PP512 - Contralateral Noise in Long-Latency Auditory Evoked Potentials in Normal Hearing Workers

Workers exposed to occupational noise may have changes in the central auditory nervous system, possibly related to hidden hearing loss (cochlear synaptopathy). This potentially impairs sound processing, especially in the presence of background noise. The objective of this study was to electrophysiologically assess normal hearing workers with LLAEP, with and without contralateral noise. They had changes in N2 latency with noise and P3 amplitude with and without noise, which suggests cortical impairments in normal hearing workers exposed to occupational noise.

Summary:

Rationale: It is important to thoroughly assess the auditory pathway of workers exposed to occupational noise because it can affect both their peripheral and central portions. Authors suggest that occupational noise exposure may cause hidden hearing loss (cochlear synaptopathy), as synapses between inner hair cells and auditory nerve fibers are permanently damaged, though not affecting hearing thresholds. This suggests that noise exposure damages fibers with low spontaneous firing rates, thus impairing sound processing, especially in the presence of background noise. Hence, it is important to verify whether noise exposure causes synaptopathy in humans and which procedures would be more sensitive to detect it – for instance, by assessing the central auditory nervous system (CANS) with electrophysiological procedures, such as long-latency auditory evoked potentials (LLAEP) in the presence of background noise, which is when hidden hearing losses may be more evident.

Objectives: To assess CANS with LLAEP with and without noise in normal hearing workers exposed to occupational noise.

Design: The study included 60 male workers with normal hearing thresholds – 30 in the Study Group (SG - exposed to occupational noise), with a mean age of 35.6 years, and 30 in the Control Group (CG - not exposed to occupational noise), with a mean age of 35.3 years. Before the electrophysiological tests, they were submitted to medical history surveys, otoscopy, acoustic immittance measures, pure-tone audiometry, and speech audiometry. LLAEP was performed with tone-burst stimuli with and without contralateral noise at a 15-dB signal-to-noise ratio (stimulation at 75 dBnHL and contralateral noise at 60 dBnHL).

Results: Decreased P3 amplitude was found in LLAEP without noise in SG in both ears, in comparison with CG (p-value 0.001 – right ear [RE]; p-value 0.002 – left ear [LE]). Decreased P3 amplitude was likewise found in SG with contralateral noise, comparing the groups, though statistically significant only in LE (p-value 0.010). SG also had the highest N2 latency in RE (p-value 0.021).

Conclusions: Normal hearing workers exposed to occupational noise performed worse in N2 with noise and P3 cognitive component both with and without noise than non-exposed workers. This suggests impairments in the cortical auditory pathway, affecting auditory discrimination skills, temporal processing, attention, and memory. Thus, the findings of the present study corroborate the theory that noise exposure is harmful to CANS, despite the normal hearing thresholds. They also indicate that LLAEP with and without noise is sensitive to investigating hidden hearing losses.