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First, Do No Harm: Temporary Threshold Shift Screening Is Not Worth the Risk

Posted on by Christa L. Themann, MA, CCC-A

 

Recently, a study by Dr. Hanns Moshammer and colleagues on “The Early Prognosis of Noise-Induced Hearing Loss” garnered national media attention.[1] Their research, published in Occupational and Environmental Medicine, [2] recommended routine implementation of a temporary threshold shift (TTS) screening test to identify workers particularly at risk of developing noise-induced hearing loss (NIHL) from occupational exposure to hazardous noise. NIHL is one of the most common work-related conditions in the United States. Susceptibility to NIHL varies across individuals, but unfortunately, no methods are available to predict risk for a particular worker.

Moshammer et al. exposed newly-hired employees to a 20 minute, high-intensity, low frequency experimental noise and measured the resulting TTS. Then they followed the workers over time to see who ultimately developed a permanent threshold shift (PTS). The authors reported that a TTS of 14 dB or more measured 2.5 minutes after the experimental exposure identified workers at greater risk for PTS. They recommended routine implementation of their TTS procedure to screen for susceptibility to noise in occupational hearing loss prevention programs.

However, we do not believe the study results support this recommendation. The false-positive rate in the study was 30%, which means that – if this procedure was implemented – nearly one-third of workers would be incorrectly told that they are particularly at risk for NIHL. This could cause unnecessary alarm and open the door to potential discrimination in work assignments, promotions, etc. The false-negative rate in the study was 18%. These mis-identified workers would be incorrectly told they were not at risk, potentially creating a false sense of security and leading to laxity in the use of hearing protective devices.

The hypothesis that TTS might predict future PTS is not new. However, extensive research over the past 80 years has produced mixed results regarding the relationship between TTS and PTS.[3-6] No simple, robust relationship between TTS and PTS has been found. One reason is that many factors in addition to individual susceptibility influence the amount of PTS a person develops. These factors include the type of occupational noise exposure, the level of protection obtained from hearing protectors, additional noise exposure outside of work, other hearing risks such as ototoxicants and trauma, general health conditions, and biological factors including age, gender and race.[4, 7] Furthermore, recent animal research indicates that the underlying mechanisms for PTS and TTS may be different and unrelated [8]. All of these issues could explain why a consistent relationship between TTS and PTS has been elusive. While Moshammer et al.’s work contributes to the literature on the topic, their findings must be considered in light of the whole body of research. Recommendation of a particular TTS screening procedure on the basis of their results alone is premature.

A long-standing principle of medical ethics is “First, do no harm.” New evidence indicates that TTS-inducing exposures may cause an irreversible loss of neural synapses and degeneration of the cochlear nerve even after hearing thresholds completely recover.[9] This means that a TTS screening test could possibly cause permanent auditory damage to workers.  In addition, the TTS exposure used in the study exceeded NIOSH-recommended noise exposure limits[10] as well as the legal exposure limits in many countries, including Australia, Finland, France, Germany, Italy, the Netherlands, Norway, Sweden and the UK.[11] Some workers in the Moshammer study experienced alarming temporary hearing shifts – as much as 38 dB. We must have very good reasons to purposely put someone’s hearing at risk before we recommend TTS screening as routine practice in hearing loss prevention programs.

NIHL is an important occupational health issue, and the ability to predict who is most at risk could certainly help reduce the burden of hearing loss among American workers. But prevention is more important than prediction, and we already know how to prevent NIHL through reduction of noise levels and consistent use of properly fit hearing protection devices. Further research on susceptibility and the utility of TTS screening has a place and could eventually lead to improved approaches to protecting the millions of workers exposed to noise on-the-job.   However, prognostic TTS screening is not warranted at this time.

For additional discussion of issues related to TTS screening, see our commentary “Early Prognosis of Noise-Induced Hearing Loss: Prioritising Prevention over Prediction” in Occupational and Environmental Medicine. For more information on the risks of occupational noise exposure and how to prevent work-related hearing loss, see the NIOSH noise and hearing loss prevention topic page.

 

Christa L. Themann, MA, CCC-A, is an audiologist in the NIOSH Division of Applied Research and Technology.

 

References

  1. Kennedy M (2014). New test may predict worker hearing loss. Reuters. Available online at http://www.reuters.com/article/2014/08/29/us-workers-noise-hearing-test-idUSKBN0GT1X120140829.
  2. Moshammer H, Kundi M, Wallner P, Herbst A, Feuerstein A, Hutter HP. Early prognosis of noise-induced hearing loss. Occup Environ Med 2014; Online First: 25 July 2014 doi:10.1136/oemed-2014-102200.
  3. Melnick W. Human temporary threshold shift (TTS) and damage risk. J Acoust Soc Am. 1991 Jul;90(1):147-54.
  4. Henderson D, Subramaniam M, Boettcher FA. Individual susceptibility to noise-induced hearing loss: an old topic revisited. Ear Hear 1993;14:152–168.
  5. Ward WD. Endogenous factors associated with susceptibility to damage from noise. Occup Med 1995:10(3):561-75.
  6. Quaranta A, Portalatini P, Henderson D. Temporary and permanent threshold shift: an overview. Scand Audiol Suppl 1998;48(Suppl 48):75–86.
  7. Humes LE. Noise-induced hearing loss as influenced by other agents and by some physical characteristics of the individual. J Acoust Soc Am 1984;76:1318–1329.
  8. Nordmann AS, Bohne BA, Harding GW. Histopathological differences between temporary and permanent threshold shift. Hear Res 2000;139:13–305.
  9. Kujawa SG, Liberman MC. Adding insult to injury: cochlear nerve degeneration after “temporary” noise-induced hearing loss. J Neurosci 2009;29:14077–14085.
  10. NIOSH [1998]. NIOSH criteria for a recommended standard: occupational noise exposure; revised criteria. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 98–126.
  11. Suter AH. Standards and regulations. In: Suter AH ed. 47: Noise, in: Stellman JM ed. Encyclopedia of Occupational Health and Safety. Geneva, International Labor Organization, 2011. http://www.ilo.org/oshenc/part-vi/noise/item/755-standards-and-regulations (accessed 5 Sept 2014).
Posted on by Christa L. Themann, MA, CCC-A

