Asymmetry of Occupational Noise Induced Hearing Loss: An Electrophysiological Approach.

 The question as to whether occupational noise exposure causes symmetrical or asymmetrical hearing loss is still controversial and incompletely understood.  Two electrophysiological methods (cortical evoked response audiometry: CERA and auditory steady state responses: ASSR) were used to address this issue.  156 subjects with a well-documented history of noise exposure, a wide range of noise induced hearing loss (NIHL) and without middle ear pathology underwent both a CERA and an ASSR examination in the context of an exhaustive medicolegal expert assessment intended for possible compensation.  Whatever the method (CERA or ASSR), the average electrophysiological hearing thresholds (1-2-3 kHz) are significantly worse in the left ear. The right - left differences in CERA and ASSR thresholds are strongly correlated with each other. No significant effect of frequency is found. No correlation is observed between right - left differences in hearing thresholds and either age or degree of hearing loss.  In NIHL, there is an actual average right - left difference of about 2.23 dB, i.e., 3.2%, the left ear being more impaired.