INTRODUCTION
We first looked at asymmetry in noise-induced hearing loss (NIHL) claims in edition 24 of BC Disease News. This week, we revisit the issue in the context of recent medical studies which consider whether asymmetry is a consequence of noise exposure and if so when.
Next week we look at how the courts have dealt with asymmetry with a review of the relevant NIHL case law.
WHAT IS ASYMMETRY?
Differences in thresholds at corresponding frequencies between the ears is inevitable as a consequence of the variability of pure tone audiometry. Small differences in thresholds are bound to occur simply as a result of the imprecise nature and errors of audiometry, as was outlined in the recent feature on audiometric variability in edition 160 of BC Disease News here.
However where the hearing thresholds between the ears of an individual differ significantly at corresponding frequencies than is referred to as asymmetrical hearing loss with the audiograms displaying asymmetry. What is a significant difference? As defined by Alberti[i] if the threshold differences are greater than 10dB between the ears - the generally accepted level of normal audiometric variability - then this can be defined as asymmetry.
NIHL on the other hand has been described as being typically bilateral and is usually considered to cause similar amounts of damage to both ears5, since most occupational noise is symmetrical.[ii]
How common is asymmetry in the non-exposed population and what are its causes? How does the presence of asymmetry sit with a diagnosis of NIHL? Can noise exposure give rise to asymmetry and if so in what circumstances?
GENERAL INCIDENCE OF ASYMMETRY
A 2009 study by Lutman and Coles,[iii] sought to determine the incidence of asymmetry in non-noise exposed individuals. Audiograms from 2679 participants were analysed. Lutman and Coles estimated that around 1 % of the UK population aged 18-80 years have an asymmetry of 15 dB or more, based on the average of 0.5, 1, 2 and 4 kHz and the prevalence of asymmetry was greater in older people than in younger people.
CAUSES OF ASYMMETRICAL HEARING LOSS
Frequent ear infections, Ménière disease and ear surgery may result in asymmetry.[iv]
The causes of asymmetry were identified in the CLB Guidelines 2000 at Section 12 and Note 11 of the Guidelines as follows:
The Guidelines also recognise that in some cases the cause of asymmetry will be unknown.
In their 2009 study by Lutman and Coles on asymmetry, for participants with the largest asymmetry, detailed examination of the original data, and consideration of factors such as family history of deafness, head injury, ear injury, administration of ototoxic medication, general health factors including hypertension and history of infectious diseases, failed to reveal a potential cause of the asymmetry. This led Lutman and Coles to conclude that many hearing asymmetries in the general population can be expected to be of unknown causation.
It seems then that unknown causation of asymmetry in a population is common and so it’s presence alongside NIHL does not mean that it has been caused by asymmetrical noise exposure - unless there is good evidence to show otherwise.
ASYMMETRY AND FIREARMS NOISE
Many studies support the conclusion that exposure to firearms noise often results in asymmetry with the left ear worse affected than the right. This is said to arise from the ‘head shadowing’ effect and the right ear being ‘protected’ within the shoulder of the person using the firearm.
Cox and Ford assessed the asymmetry of 225 soldiers exposed to a variety of weapon noise who had deterioration of hearing.[v] At 0.5 and 1 kHz the asymmetry was 10 dB or less in 90 % of cases, and the hearing thresholds rarely exceeded 25 dB. However, the degree of hearing loss and asymmetry increased as the frequency increased, and the average loss at 2, 3, 4 and 6 kHz was significantly greater in the left ear.
Chung and colleagues also analysed audiograms of 29,953 workers with histories of shooting in addition to occupational noise exposure[vi] and they found that shooting was associated with AHL. In this study asymmetry was significant only at 2 kHz and above and most prominent at 4 kHz. The difference between the ears increased with longer shooting history.
Additionally, Prosser found that among 133 railway workers who also hunted for sport outside of work and 82 workers who did not, hunters had significantly worse hearing than non-hunters in the ear closest to the barrel of the gun. [vii]
ASYMMETRY AND OCCUPATIONAL (NON-FIREARMS) EXPOSURE
Alberti et al, 1979,[viii] looked at 1,873 consecutive patients referred for compensation assessment for presumed industrial hearing loss, and found that 281 (15 %) had an average difference in hearing threshold between the ears of 15 dB at 0.5, 1, 2 and 4 kHz.