9 comments on “First, Do No Harm: Temporary Threshold Shift Screening Is Not Worth the Risk”

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    I could not agree more with these comments. I was a bit shocked to see that the authors in answer to my comments just reiterated that “an exposure from a 20 min TTS test, can have consequences on the integrity of the auditory system”. They mean to say can cause permanent hearing loss. Then they continue and say “But compared to the years of occupation in noisy occupational settings with hours of daily exposure, we do not think it unethical to perform such a TTS test that may prove useful in individual counselling”. Translated to plain language this would mean that you would induce hearing loss in some to prevent it in others. I don’t think this is a tenable position. There is only one conclusion possible: TTS-tests should not be performed.

    A screening test with 82 % sensitivity and 53 % specificity and an area under the ROC curve of 0.67 (CI: 0.58 to 0.75) leaves a great deal to be desired. The predictive accuracy of a positive result from this test would not be sufficient for widespread application. For the positive predictive value of such a test to be greater than 50 %, the prevalence of hearing impairment among exposed workers would need to be at or above 37 % at the time of outcome assessment. Among a population of workers with a 37 % prevalence of hearing impairment, there should be no question that the benefits of redoubled prevention efforts would outweigh by far any test having a 50 % chance of identifying a worker with increased susceptibility (and the complementary 50 % chance of the test resulting in a false positive).

    The need to accurately predict an individual’s degree of susceptibility to noise induced hearing loss (NIHL) in high level industrial noise environments is an important objective for hearing conservation purposes. The temporary threshold shift (TTS) test promoted by Moshammer et al, however, is not a reliable or practical method to accomplish this objective. More specifically, I concur with the viewpoints by Themann CL, et al. regarding the utility of the TTS test procedure, and associated study results, presented by Moshammer et al. to predict one’s risk for developing NIHL as follows:

    The relationship between TTS and PTS is not straightforward, (i.e, each may be related to different underlying physiological mechanisms), it will be difficult to obtain accurate TTS test measures due to confounding procedural variables in most industrial settings, and TTS testing may pose a risk in those highly susceptible to noise exposure. Additionally, the use of a continuous noise to create TTS in the Moshammer et al. study, ignores the potential differences posed by various hazardous types of noise (e.g., Gaussian and non-Gaussian, intermittent and time varying, etc.) in creating NIHL. The reliability of the test results in the Moshammer et al. study (e.g., 82% sensitivity and 70% specificity) is also not sufficient to support the use of the TTS test as a predictor of NIHL due to high false positive and negative rates.

    While a reliable test to predict one’s susceptibility to NIHL remains elusive, the need to better understand the potential hazardous effects of different types of industrial noise on hearing is of paramount importance for hearing conservation purposes.

    1.Themann CL, et al. Early prognosis of noise-induced hearing loss: prioritizing prevention over prediction Occup Environ Med Month 2014 Vol 0 No 0

    2. Moshammer H, Kundi M, Wallner P, et al. Early prognosis of noise-induced hearing loss. Occup Environ Med 2014. Published Online First: 25 July 2014. doi:10.1136/oemed-2014-102200

    I was surprised to read the article summary of the study by Moshammer et al. I did not read the original article – but if I had I would be curious to know if the study they described was IRB approved by an institutional review board. My own opinion is that I would not have approved such a study if serving on the IRB board.

    Further, to expose workers to high intensity low frequency noise that was demonstrated in their study to produce PTS is unconscienable in my opinion. Two persons have already expressed their opinions about the study and recommendation – and I agree. Further, Christy Themann’s recommendation to “do no harm” is a common sense response to attempting to predict workers at greater risk for PTS – and known solutions to preventing PTS are scientifically and practically much more defensible.

    The scientific data do not support TTS as a predictor of PTS. Animal studies (e.g., Kujawa & Liberman, 2006, 2009; Lin et al., 2011; Furman et al., 2013, and others) demonstrate that irreversible neural degeneration occurs after noise exposures that produce TTS. This neural degneration is believed to contribute not only to hearing loss, but also to tinnitus, hyperacusis and difficulties with speech recognition in noise. Given these factors, I believe it is unconscionable to knowinlgy submit human subjects to procedures that produce TTS, and I wonder what life-long effects the subjects in this study will suffer as a consequence of their participation.

    The need to accurately predict an individual’s degree of susceptibility to noise induced hearing loss (NIHL) in high level industrial noise environments is an important objective for hearing conservation purposes. The temporary threshold shift (TTS) test promoted by Moshammer et al, however, is not a reliable or practical method to accomplish this objective.

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