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Recently, we showed that individual differences in OAE levels may be a useful predictor of NIHL (Lapsley Miller et al., 2006), but it is unknown whether these findings reflect innate or acquired risk factors. Regardless, the ability to predict those personnel more at risk for NIHL is a significant step towards reducing its incidence.
In this study, OAEs and hearing levels were measured in a group of 338 sailors stationed on an aircraft carrier. Measurements were made before and after a six-month operational deployment, during which there were approximately 6,000 aircraft launches and recoveries. The sailors in the study all worked in hazardous noise environments. Fifteen sailors (18 ears) had confirmed PTS after the deployment. Despite the fact that all the sailors in the study had normal hearing, many had very low-level or no OAEs.
We were interested to see if having low-level OAEs at pre-deployment testing was predictive of PTS at post-deployment testing. To do this, we used Bayesian statistics to calculate the PTS rate (which was approximately 3%) and the positive predictive value, which in this case is the conditional probability of a PTS given a positive OAE test result, defined to be an OAE level below a cutoff value (Zhou, Obuchowski, & McClish, 2002). The positive predictive value is very useful because it allows clinicians to assign an actual probability of PTS based on a test result.
To calculate the positive predictive value, the sailors were split into two groupsСthose who acquired a PTS and those who did not. A cutoff was chosen based on OAE amplitude. For the two groups, we counted how many sailors also had a pre-deployment OAE level less than the cutoff. We then calculated the positive predictive value for that cutoff. By systematically varying the cutoff over the range of OAE amplitude, the entire positive predictive value function can be plotted. The results are shown in Figure 1 [PDF] for the best OAE predictor, which was the TEOAE amplitude in the 4 kHz half-octave band. The straight dotted line indicates the PTS rate. The solid line shows the calculated positive predictive value. For our population, an OAE test result less than 0 dB SPL indicated an increased risk for PTS. This risk increased markedly as TEOAE amplitude went below -5 dB SPL.
This example provides only a proof of concept; it does not provide risk levels for use in the clinic. This is because risk depends on the specific noise exposure, length of exposure, population demographics, and other factors. Our results apply only to the population of male sailors with normal hearing on aircraft carriers who are deployed for six months. Furthermore, these results were based on 524 ears with no PTS and 17 ears with PTS (eight of whom had noise floor substitutions made for missing TEOAEs).
These are not enough cases on which to build norms; we encourage others to do similar studies with other populations and noise exposures.
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