Using 6 kHz as an Anchor Point: Percy v Northern Gas Networks & 2 Ors. (Newcastle County Court, 15 June 2018)

Case judgment has recently been handed down in Percy v Northern Gas Networks & 2 Ors. (Newcastle County Court, 15 June 2018). His Honour Judge Freedman ruled on the correct interpretation of the Coles Guidelines, in respect of bulge analysis, for the purpose of ensuring correct diagnosis of NIHL.

The 73 year-old claimant alleged that he had been exposed to excessive noise during his employment with all 3 defendants. However, the claim against the 2nd defendant was dismissed.

In his employment with the 1st defendant, between 1979 and 1987, he worked as a fitter. In his employment with the 3rd defendant, at various times during the 1980’s and 1990’s, he worked as a shot blaster.

The 1st and 3rd defendants conceded breach of duty.

Instructed engineer, Mr Worthington, found that it was ‘highly likely that the daily noise exposure level was well in excess of 90 dB(A) LEP,d during his employment with the 1st and 3rd defendants’.

In respect of medical evidence, Mr Parker was instructed for the defendants, while Mr Johnson was instructed on behalf of the claimant. The claimant’s hearing was represented on 2 audiograms, one produced in 2013 and another, produced in 2015.

According to the 2013 audiogram, the claimant’s mean bilateral hearing threshold level (HTL) across 1-3 kHz was 33.6 dB. Over the same frequency range, according to the 2015 audiogram, hearing losses were calculated as 42.0 dB. His hearing loss was significant and he had mild tinnitus.

Although R1 and R2 of the CLB Guidelines were satisfied in both audiograms, the medical experts were in disagreement over the correct anchor points to use when undertaking bulge analysis of audiograms, i.e. compliance with R3(a).

Paragraphs 7.1 and 7.2 of the CLB Guidelines state that for R3(a) to be satisfied:

‘Evidence of probable presence of NIHL is considered to be present if there is a downward notch in the audiogram in the 3-6 kHz range that is large enough to be identifiable with a degree of confidence ... Evidence for NIHL is also provided on the audiogram by sufficiently large relative bulge downwards and to the left in the 3-6 kHz range… in a considerable portion of NIHL cases, especially after the age of about 50 years, the characteristic high frequency notch is missing.  This is usually due to the additional presence of high frequency impairment of other causation … Typically, that has the effect of converting a noise-induced audiometric notch into a bulge …  In other cases it may reduce the notch to a size (e.g. 5 dB) that is not significant as a notch.  Nevertheless, it will add to the size of a potential bulge and should be examined closely to see if it classifies as a bulge…’

When calculating notching / bulging, upper and lower anchor points are used.

If the claimant’s expert was deemed to have used the appropriate anchor points, bulges in both audiograms were ‘Coles-compliant’. If, however, the defendant expert’s different anchor points were more suitable, they were not ‘Coles-compliant’.

The Anchor Point Issue

Generally, 8 kHz is used as the upper anchor point when calculating the size of notches/bulges. There are, however, certain circumstances when 6 kHz is the legitimate upper anchor point.

Initially, Mr Johnson used the 8 kHz anchor point in his analysis of bulging in the 2013 audiogram.

In the 2013 audiogram, the claimant’s HTL at 8 kHz was 85 dB. Whereas, in the 2015 audiogram, no HTL could be recorded (inferring maximum HTL of 100 dB tested).

After having reviewed the 2015 audiogram, he revisited his calculations and proceeded to use 6 kHz as the upper anchor point. His change of mind was provoked by the ‘precipitous drop-off’ of more than 15 dB at 8 kHz between the original audiogram and the later audiogram. Mr Johnson also observed a 21 dB difference between 6 and 8 kHz on the 2013 audiogram, where the expected AAHL (50th percentile) was 9 dB.

Altered anchor points for avoidance of Coles calculation distortion yielded a ‘Coles-compliant’ bulge in the 2013 audiogram.

While Mr Parker accepted that bulges could be discerned using a 6 kHz upper anchor point, he saw no justification, in this instance, for departing from the standard approach, i.e. the use of 8 kHz as the upper anchor point could still provide a reliable interpretation. He also argued that the CLB Guidelines does not advocate comparison between audiograms to diagnose NIHL.

Counsel for the claimant argued that it was ‘illogical to ignore the second, later audiogram, particularly where there is already evidence of disproportionate high-frequency losses’. To do so would constitute a failure ‘to accord proper weight to the significant changes which occurred between 2013 and 2015’.

HHJ Freedman concluded, on his interpretation of Note 10, that:

‘… in relation to the ‘precipitous drop-off’ point … it is not being contemplated that 6 kHz can be used to assess whether there is a bulge in 2013, when there has been diminution in values at 8 kHz over the ensuing two years’.

