Weak Middle-Ear-Muscle Reflex in Humans with Noise-Induced Tinnitus and Normal Hearing May Reflect Cochlear Synaptopathy

Abstract

Chronic tinnitus is a prevalent hearing disorder, and yet no successful treatments or objective diagnostic tests are currently available. The aim of this study was to investigate the relationship between the presence of tinnitus and the strength of the middle-ear-muscle reflex (MEMR) in humans with normal and near-normal hearing. Clicks were used as test stimuli to obtain a wideband measure of the effect of reflex activation on ear-canal sound pressure. The reflex was elicited using a contralateral broadband noise. The results show that the reflex strength is significantly reduced in individuals with noise-induced continuous tinnitus and normal or near-normal audiometric thresholds compared with no-tinnitus controls. Due to a shallower growth of the reflex strength in the tinnitus group, the difference between the two groups increased with increasing elicitor level. No significant difference in the effect of tinnitus on the strength of the middle-ear muscle reflex was found between males and females. The weaker reflex could not be accounted for by differences in audiometric hearing thresholds between the tinnitus and control groups. Similarity between our findings in humans and the findings of a reduced middle-ear muscle reflex in noise-exposed animals suggests that noise-induced tinnitus in individuals with clinically normal hearing may be a consequence of cochlear synaptopathy, a loss of synaptic connections between inner hair cells (IHCs) in the cochlea and auditory-nerve (AN) fibers that has been termed hidden hearing loss.

The search for noise-induced cochlear synaptopathy in humans: Mission impossible?

“…Decreased responses for two other physiological measures that are sensitive to synaptopathy in mouse models, the MEMR and the EFR (Shaheen et al 2015;Valero et al 2016;Valero et al 2018), have been associated with tinnitus in humans as well. In individuals with tinnitus who have normal or near-normal audiograms, Wojtczak et al (2017) observed a weakened MEMR relative to age-and sex-matched controls. Paul et al (2017) showed EFR reductions for individuals with normal audiograms and tinnitus compared to those without tinnitus.…”

“…In the Schaette and McAlpine (2011) study there was a small audiometric threshold elevation (3.5 dB) in the tinnitus group at 12 kHz, and thresholds at higher frequencies were not reported. Wojtczak et al (2017), who reported a large reduction in the acoustic MEMR amplitude in their tinnitus participants compared to controls, also observed substantial audiometric differences between groups. Although the effect of group was still highly significant after controlling for audiometric threshold, the pure tone threshold measurements were limited to a minimum of dB HL, which may have biased thresholds for the controls upwards.…”

Effects of noise exposure on auditory brainstem response and speech-in-noise tasks: a review of the literature

“…With respect to the differential diagnosis of cochlear synaptopathy, the amplitude of the middle ear muscle reflex has been successfully associated with cochlear synaptopathy in mouse models (Valero, Hancock and Liberman 2016;Valero et al 2018). Middle ear muscle reflexes are weak, or absent, in a subset of the population (Flamme et al 2017;McGregor et al 2018), leading several investigators to suggest that noise exposure resulting in cochlear synaptopathic injury could provide one explanation for individual variability in human participants (Wojtczak, Beim, and Oxenham 2017;McGregor et al 2018). There is also the thought that the amount of energy reaching the cochlea is a function of the wide band reflectance or admittance.…”

Non-Invasive Assays of Cochlear Synaptopathy -- Candidates and Considerations

“…Because these electrophysiological measures of subcortical coding are largely unaffected by top-down effects related to an individual's state of arousal or attention (Varghese et al, 2015;Kuwada et al, 2002;Cohen & Britt, 1982;Thornton et al, 1989;See Section 3.6), these results suggest that that the fidelity of "bottom-up" neural processing very early along the auditory pathway can contribute to complex perceptual function. Finally, individuals experiencing tinnitus despite normal audiograms reportedly exhibit subtle differences in the ABR (Shaette & McAlpine, 2011), the EFR (Paul et al, 2017), and the acoustically evoked middle-ear muscle reflex (Wojtczak et al, 2017). Such results are consistent with the notion that cochlear synaptopathy contributes to important aspects of auditory coding in humans.…”