Chronic Conductive Hearing Loss Leads to Cochlear Degeneration

Abstract

Synapses between cochlear nerve terminals and hair cells are the most vulnerable elements in the inner ear in both noise-induced and age-related hearing loss, and this neuropathy is exacerbated in the absence of efferent feedback from the olivocochlear bundle. If age-related loss is dominated by a lifetime of exposure to environmental sounds, reduction of acoustic drive to the inner ear might improve cochlear preservation throughout life. To test this, we removed the tympanic membrane unilaterally in one group of young adult mice, removed the olivocochlear bundle in another group and compared their cochlear function and innervation to age-matched controls one year later. Results showed that tympanic membrane removal, and the associated threshold elevation, was counterproductive: cochlear efferent innervation was dramatically reduced, especially the lateral olivocochlear terminals to the inner hair cell area, and there was a corresponding reduction in the number of cochlear nerve synapses. This loss led to a decrease in the amplitude of the suprathreshold cochlear neural responses. Similar results were seen in two cases with conductive hearing loss due to chronic otitis media. Outer hair cell death was increased only in ears lacking medial olivocochlear innervation following olivocochlear bundle cuts. Results suggest the novel ideas that 1) the olivocochlear efferent pathway has a dramatic use-dependent plasticity even in the adult ear and 2) a component of the lingering auditory processing disorder seen in humans after persistent middle-ear infections is cochlear in origin.

Developmental Conductive Hearing Loss Reduces Modulation Masking Release

“…There is evidence that CHL through malleus removal does not lead to sensorineural hearing loss ( Tucci et al., 1999 ). However, a recent study investigated the vulnerability of cochlear innervation following CHL through otitis media in mice ( Liberman, Liberman, & Maison, 2015 ). Oval window removal can cause loss of inner hair cell afferent innervation, but this effect is only pronounced for cochleotopic positions above 5 kHz ( Liberman et al., 2015 ).…”

Perinatal thiamine deficiency causes cochlear innervation abnormalities in mice

“…10). In the adult ear, prior studies from our group have shown that 1) surgical lesions to the LOC and MOC systems can lead to synaptopathy in the IHC area (Maison et al, 2013; Liberman et al, 2014) and that chronic sound deprivation by removal of the eardrum can also lead to dramatic loss of LOC innervation density as well as IHC synaptopathy (Liberman et al, 2015). …”

Cortical and Sensory Causes of Individual Differences in Selective Attention Ability Among Listeners With Normal Hearing Thresholds

“…A growing number of animal studies show that noise exposure can cause a loss of auditory nerve fibers (ANFs) without elevating hearing detection thresholds or affecting the audiogram (Kujawa & Liberman, 2015;Liberman, 2015;Liberman, Liberman, & Maison, 2015). Noise exposure that leaves cochlear mechanical responses intact can produce a rapid loss of as many as 40%-60% of the ANF synapses driven by cochlear inner hair cells (cochlear synaptopathy), which carry the ascending signal up the auditory pathway (Kujawa & Liberman, 2006.…”