The judge went on to conclude that ‘exceptional’ disparity between HTL’s at 6 and 8 kHz could justify a departure from an 8 kHz anchor point. Using the 25th percentile, the difference between AAHL and actual loss was 13 dB and even though this was deemed to be ‘significant’, it was not enough to depart from the usual anchor point.

A key factor of the judge’s ratio on this issue was that the claimant’s expert shifted his opinion retrospectively, despite feeling able to interpret the 2013 audiogram using 8 kHz as the upper anchor point at the time. In any event, the CLB Guidelines do not explicitly refer to comparisons of audiograms produced at different times.

Mr Parker was therefore right to use 8 kHz as the upper anchor point.

Having ascertained the correct methodology for calculating bulges/notches, it was necessary to consider whether ‘Coles-compliant’ bulges were present.

The Bulge Calculation Issue

Under paragraph 7.6 of the CLB Guidelines, it states that:

‘A high-frequency bulge in the air conduction audiogram that is sufficiently large to be indicative of the probable presence of NIHL is defined as follows.  Such a bulge is present if the HTL at 3 and/or 4 and/or 6 kHz, after any due correction for earphone type…is at least 10 dB greater relative to the comparison values for age-related hearing loss at corresponding frequencies.  If an average of two or more HTL measurements can be used, the 10dB figure may be slightly reduced… Occasionally, the bulge extends to involve 2 kHz or even 1 kHz’.

Mr Johnson argued that there was a ‘Coles-compliant’ bulge, irrespective of Mr Parker’s preferred anchor point analysis, as the bulge at 4 kHz was at least 10 dB greater than those at 3 or 6 kHz. Although ‘notching’ at 4 kHz did not meet the 10 dB criterion, he maintained that ‘it was appropriate to look at comparative values as opposed to absolute values’. There was a recovery on both sides (3 and 6 kHz) of the calculated bulge values at 4 kHz, which he argued was all that mattered.

Claimant counsel submitted that the identification of a notch requires the comparison of HTL’s with ‘adjacent thresholds’ and that the same should apply to bulge calculations.

The defendants’ expert contested this assertion, stating that recoveries at ‘adjacent thresholds’ are only relevant if there is a notch and not if there is a bulge.

HHJ Freedman, again, preferred the analysis of Mr Parker, reasoning, at paragraph 31:

‘… it is clear that to determine the presence or otherwise of a bulge, hearing threshold levels should be compared “relative” to the comparison values for age-related hearing loss at corresponding frequencies.  As Mr Cooper points out, this does not suggest a comparison with adjacent frequencies in the bulge line, in the manner undertaken by Mr Johnson. The Guidelines are comparing the claimant’s hearing threshold levels with his predicted age hearing loss to determine if there is a difference of at least 10db at the frequencies which are usually affected by noise’.

As such, ‘corresponding frequencies’ does not equate to ‘adjacent frequencies’.  If this was intended by the CLB Guidelines, then it would have been ‘overtly stated’.

When asked, Mr Parker answered that he was unaware of any other expert carrying out a bulge analysis in the way that Mr Johnson purported and HHJ Freedman remarked that he was struck by Mr Johnson’s unsatisfactory response to the same question.

‘In short, the guidance which has been given is that if there is no notch, it is necessary to see if it has been ‘converted’ into a bulge.  In other words, it is only if there is no notch that it is then necessary to undertake a bulge analysis ... to determine the presence or otherwise of a bulge, hearing threshold levels should be compared “relative” to the comparison values for age-related hearing loss at corresponding frequencies ... this does not suggest a comparison with adjacent frequencies in the bulge line ... The Guidelines are comparing the claimant’s hearing threshold levels with his predicted age hearing loss to determine if there is a difference of at least 10db at the frequencies which are usually affected by noise’.

Accordingly, it was HHJ Freedman’s view that Mr Parker’s approach reflected the correct interpretation and application of the Guidelines.

Conclusion

On the balance of probabilities, NIHL was not established and the claim against the 1st and 3rd defendants was dismissed.

A factor of potential influence, during the course of the trial, was that oral expert evidence was concurrently adduced, otherwise known as expert ‘hot tubbing’.

We previously reported, in edition 211 of BC Disease News (here), that the Civil Procedure Rule Committee (CPRC) had amended Practice Direction 35 to permit the court to direct ‘experts from like disciplines to give their evidence and be cross - examined on an issue - by - issue basis, so that each party calls its expert or experts to give evidence in relation to a particular issue, followed by the other parties calling their expert or experts to give evidence in relation to that issue …’

Through ‘hot-tubbing’, if an expert is more convincing, it is arguably more visible than if witnesses are questioned on a traditional basis.