Tinnitus and decreased sound tolerance in military Veterans: Relationship with cochlear deafferentation.

Summary

Compared 22 Veterans with constant tinnitus and 19 with decreased sound tolerance, all with normal audiograms, against 165 non-Veteran controls with minimal noise exposure, using four physiological measures. Both Veteran groups showed reduced ABR wave I amplitudes and envelope following response magnitudes, the two markers most closely tied to cochlear deafferentation, along with reduced high frequency DPOAEs indicating subclinical outer hair cell dysfunction that a standard audiogram misses. The wideband middle ear muscle reflex performed poorly: it was smaller in the decreased-sound-tolerance group, but in the tinnitus group it was inconsistent, with some Veterans showing unusually large reflexes despite reduced ABR and EFR, so the authors conclude it is not a reliable deafferentation marker. Wave V/I ratios tended to be elevated in Veterans, which the authors read as increased central gain in the auditory brainstem possibly contributing to both symptoms.

Key findings

• ABR wave I amplitudes and envelope following response magnitudes were reduced in Veterans with constant tinnitus and in Veterans with decreased sound tolerance, relative to 165 low-noise-exposure controls, despite normal audiograms.
• High frequency DPOAEs were also reduced, so these ears carry subclinical outer hair cell dysfunction alongside the deafferentation rather than purely synaptic damage.
• The wideband middle ear muscle reflex was not a dependable marker: some Veterans with tinnitus had large MEMRs even though their ABR and EFR both pointed to deafferentation.
• ABR wave V/I ratios tended to be elevated in Veterans, consistent with increased central gain in the auditory brainstem compensating for reduced peripheral input.