of DPOAEs in thenoise-exposed mining population.MethodologyThe current study was divided into two phases: phase one’s objectives entailed theinvestigation of the characteristics of DPOAEs in a noise-exposed miningpopulation; phase two aimed to develop a multivariate regression model that wouldfacilitate the prediction of the hearing threshold levels from the DPOAE levels inthis population.ObjectivesThe objectives in phase one of the study were to investigate the bivariatecorrelations between DPOAE levels and air-conduction hearing threshold levels innoise-exposed gold miners, for the three stimulus procedures. The study alsoaimed to investigate the bivariate correlations between various pure-tone averages(PTA) and the DPOAE averages of f2 frequencies closest to those pure-tonefrequencies. Similarly, the Speech Recognition Thresholds (SRT) were correlatedwith DPOAE averages of f2 frequencies closest to the PTA.xxThe study further aimed to investigate the characteristics of DPOAEs in noiseexposedgold miners by comparing the average DPOAE levels for different agecategory groups, different ethnic groups and for different occupation types. Finally,phase one aimed to describe the characteristics of emission level and noise floordifferences (DP-NF) in a DPOAE database of a noise-exposed gold miningpopulation.Phase two of the study had the objective of developing a multivariate predictionmodel using stepwise regression analysis to identify which of the DPOAEfrequencies produced the best prediction of the audiogram frequencies whenmultivariate inputs were used for each stimulus procedure. The objective was alsoto evaluate the use of the predicted audiograms’ calculated percentage loss ofhearing (PLH) with that of the actual PLH.This retrospective record review used an audiological database from a mine in theNorth West province of South Africa that contained 4800 records. The requiredsample size to be representative of the population was statistically determined. Therecords were randomly selected resulting a sample size for the FB2-S group of161, for the FB1-S group of 177 and the FB1-S group of 155 respectively. Thehearing loss characteristics in the samples ranged from normal to profound losseswith the majority being mild to moderate hearing losses.ResultsThe 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 airconduction hearing threshold levels. Similarly, Frequency Band 1-Single (FB1-S)and Frequency Band 2-Single (FB2-S) also showed negative correlations (rangingfrom -0.203 to -0.609 and -0.274 to -0.738 respectively). These correlationstrengths have been confirmed previously by other published studies.xxiCorrelations between groups of frequencies on an audiogram and averaged matchgroups of DPOAE frequencies by intensity levels, both for PTA and SRT, rangedbetween -0.323 and -0.661. No statistically significant differences were foundbetween 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 differenceswere found between the DPOAE levels and the occupation types of mining teammembers, stopers and drillers. There was, however, a relational finding of aprogressive 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 NFlevels in the sample ranged from 0.1 to -16.8 dB SPL with the mean DP-NFdifference ranges form 0.4 to 9.3 dBSPL. More than 60% of the data collectedresulted in a DP-NF of less than 10 dB SPL.The simple correlation relationship between hearing threshold levels and DPOAEsdid not sufficiently explain the variance within the sample and due to the fact that anumber of the independent variables in the sample were highly correlated, therewas a call to use a method that allows for multicolinearity (i.e. stepwise regressionanalysis) in order to develop a prediction model. Consequently, phase two of thestudy was able to compare actual air-conduction hearing threshold levels withthose calculated with the prediction model, and then calculate predictedpercentage loss of hearing (PLH) with actual PLH found in the noise-exposed goldminers.In phase two, with the use of the predictive models, the predicted hearing thresholdlevels were found to differ from the actual thresholds by no more than 7dB HLacross all frequencies (average of 5 dB HL for FB1-R, 2 dB HL for FB1-S and 3 dBHL for FB2-S). The differences for each audiogram frequency between the actualand the predicted thresholds are represented on scatter plots in phase two of thethesis. The PLH of the predicted audiograms was calculated using the weightedxxiitables prescribed by the Compensation for Occupational Diseases and Injuries Act(COIDA). A comparison of the predicted PLH with the actual PLH indicated that thepredicted PLH ranged between minus 1.3% PLH and plus 6.7% PLH of the actualPLH.Results of the study are discussed with regards to the clinical implications, and theimplications for training occupational audiologists in South Africa. The results ofthis study will improve and inform practice in the mining environment and in thefield of compensation for NIHL. By developing a reliable prediction tool which isimplemented on an objective test proven to document the extent of damageincurred from noise-exposure, a clinician will gain greater confidence in anaccurate diagnosis, thereby further safeguarding a vulnerable population. Theresults from this study are highly relevant to the mining industry and will add valueto the industrial development of South Africa by informing the policy on hearingconservation and compensation, thereby increasing the awareness of the need forimproved occupational health and safety conditions and sustainable developmentin the mining industry.

[发布机构] University of the Witwatersrand