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

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).