Chung et al, 1983,[ix] considered1461 audiometric records of claims for NIHL, it was found that 69 (4.7 %) had a well-defined pattern or hearing loss, in which only 2 kHz is asymmetrical by 20 dB or more.
Barrs et al., 1993,[x] analysed the audiological tests of 246 workers who underwent otologic and audiologic testing as part of a worker’s compensation claim for work-related NIHL and asymmetric hearing loss was found in 28 patients (20%) with asymmetry being defined as an average difference of at least 15 dB at 0.5,1,2 and 3 kHz.
More recent studies also suggest that asymmetry can be a consequence of general occupational noise exposure and can arise without the ‘head shadowing’ effect suggested in firearms exposure studies. The competing theory put forward in the recent studies is that the left ear is simply physiologically more susceptible to noise damage than the right ear.
Broste found that hearing loss at higher frequencies was also observed more often in the left ear of farming high school students.[xi] He suggested this was due to drivers habitually observing their work by looking over their right shoulder and therefore shielding their right ear from the noise of the engine. This ‘head shadowing’ theory and has been doubted by Berg and colleagues in their 2014 paper in which they highlighted that there is little published evidence to support the theory.[xii] Instead they conclude that the physiological explanation of a left ear being more susceptible to NIHL is a more credible alternative.
A study by Fenandes and Fernandes in 2010 claims the incidence of asymmetric hearing loss in the unexposed population is significantly lower than that incidence among the noise-exposed population it must therefore be the case that noise exposure causes asymmetrical hearing loss. (We consider the Fernandes and Fernandes study in more detail next week).
SO DOES OCCUPATIONAL NOISE EXPOSURE CAUSE ASYMMETRY?
This was revisited recently by Dobie, in a study published in 2014, which aimed to determine whether occupational noise exposure increases audiometric asymmetry.[xiii] The paper notes that asymmetry is common in both clinic samples and the general population, particularly at higher frequencies and when both ears tend to have higher thresholds.
Data for this study were taken from the Occupational Noise and Hearing Survey (ONHS), carried out by the National Institute for Occupational Safety and Health (NIOSH, United States). Audiometry was obtained for 2044 men, mostly from printing and steel fabrication plants, who were divided into four groups:
It was found that there was more asymmetry when average thresholds were higher. Also, age was a strong predictor of average thresholds, but had little if any effect on asymmetry.
Both the noise exposed men and those who failed the screening criteria had significantly worse binaural average hearing than those who were not noise exposed, with left ears being worse than right ears in all four groups by around 1 to 2 dB across the frequencies 0.5,1,2 and 3 kHz and 2 to 4 dB worse across 3, 4 and 6 kHz.
The most important finding from this paper is that there were no significant differences in asymmetry related to occupational noise exposure. There were no significant differences between NE and NNE groups after controlling for overall average amount of hearing loss. Though asymmetry increases with greater overall hearing loss, when this was taken into account, men with occupational noise exposure did not have more asymmetry compared with that in men in the same workplaces who were not exposed to noise.
The author offers several examples to demonstrate how occupational noise could feasibly affect audiometric asymmetry, in spite of the findings of this study. A worker who originally had asymmetry might suffer more NIHL in the better ear than the worse ear, and thus his audiogram would become more symmetrical. Another worker with very asymmetric noise exposure may develop an asymmetrical NIHL. However, the findings from this study suggest that, for most workers in general industry, occupational noise does not cause or contribute to asymmetrical hearing loss.
A 2016 review by John Phillips and colleagues aimed to collate the overall prevalence of unexplained asymmetrical hearing loss in subjects with NIHL and to provide a balanced argument regarding causality.[xiv] The analysis suggested that 2.4 to 22.6 % of subjects with NIHL had asymmetrical hearing loss, with the left side having the greater deficit in the majority of reported cases (around 60-80 %). The two theories of asymmetry development, a physiological increased susceptibility to NIHL in the left ear and preferential protection of the right ear, were discussed. It is of interest that the majority of subjects exhibiting greater loss in the left ear occurred even when taking handedness into account i.e. which was the dominant hand. The reviewers suggest that this casts doubt on the “head shadowing effect”. [However, “head shadowing could occur independently of handedness, such as turning the head over the right shoulder in a tractor because the controls for trailing equipment are on the right of the driver as shown in Broste].
