Neurophysiological effects of noise-induced hearing loss

Abstract

Noise over-exposure is known to cause sensorineural hearing loss (SNHL) through damage to sensory hair cells and subsequent cochlear neuronal loss. These same histopathological changes can result from other etiological factors (e.g., aging, ototoxic drugs). Audiologically, each is currently classified as SNHL, despite differences in etiology. Functionally, noise-induced hearing loss can differ significantly from other SNHL etiologies; however, these differences remain hidden from current audiological diagnostic tests. For example, noise-induced mechanical damage to stereocilia on inner- and outer-hair cells results in neurophysiological responses that complicate the common view of loudness recruitment. Also, mechanical effects of noise overexposure appear to affect the tip-to-tail ratio of impaired auditory-nerve-fiber tuning curves in ways that create a much more significant degradation of the cochlear tonotopic representation than does a metabolic form of age-related hearing loss. Even temporary threshold shifts due to moderate noise exposure can cause permanent synaptic loss at inner-hair-cell/auditory-nerve-fiber synapses, which reduces neural-coding redundancy for complex sounds and likely degrades listening in background noise. These pathophysiological results suggest that novel diagnostic measures are now required to dissect the single audiological category of SNHL to allow for better individual fitting of hearing aids to restore speech intelligibility in real-world environments. [Work supported by NIH.]