Adding Insult to Injury: Cochlear Nerve Degeneration after “Temporary” Noise-Induced Hearing Loss

TLDR

Intense sound exposure can cause temporary or permanent hearing loss, and recovery of hearing thresholds has been assumed to mean that inner ear structures are fully restored, with no lasting effects. Moderate, reversible threshold shifts from noise exposure actually cause acute loss of cochlear nerve terminals and delayed nerve degeneration, leading to progressive auditory deficits that can underlie tinnitus, hyperacusis, and difficulty hearing in noise.

Abstract

Overexposure to intense sound can cause temporary or permanent hearing loss. Postexposure recovery of threshold sensitivity has been assumed to indicate reversal of damage to delicate mechano-sensory and neural structures of the inner ear and no persistent or delayed consequences for auditory function. Here, we show, using cochlear functional assays and confocal imaging of the inner ear in mouse, that acoustic overexposures causing moderate, but completely reversible, threshold elevation leave cochlear sensory cells intact, but cause acute loss of afferent nerve terminals and delayed degeneration of the cochlear nerve. Results suggest that noise-induced damage to the ear has progressive consequences that are considerably more widespread than are revealed by conventional threshold testing. This primary neurodegeneration should add to difficulties hearing in noisy environments, and could contribute to tinnitus, hyperacusis, and other perceptual anomalies commonly associated with inner ear damage.