Audiometric notch as a sign of noise induced hearing loss
D I McBridea b, S Williamsb
a Institute of
Occupational Health, University of Birmingham, UK, b New Zealand Occupational and Environmental
Health Research Centre, University of Otago, Department of Preventive
and Social Medicine, PO Box 913, Dunedin, New Zealand
Correspondence to: Dr D I McBride mcbride{at}gandalf.otago.ac.nz.
Accepted 25 July 2000
OBJECTIVES
To
investigate the relation between different types of exposure to noise
and a classic sign of noise induced hearing loss (NIHL), the
audiometric notch.
METHODSThe study
sample had exposure to both continuous and impulse noise and was drawn
from a population of electrical transmission workers. Audiograms, taken
as part of a hearing conservation programme, were read by three
clinicians experienced in the assessment of NIHL. Working independently
and using their clinical judgment, they were asked to identify
localised increases in the threshold of hearing (audiometric notches)
which they would attribute to noise, had a suitable history of exposure
been elicited. Prevalent cases of NIHL were identified by the presence
of a notch in either ear. Risk factors for NIHL were assessed by a
questionnaire which sought information about exposure to air blast
circuit breaker noise; firearms; explosions, and continuous noise. The
odds of exposure to these factors in those with and without hearing
loss were calculated, and odds ratios (ORs) and 95% confidence
intervals (95% CIs) were estimated by logistic regression.
RESULTSOf the 648 questionnaires sent out 357 were returned, a response rate of 55%. Of
these, at least two out of the three assessors identified 175 (49%)
people with a notch at any audiometric frequency. There was no
association between these cases and the NIHL risk factors identified by
the questionnaire, but a further frequency specific analysis showed a
small proportion of people (15 (4%)) with notches at 4 kHz who had the
expected associations with exposure to noise and a significant OR for
firearms of 4.25 (95% CI 1.28 to 14.1). The much larger proportion of
people with 6 kHz notches (110 (31%)) did not show these associations.
CONCLUSIONSTo
diagnose 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.
Keywords: hearing loss; noise induced; noise; occupational
© 2001 by Occupational and Environmental Medicine
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