Keoghs Insight

Author

Kirsty Wareing

Listen on repeat

AWARE13/03/2015
Disease Aware Issue 6

Audiograms are the basis of any diagnosis of noise-induced hearing loss (NIHL). When a pure-tone audiogram is carried out, hearing is measured at a range of frequencies in each ear (typically 250 Hz to 8 kHz). The British Society of Audiology (BSA) sets out guidelines for conducting an audiogram in order to obtain an accurate reading, including:

  • The level of background noise must be below 35 dB. The machine must be tested and calibrated within the last 12 months.
  • The person being tested should also respond to the tone at the least suggestion of a signal rather than when a signal is definitely heard.

The Coles, Lutman and Buffin (CLB) guidelines set out the most commonly accepted criteria required to be met for a diagnosis of NIHL to be made.

These are commonly referred to as R1, R2 and R3, and are as follows:

  • R1 is high-frequency impairment, where a single measurement at a hearing threshold level (HTL) of 3 kHz to 6 kHz is at least 10 dB greater than the HTL at 1 kHz or 2 kHz
  • R2 is noise exposure: the claimant is required to prove exposure to a noise immission [the noise an individual receives] level (NIL) of at least 100 dB
  • R3 is the presence of an audiometric configuration, i.e. a notch or a bulge, of at least 10 dB in the 3 to 6 kHz range of an audiogram.

The question for any defendant (and indeed the court), is how reliable can a single audiogram be for a diagnosis of NIHL? On what basis can a defendant seek a second audiogram?

Firstly, a defendant may not be satisfied that the audiogram was conducted under suitable conditions, as per the BSA guidelines.

For example, a claimant’s audiogram may have been conducted at a hotel or in another area where the ambient noise level was not below 35 dB (or the level is uncertain).

There may be questions as to the reliability of the audiogram or indeed the claimant.

Alternatively, the qualifications (or previous audiograms) of the audiometrist may be open to question.

There is however a valid argument that states that NIHL should not be diagnosed on the basis of a single audiogram.

There are various factors which can cause audiometric error. Two or more comparable tests provide a far more accurate picture of someone’s hearing.

Stephens (1981)1 found 38 sources of variance in audiometric testing and the 'Black Book' 2 describes the main sources of audiometric error (Chapter 5, section 5.2.1), inter alia, as:

  • Calibration error of the audiometer. The manufacturing specifications allow volume performance within a tolerance of +/-3dB
  • Unpredictable interaction between different ear types and earphones
  • Accuracy of the audiometer tone frequencies, ie. the audiometer may in effect be testing at a different frequency to that indicated
  • Background noise in the test room
  • Fitting of the headphones / bone vibrator
  • Miscellaneous noise - such as clothes rubbing on transducer cables
  • Individual motivation / ability / attention
  • Fatigue, colds, excess wax, temporary effects of preceding noise
  • Real fluctuation in hearing sensitivity.

The ‘Black Book’ goes on to state that:

  • Repeatability varies from person to person
  • Repeatability is best at 1 and 2 kHz and poorer outside these limits especially at 6 kHz
  • With 5 dB measurement steps then audiometric variability within the same test (intra-test variability) may be within +/- 10dB.

A further audiogram may also be required where there are clear gaps in the air-bone readings or where the claimant has greater than expected losses at the lower frequencies, as these may be indicative of conductive causes of hearing loss.

Gaps in the air-bone readings suggest that the inner ear and central auditory pathways are normal and that the hearing loss is localised to the middle and outer ear.

This causes an interruption in effective sound conduction (i.e. a conductive hearing loss). Conductive hearing loss is not NIHL.

All this means that a diagnosis of NIHL based on a single audiogram, must be considered unsafe.

The question of repeat audiograms also assists in claims of suspected exaggeration. It is very difficult for a claimant to exaggerate the level of hearing loss by the same degree in two or more separate tests (particularly spaced apart).

Defendants should, in instances where there is a single audiogram, be routinely asking for a second audiogram - even in cases where the medical evidence itself is not being challenged.

The costs involved are limited - usually in the region of £150, whilst given the short time frame required to arrange and carry out the tests, there is usually no impact on a court timetable.

A second audiogram will assist in assuring a court and a defendant, that causation has been properly established and a diagnosis is safe.

If there are any doubts about the accuracy of a single audiogram, a second must be considered the prudent course of action prior to settlement of a claim. As ever, this comes with a qualification.

If the first audiogram appears satisfactory, and where there are good reasons to suspect that the individual may well have been exposed to excessive noise, a second audiogram may actually show a worse picture, and increase damages.