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

摘要

BackgroundudThe noise-exposed population in the mining industry in South Africa poses uniqueudproblems to the occupational audiologist working in this environment, due to theudbroad linguistic and cultural diversity in the audiology and mining environment.udUnfortunately, the problems are also exacerbated by a high incidence ofudpseudohypacusis within this population who are incentivised by compensation forudNIHL. A solution to these specific problems would be the reliable and valid use ofudan objective test of function such as the DPOAE. The rationale for the studyudtherefore was to extend the body of knowledge about the use of DPOAEs in theudnoise-exposed mining population.udMethodologyudThe current study was divided into two phases: phase one’s objectives entailed theudinvestigation of the characteristics of DPOAEs in a noise-exposed miningudpopulation; phase two aimed to develop a multivariate regression model that wouldudfacilitate the prediction of the hearing threshold levels from the DPOAE levels inudthis population.udObjectivesudThe objectives in phase one of the study were to investigate the bivariateudcorrelations between DPOAE levels and air-conduction hearing threshold levels inudnoise-exposed gold miners, for the three stimulus procedures. The study alsoudaimed to investigate the bivariate correlations between various pure-tone averagesud(PTA) and the DPOAE averages of f2 frequencies closest to those pure-toneudfrequencies. Similarly, the Speech Recognition Thresholds (SRT) were correlatedudwith DPOAE averages of f2 frequencies closest to the PTA.udxxudThe study further aimed to investigate the characteristics of DPOAEs in noiseexposedudgold miners by comparing the average DPOAE levels for different ageudcategory groups, different ethnic groups and for different occupation types. Finally,udphase one aimed to describe the characteristics of emission level and noise flooruddifferences (DP-NF) in a DPOAE database of a noise-exposed gold miningudpopulation.udPhase two of the study had the objective of developing a multivariate predictionudmodel using stepwise regression analysis to identify which of the DPOAEudfrequencies produced the best prediction of the audiogram frequencies whenudmultivariate inputs were used for each stimulus procedure. The objective was alsoudto evaluate the use of the predicted audiograms’ calculated percentage loss ofudhearing (PLH) with that of the actual PLH.udThis retrospective record review used an audiological database from a mine in theudNorth West province of South Africa that contained 4800 records. The requiredudsample size to be representative of the population was statistically determined. Theudrecords were randomly selected resulting a sample size for the FB2-S group ofud161, for the FB1-S group of 177 and the FB1-S group of 155 respectively. Theudhearing loss characteristics in the samples ranged from normal to profound lossesudwith the majority being mild to moderate hearing losses.udResultsudThe findings of phase one showed negative correlations ranging from -0.327 toud-0.573 for Frequency Band 1- Replicated (FB1-R) between DPOAE levels and airudconduction hearing threshold levels. Similarly, Frequency Band 1-Single (FB1-S)udand Frequency Band 2-Single (FB2-S) also showed negative correlations (rangingudfrom -0.203 to -0.609 and -0.274 to -0.738 respectively). These correlationudstrengths have been confirmed previously by other published studies.udxxiudCorrelations between groups of frequencies on an audiogram and averaged matchudgroups of DPOAE frequencies by intensity levels, both for PTA and SRT, rangedudbetween -0.323 and -0.661. No statistically significant differences were foundudbetween the DPOAE measurements and ethnic groups of African and Caucasianud(Sample size of 175 for FB1-S, 137 for FB1-R and 161 for FB2-S). No differencesudwere found between the DPOAE levels and the occupation types of mining teamudmembers, stopers and drillers. There was, however, a relational finding of audprogressive decrement of DPOAE intensity levels by decade of age increaseud(Sample size of 37 for FB1-S, 45 for FB1-R and 155 for FB2-S).udMean DP levels in this population ranged from 1.5 to -14 dB SPL, and mean NFudlevels in the sample ranged from 0.1 to -16.8 dB SPL with the mean DP-NFuddifference ranges form 0.4 to 9.3 dBSPL. More than 60% of the data collectedudresulted in a DP-NF of less than 10 dB SPL.udThe simple correlation relationship between hearing threshold levels and DPOAEsuddid not sufficiently explain the variance within the sample and due to the fact that audnumber of the independent variables in the sample were highly correlated, thereudwas a call to use a method that allows for multicolinearity (i.e. stepwise regressionudanalysis) in order to develop a prediction model. Consequently, phase two of theudstudy was able to compare actual air-conduction hearing threshold levels withudthose calculated with the prediction model, and then calculate predictedudpercentage loss of hearing (PLH) with actual PLH found in the noise-exposed goldudminers.udIn phase two, with the use of the predictive models, the predicted hearing thresholdudlevels were found to differ from the actual thresholds by no more than 7dB HLudacross all frequencies (average of 5 dB HL for FB1-R, 2 dB HL for FB1-S and 3 dBudHL for FB2-S). The differences for each audiogram frequency between the actualudand the predicted thresholds are represented on scatter plots in phase two of theudthesis. The PLH of the predicted audiograms was calculated using the weightedudxxiiudtables prescribed by the Compensation for Occupational Diseases and Injuries Actud(COIDA). A comparison of the predicted PLH with the actual PLH indicated that theudpredicted PLH ranged between minus 1.3% PLH and plus 6.7% PLH of the actualudPLH.udResults of the study are discussed with regards to the clinical implications, and theudimplications for training occupational audiologists in South Africa. The results ofudthis study will improve and inform practice in the mining environment and in theudfield of compensation for NIHL. By developing a reliable prediction tool which isudimplemented on an objective test proven to document the extent of damageudincurred from noise-exposure, a clinician will gain greater confidence in anudaccurate diagnosis, thereby further safeguarding a vulnerable population. Theudresults from this study are highly relevant to the mining industry and will add valueudto the industrial development of South Africa by informing the policy on hearingudconservation and compensation, thereby increasing the awareness of the need forudimproved occupational health and safety conditions and sustainable developmentudin the mining industry.