Cochlear Synaptopathy and Noise-Induced Hidden Hearing Loss

Abstract

Recent studies on animal models have shown that noise exposure that does not lead to permanent threshold shift (PTS) can cause considerable damage around the synapses between inner hair cells (IHCs) and type-I afferent auditory nerve fibers (ANFs). Disruption of these synapses not only disables the innervated ANFs but also results in the slow degeneration of spiral ganglion neurons if the synapses are not reestablished. Such a loss of ANFs should result in signal coding deficits, which are exacerbated by the bias of the damage toward synapses connecting low-spontaneous-rate (SR) ANFs, which are known to be vital for signal coding in noisy background. As there is no PTS, these functional deficits cannot be detected using routine audiological evaluations and may be unknown to subjects who have them. Such functional deficits in hearing without changes in sensitivity are generally called “noise-induced hidden hearing loss (NIHHL).” Here, we provide a brief review to address several critical issues related to NIHHL: (1) the mechanism of noise induced synaptic damage, (2) reversibility of the synaptic damage, (3) the functional deficits as the nature of NIHHL in animal studies, (4) evidence of NIHHL in human subjects, and (5) peripheral and central contribution of NIHHL.

Current insights in noise-induced hearing loss: A literature review of the underlying mechanism, pathophysiology, asymmetry, and management options

“…NIHL can be associated with a decrease in speech recognition scores in quiet as well as in background noise, even in the setting of a normal pure tone audiogram [16]. This is probably related to the synaptopathic mechanisms, as discussed previously [7, 8, 16] and reduced temporal processing skills [70] as a result of noise-induced affected connections between inner hair cells and low spontaneous rate auditory nerve fibres, which are important for temporal processing [8]. In order to quantify noise-induced damage, it is recommended that speech recognition tests in quiet and in noise should be performed in addition to pure tone thresholds [7].…”

Inner Hair Cell Loss Disrupts Hearing and Cochlear Function Leading to Sensory Deprivation and Enhanced Central Auditory Gain

“…There is currently tremendous scientific and clinical interest in a form of hidden hearing loss termed synaptopathy that affects the synaptic ribbon at the base of the IHC, glutamatergic receptors located on type I auditory nerve terminals and neurotrophin3 which provides trophic support for SGN (Liberman et al, 2011; Kujawa and Liberman, 2015; Shaheen et al, 2015; Viana et al, 2015; Shi et al, 2016a,b; Suzuki et al, 2016). Identifying the unique perceptual deficits associated with this condition will provide additional tools for identifying individuals with normal clinical audiograms that nonetheless have significant auditory processing disruptions.…”

Biomarkers for Hearing Dysfunction: Facts and Outlook

“…2 ). In humans, the absolute latencies and supra-threshold amplitudes of waves, ABR wave I (distal peripheral portion of the AN [90] ), ABR wave III (fibers from cochlear nucleus to caudal auditory pons and superior olivary complex [91] ), ABR wave VI (medial geniculate body or the auditory thalamus), or ARB wave V (believed to correspond with the IC in the midbrain [92,93] ) can be analyzed for various stimulus intensity levels and, for example, recording time of ABR measurements that should be set at >10 ms. An increasing body of evidence demonstrates that, due to altered leisure behavior and demographic crisis, the hidden hearing loss and auditory neuropathy are likely widely prevalent over time [94][95][96] , contributing to presbycusis [62,64,97] or hyperacusis and tinnitus [21,22,98] .…”