Re-evaluating NIHL Diagnosis and Prevention Guidelines

NIHL IN MEDICOLEGAL SETTINGS

COCHRANE REVIEW UPDATES

Just before Christmas last year, we learned with great sadness that Ross Coles, PhD, had died at the age of 89. He was an outstanding leader in audiology and launched the first university degree for audiology in the United Kingdom in 1972 at the University of Southampton. So many people across the world (myself included!) are proud alumni of this MSc in Audiology program. His main interests were in diagnostic audiology, tinnitus, and noise-induced hearing loss (NIHL). He was a medical expert involved in the early trials establishing the principle that NIHL qualified for compensation in English law. In one such trial, he was in the witness box for six weeks. The work of Coles was so pioneering in this field that he co-authored the two most well-known publications used by judges and lawyers affectionately known as “The Black Book” (King, Coles, Lutman, & Robinson, 1992) and the “CLB Guidelines” (Clin Otolaryngol Allied Sci. 2000 Aug;25(4):264).

noise-induced hearing loss
noise-induced hearing loss

Excessive noise level from a steam leak in a power station
Excessive noise level from a steam leak in a power station

NIHL IN MEDICOLEGAL SETTINGS

Thinking about my own memories of Coles led me to consider his work in formalizing the guidelines for the diagnosis of NIHL in a medicolegal context. These cases usually involve a claimant who alleges that his or her hearing has been damaged by exposure to noise, usually at work. To address this case, audiometry is first done to establish if hearing loss is present. If there is hearing impairment, the court needs to be convinced that it was caused by the alleged noise exposure, on the balance of probability. Hearing damage from noise generally presents as a notch or dip in the audiogram around 4 kHz. However, there may be age-related and other causes of hearing loss that add to the noise-induced component. As such, the notch tends to disappear, leaving the noise-induced component as a bulge under the age-related hearing loss. The bulge is identified by comparing the 4 kHz threshold with anchor points usually at 1 and 8 kHz. However, there will often be some NIHL at the anchor frequencies, so this can mask the extent of noise damage. The challenge, therefore, is to establish how much of the hearing loss is noise-induced and how much is from other sources. In Coles’ last published paper, the authors offer guidelines to quantify the extent of hearing loss caused by noise (Clin Otolaryngol. 2016 Aug;41(4):347).

One approach may be to compare the claimant's hearing with the average hearing levels of people of the same age, although of course using average data does not allow for individual variation in hearing levels and will inevitably under- or overestimate in many cases. If we only knew what the age-associated hearing loss (or hearing loss from other causes) was in each case, then we would also know the noise-induced component. The original guidelines (Clin Otolaryngol Allied Sci. 2000) used the anchor points at 1 and 8 kHz to estimate the age-induced hearing loss, acknowledging that the age factor was overestimated (due to noise damage also at the anchor points). In their detailed and comprehensive paper, Coles, et al., spelled out a new procedure to quantify NIHL in more detail (Clin Otolaryngol. 2016). While the first analysis is the same as in the original paper, they proposed a second four-stage calculation to estimate the extent of noise damage at the anchor points, created modified anchor points, fit the age-related contour to this, thus leaving NIHL as the difference between the audiometric thresholds and the modified age-associated hearing loss contour. Although this sounds complicated, it is in fact straightforward to do and the paper included three worked examples. While no method is perfect and this process can only give a best estimate, it is useful to have a scientific approach to identifying how much hearing loss was likely caused by noise.

COCHRANE REVIEW UPDATES

Millions of people worldwide are exposed to dangerous levels of noise. The World Health Organization said that noise at work is responsible for 4.2 million lost years of healthy life (WHO, 2002). A recent Cochrane systematic review titled “Interventions to prevent occupational noise-induced hearing loss” assessed the effectiveness of non-drug interventions in preventing occupational noise exposure or occupational hearing loss (Cochrane Database Syst Rev. 2017 Jul 7;7:CD006396). By methodically collecting and summarizing as many compatible research papers as possible, a systematic review offers the highest level of evidence. This review was the second update of a Cochrane review originally published in 2009 (Cochrane Database Syst Rev. 2009 Jul 8;(3):CD006396). The review aimed to explore two questions:

  • Can noise exposure at work be prevented?
  • Can hearing loss associated with noise be prevented?

Engineering controls, legislation, administrative controls, personal hearing protection, and hearing surveillance were considered. Twenty-nine studies were included. The full article is 172 pages long, but a plain language summary is also available.

Unfortunately, the review results show that much work is still needed to collect robust evidence. First, there was very low-quality evidence suggesting that stricter legislation reduced noise at work. Second, several engineering case studies described reductions in noise level from buying new machinery, segregating noise sources, or installing panels around the sources. However, no long-term follow-up was done on any of the studies. Furthermore, the studies did not include noise dose measurement of the people working at the places investigated. Finally, moderate-quality evidence was found that supported the benefit of teaching people how to insert ear plugs correctly in reducing noise exposure, but more long-term monitoring and follow-up are needed. The review also found very low-quality evidence that using hearing protection as part of a hearing loss prevention program was helpful.

However, it is important to remember that the lack of high-quality evidence does not mean that these interventions are not working; it simply means we need more carefully controlled studies.

In a conversation with Reuben Peckham, a consulting engineer at 24 Acoustics who advises clients on noise reduction and serves as an expert witness in NIHL claims, he said that engineering control methods are very often available to reduce noise levels (and hence noise exposure). However, these are sometimes not practical due to the cost involved or the resulting loss in performance or production. Legislation in the United Kingdom and Europe says that actions must be taken to reduce noise exposure, with hearing protection being the last resort if noise exposure can't be reduced through other methods. Relying foremost on hearing protection is not ideal because many companies don't realize that using ear muffs along with other personal protective equipment such as protective helmets or eye protection can reduce the efficiency of their fit, thereby significantly limiting their protective function.

There's still a long way to go to successfully prevent hearing damage from noise. Monetary compensation, sadly, cannot make up for the immense consequences of damaged hearing. As such, conducting accurate reviews of available evidence is vital to making the best estimate of the amount of noise damage affecting different populations—just the right job for an interested audiologist with great attention to detail and a systematic, evidence-based approach.

Helen Cullington, PhD
Helen Cullington, PhD

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