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

Military Veterans are at high risk for noise-induced cochlear synaptopathy, a type of cochlear deafferentation. Predicted perceptual consequences of deafferentation include tinnitus and hyperacusis. The objective of this study was to determine if compared to young non-Veteran controls with minimal noise exposure, Veterans with normal audiograms who report constant tinnitus and/or decreased sound tolerance (DST) show reductions in three physiological indicators of deafferentation: (1) auditory brainstem response [ABR] wave I amplitude, (2) envelope following response [EFR] magnitude, and (3) wideband middle ear muscle reflex [MEMR] magnitude. Outer hair cell (OHC) function was also evaluated using distortion product otoacoustic emissions [DPOAEs].

Mean ABR wave I amplitudes, EFR magnitudes, and high frequency DPOAEs were reduced in a sample of 22 Veterans with tinnitus and 19 Veterans with DST relative to 165 controls. On average, MEMR magnitudes were also smaller for Veterans with DST than for controls. While many Veterans with tinnitus had smaller MEMRs than controls, a small number had large MEMRs. ABR wave V/I amplitude ratios tended to be elevated for Veterans compared to controls.

ABR, EFR, and DPOAE measurements were suggestive of both cochlear deafferentation and subclinical high frequency OHC dysfunction in Veterans with constant tinnitus or DST. Reductions in MEMR magnitude were not consistently observed in Veterans with tinnitus who had reduced ABR and EFR measurements, indicating that the MEMR may not be a reliable indicator of cochlear deafferentation. Increased central gain in the auditory brainstem may have contributed to the perception of tinnitus and DST.