Aging after noise exposure: acceleration of cochlear synaptopathy in "recovered" ears.
Level 5 - mechanism / opinion, no new human data
Animal laboratory experiment without human data
PubMed 25972177 · doi:10.1523/JNEUROSCI.5138-14.2015
What was done
Adult mice were exposed to noise at 8-16 kHz for 2 hours at either 100 dB SPL (causing permanent synaptic damage without acute hair cell loss) or 91 dB SPL (causing temporary threshold shifts without acute synaptopathy). Animals were evaluated from 1 hour up to approximately 20 months after exposure. Cochlear function was assessed using distortion product otoacoustic emissions and auditory brainstem responses (ABRs). Cochlear whole mounts and plastic sections were examined to quantify hair cells, cochlear neurons, and synaptic connections.
What was found
The 100 dB exposure caused 35-50 dB threshold shifts at 24 hours that resolved by 2 weeks, but synaptic counts and high-frequency ABR amplitudes remained reduced by up to approximately 45%. As exposed animals aged, synaptopathy worsened compared to controls and spread to lower frequencies, followed by proportional ganglion cell losses. Threshold shifts re-emerged more than 1 year post-exposure, becoming up to 18 dB greater than controls by approximately 20 months, with outer hair cell loss exacerbated by approximately 10% at 32 kHz. In contrast, 91 dB exposure showed no acceleration of synaptic loss or functional decline up to at least 1 year post-exposure.
Why it matters
These findings show that a single acoustic trauma producing acute synaptopathy accelerates late-life neurodegeneration and hearing threshold elevation long after apparent recovery.
Limits
The study was conducted in mice, limiting direct translation to humans. The abstract does not provide the total sample size (n).
Cited by
- supports Temporary threshold shifts after acoustic trauma can result in permanent loss or degeneration of synapses connecting sensory hair cells to auditory neurons, known as hidden hearing loss.