Quantification of Noise Induced Hearing Loss – A Game Changer?

“Guidelines on the diagnosis of noise-induced hearing loss for medical legal purposes”¹ has become the definitive paper on which the diagnosis of noise induced hearing loss (NIHL) in compensation cases is made. However, there has hitherto not been a universally accepted method of quantifying the degree of NIHL once the diagnosis has been made. The original authors in a new publication “Guidelines for quantification of noise induced hearing loss in a medical legal context”² describe such a method.

The original diagnosis guidelines require 3 criteria to be satisfied. R1 is the presence of a high frequency sensorineural impairment. R2 is sufficient noise exposure. R3 is the presence of an acoustic notch or an audiometric bulge pattern on the puretone audiogram. The diagnosis guidelines are not however definitive as the method of age associated hearing loss (AAHL) calculation is open to several interpretations and the guidelines do not quantify the degree of NIHL. The new quantification guidelines address these issues.

They specify that the age used for estimating the AAHL is the nearest 5 year division, and that the 25th, 50th or 75th percentile is used depending on the best fit to the 1 and 8 kHz anchor points. It also requires the adjusted AAHL to be calculated using the “interpolation” rather than the “equally spaced” method. Thus a more uniformed calculation method is achieved.

The quantification of the NIHL is obtained by modifying the anchor points of 1 and 8 kHz to remove any NIHL component, adjusting the AAHL accordingly and subtracting from the measured threshold to produce a NIHL value in decibels. This is known as the “Full method”.

An alternative “Short method” can be applied in which the average bulge over 1, 2 and 3 kHz is modified by 1.32 to produce an NIHL value in decibels. This is, however, only a close approximation to the “full method” value and is particularly inaccurate if there is a deep notch at 4 kHz or the threshold at 3 kHz is worse than at 4 kHz.

The advantages of the new quantification guidelines are that it follows on naturally from the calculations used to establish the diagnosis of NIHL and produces an NIHL figure in decibels. Its disadvantages are that it is time consuming and in common with all NIHL methods of calculation, still relies on an initial averaged AAHL database.

The new quantification guidelines are “in press” and are already available on line with a printed publication date shortly. They are increasingly being used in original reports for the court and part 35 questions often now request the new calculation. They are yet to be tested in Court, but like their predecessor, the diagnosis guidelines, they are likely to gain widespread acceptance on the basis that it is “the worst possible method apart from all the rest”.

References:
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¹ Guidelines on the diagnosis of Noise Induced Hearing Loss for medical legal purposes by RRA Coles, ME Lutman and JT Buffin. Clinical Otolaryngology. Volume 25. (4) pages 264-273 (2000).

² Guidelines for quantification of noise induced hearing loss in a medical legal contexts by ME Lutman, RRA Coles and JT Buffin. Clinical Otolaryngology. (2016. In Press).

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