Adding insult to injury: cochlear nerve degeneration after "temporary" noise-induced hearing loss.

Summary

Recovery of hearing thresholds after loud noise exposure has long been assumed to mean the ear escaped lasting damage. Using cochlear function tests and confocal imaging in mice, this study showed that acoustic overexposures causing moderate but fully reversible threshold elevation leave sensory hair cells intact, yet cause acute loss of afferent nerve terminals and delayed degeneration of the cochlear nerve itself. This primary neural degeneration is invisible to conventional threshold testing but should add to difficulty hearing in noisy environments, and could contribute to tinnitus, hyperacusis, and other perceptual anomalies.

Key findings

• A fully-recovering ('temporary') threshold shift can still leave permanent nerve damage.
• Hair cells survive intact while afferent nerve terminals are acutely lost and the cochlear nerve degenerates over time.
• This 'hidden' neurodegeneration is invisible to the standard hearing test (audiogram).
• Proposed as a contributor to hearing-in-noise difficulty, tinnitus, and hyperacusis.