However, in the 2013 case of Pascoe v Ministry of Defence1, the defendant successfully refuted the claim on the basis of the configuration of the audiogram, despite the guidelines seemingly being satisfied.
The principle issue here was whether a bilateral notch at 6 kHz, in the absence of any apparent additional loss at 4 kHz, could be attributed to noise exposure.
The claimant was exposed to high levels of noise in HM Dockyards in Devonport between 1979 and 1987. Breach of duty was admitted but causation remained in dispute.
It was agreed between the parties that the audiogram in the claimant's medical evidence showed bilateral high-frequency notching at 6 kHz, which satisfied Rule 3a of the guidelines. It was also agreed that R1 and R2a were satisfied.
The claimant had an audiogram prior to his employment. This showed no trace of the 6 kHz notch. The notch was, however, apparent in audiograms dated 1992, 1993, 1995, and 2005 as well as that obtained for the medico-legal report (2012).
The audiograms also suggested deterioration post-1992, which, it was agreed, could not be attributed to noise exposure. The defendant maintained that the bilateral notch at 6 kHz was not diagnostic of noise induced hearing loss (NIHL). This was because the claimant's audiogram demonstrated no significant deterioration (beyond that to be expected due to age) at 3 kHz or 4 kHz.
The defendant’s expert asserted that 4 kHz is widely accepted to be the frequency most sensitive to damage from noise and therefore the point at which damage is most frequent.
The defendant’s expert relied on the paper Noise and its Effects 2, which states "that permanent noise induced threshold shifts are characteristically most severe at or around the 4 kHz frequency" and "the typical pattern of hearing loss that results from chronic noise exposure starts at or around 4 kHz and, with continuing exposure, progresses at this frequency then begins to involve neighbouring frequencies".
The claimant’s expert sought to explain the configuration of the claimant's audiogram with reference to the impact or impulse noise to which the claimant was exposed (whilst accepting that the claimant was also exposed to steady-state noise).
The claimant’s expert cited Sulkowski's study into Forge Hammermen 3, which suggested that, in cases of impact noise, the greatest dip is typically at 6 kHz.
The defendant countered that the subjects of the paper also showed a very substantial dip at 4 kHz. Ultimately, the court preferred the defendant’s evidence that a notch at 6 kHz, in the absence of loss at 4 kHz, was not diagnostic of NIHL.
There have been other studies to examine whether notches at 6 kHz could be caused by impact noise. Two studies into the effects of noise from firearms Gravendal & Plomp 4 and Salmivalli 5, demonstrated that the average position of the audiometric notch (or peak of the bulge) occurred at 5.9 and 5.5 kHz respectively.
Nonetheless, damage was also present on the surrounding frequencies, with evidence of loss as low at the 3 kHz frequency.
The prevalence of audiometric notching at 6 kHz is not limited to occupational impact noise. A number of studies have shown a high prevalence of notches at 6 kHz in young people.
A study in Canada 6 found that 40% of the subjects demonstrated an audiometric notch or bulge, most frequently at 6 kHz.
The study suggested a possible relationship between the notching and a number of activities; listening to music, use of machinery (snowmobiles, motorcycles and chainsaws) and use of firearms.
Of the three, the only significant associating factor was listening to music.
Further studies have found that the prevalence of NIHL amongst minors increases steadily with age.
Niskar et al 7 reported audiograms of 15.5% of 12-19 year olds showing an audiometric notch, compared to a 8.5% in 6-11 year olds.
Haapanemi 8 found a similar increase (from 6.4% in younger children to 11.5% in older children). In both studies, the frequency of greatest loss was 6 kHz.
Both studies required a higher degree of notching (16 bB and 15 dB respectively) than the guidelines before a diagnosis of NIHL was made.
As the Pascoe case suggests, there appears to be little literature to prove that an audiometric notch at 6 kHz, without evidence of damage at 4kHz, can be occasioned by noise exposure.
In cases where there is some evidence of noise damage at 4 kHz but the damage is significantly greater at 6 kHz, consideration should be given to whether the claimant was exposed to steady-state or impact noise.
Furthermore, whilst the studies of Lees et al, Niskar et al and Haapanemi focus on younger people, the year these studies were undertaken should be taken into consideration (a 16 year old at the time of the Lees study would now be in their mid 40s).
There is a lack of recent studies into the presence of 6 kHz notching in adults amongst the general population.
A 2000 study by McBride and Williams 9, suggests that the reliance on impact noise may itself be flawed. This study found no correlation between impact noise exposure and 6 kHz notching.
The authors concluded that "to make a diagnosis of NIHL, it is important to elicit a detailed and accurate history of exposure to noise: although the notch at 4 kHz is a well established clinical sign and may be valuable in confirming the diagnosis, the 6 kHz notch is variable and of limited importance".
In short, where there is only notching at 6 kHz, any claim should be treated with some suspicion.
Ben Pugsley
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