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Title: The Relative and Combined Effects of Noise Exposure and Aging on Auditory Peripheral Neural Deafferentation: A Narrative Review
Authors: Shehabi, Adnan M. 
Prendergast, Garreth 
Plack, Christopher J. 
Keywords: Cochlear Synaptopathy (CS);Noise Exposure;Presbycusis;Older deaf people;Age-Related Hearing Loss (ARHL);Auditory Brainstem Response (ABR);Auditory evoked response;Summating Potential to Action Potential Ratio (SP:AP)5;Envelope-Following Response (EFR);Middle Ear Muscle Reflex (MEMR);Speech-Perception-in-Noise (SPiN);Geriatric otolaryngology;Auditory perception
Issue Date: 2022
Publisher: Frontiers in Aging Neuroscience
Abstract: Animal studies have shown that noise exposure and aging cause a reduction in the number of 16 synapses between low and medium spontaneous rate auditory nerve fibers and inner hair cells before 17 outer hair cell deterioration. This noise-induced and age-related cochlear synaptopathy (CS) is 18 hypothesized to compromise speech recognition at moderate-to-high suprathreshold levels in 19 humans. This paper evaluates the evidence on the relative and combined effects of noise exposure 20 and aging on CS, in both animals and humans, using histopathological and proxy measures. In animal 21 studies, noise exposure seems to result in a higher proportion of CS (up to 70% synapse loss) 22 compared to aging (up to 48% synapse loss). Following noise exposure, older animals, depending on 23 their species, seem to either exhibit significant or little further synapse loss compared to their 24 younger counterparts. In humans, temporal bone studies suggest a possible age- and noise-related 25 auditory nerve fiber loss. Based on the animal data obtained from different species, we predict that 26 noise exposure may accelerate age-related CS to at least some extent in humans. In animals, noise27 induced and age-related CS in separation have been consistently associated with a decreased 28 amplitude of wave 1 of the auditory brainstem response, reduced middle ear muscle reflex strength, 29 and degraded temporal processing as demonstrated by lower amplitudes of the envelope following 30 response. In humans, the individual effects of noise exposure and aging do not seem to translate 31 clearly into deficits in electrophysiological, middle ear muscle reflex, and behavioral measures of CS. 32 Moreover, the evidence on the combined effects of noise exposure and aging on peripheral neural 33 deafferentation in humans using electrophysiological and behavioral measures is even more sparse 34 and inconclusive. Further research is necessary to establish the individual and combined effects of 35 CS in humans using temporal bone, objective, and behavioral measures.
DOI: 10.3389/fnagi.2022.877588
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