Studies included in the review showed that asymmetrical hearing loss increased with advancing age. According to the reviewers, this finding may be related to the influence of AAHL, which will act to exacerbate asymmetry. It was not possible to undertake a meta-analysis of the data from the included studies, because different definitions of asymmetry were used. Unsurprisingly, the study with the most generous definition of asymmetry-the 2010Fernandes & Fernandes gave the highest prevalence. For most studies, no data existed for number of years of employment, use of hearing protection and other risk factors such as leisure time noise exposure, smoking, hypertension, ethnicity and diabetes. Therefore, poorer hearing in noise-exposed groups may be attributed, at least in part, to these factors. If any of these factors can also contribute to asymmetrical hearing loss, another source of bias is introduced. The authors concluded that the studies reviewed provide limited evidence for the existence of asymmetrical hearing loss in subjects with occupational noise exposure. This is particularly apparent when adjusted for significant variables such as age, sex and high frequency binaural hearing loss.
CONCLUSION
Asymmetry may have a number of causes or simply be unexplained. Unexplained asymmetry seems to be fairly common within the non-noise exposed population and so there is no reason to assume that it should be uncommon within cases of NIHL – particularly where the population is older and the incidence of asymmetry is likely to increase.
Recent studies and reviews do not support the proposition that general occupational noise exposure - as opposed to unilateral firearms exposure-is a likely cause of asymmetry.
Next week, in part 2 of this feature, we consider how the courts have considered the issue of asymmetry and compatibility with a diagnosis of NIHL.
[i] Noise and the ear, Scott-Brown’s Otolaryngology, Butterworths, Chapter 18, page 627, P.W. Alberti.
[ii] Kirchner, D.B. et al. Occupational Noise-Induced Hearing Loss: ACEOEM Task Force On Occupation Hearing. J. Occup Environ Med. 54. 106-108 (2012).
[iii] Lutman, M.E. & Coles, R.A. Asymmetric Sensorineural Hearing Thresholds In The Non-Noise-Exposed UK Population: A retrospective analysis. Clin. Otalaryngol. 34, 316-321 (2009).
[iv] Dobie, R.A. Does Occupational Noise Cause Asymmetric Hearing Loss? Ear Hear. 35, 577-579 (2014).
[v] Cox, H.J & Ford, G.R. Hearing Loss Associated With Weapons Noise Exposure: When To Investigate An Asymmetrical Loss. J. Laryngol. Amp Otol. 109, 291-295 (1995).
[vi] Chung, D.Y., Gannon, R.P., Wilson, G.N & Mason, K. Shooting, sensorineural hearing loss, and workers’ compensation. J. Occup. Med. Off. Publ. Ind. Med. Assoc. 23, 481-484 (1981).
[vii] Broste, S.K., Hansen, D.A., Strand, R.L. & Stueland, D.T. Hearing Loss Among High School Farm Students. AM. J Public Health 79, 619-622 (1989).
[viii] Alberti, P.W., Symons, F. & Hyde, M.L. Occupational Hearing Loss The Significance of Asymmetrical Hearing Thresholds. Acta Otolaryngol. (Stock) 87, 255-263 (1979)
[ix] Ibid at 20.
[x] Barrs, D. M., Althoff, L. K., Krueger, W. W. O. & Olsson, J. E. Work-Related, Noise-Induced Hearing Loss: Evaluation Including Evoked Potential Audiometry. Otolaryngol. -- Head Neck Surg. 110, 177–184 (1994).
[xi] Broste, S.K., Hansen, D.A., Strand, R.;. & Stueland, D.T. Hearing loss among high school farm students. Am. J. Public Health 79, 619-622 (1989).
[xii] Berg, R.L., Pickett, W., Linneman, J.G., Wood, D.J. & Marlenga, B. Asymmetry in noise-induced hearing loss: Evaluation of two competing theories. Noise Health 16, 102 (2014).
[xiii] Ibid at 18.
[xiv] Masterton, L., Howard, J., Liu, Z. & Phillips, J. Asymmetrical Hearing Loss in Cases of Industrial Noise Exposure: A Systematic Review of the Literature. Otol. Neurotol. Off. Publ. Am. Otol. Soc. Am. Neurotol. Soc. Eur. Acad. Otol. Neurotol. 37, 998–1005 (2016).