Abstract
Auditory Neuropathy (AN) is characterized by disruption of temporal coding of acoustic signals in auditory nerve fibers resulting in alterations of auditory perceptions. Mutations in several genes have been associated to the most forms of AN. Underlying mechanisms include both pre-synaptic and post-synaptic damage involving inner hair cell (IHC) depolarization, neurotransmitter release, spike initiation in auditory nerve terminals, loss of auditory fibers and impaired conduction. In contrast, outer hair cell (OHC) activities (otoacoustic emissions [OAEs] and cochlear microphonic [CM]) are normal. Disordered synchrony of auditory nerve activity has been suggested as the basis of both the alterations of auditory brainstem responses (ABRs) and reduction of speech perception. We will review how electrocochleography (ECochG) recordings provide detailed information to help objectively define the sites of auditory neural dysfunction and their effect on receptor summating potential (SP) and neural compound action potential (CAP), the latter reflecting disorders of ribbon synapses and auditory nerve fibers.
Highlights
Auditory neuropathy (AN) is a disorder characterized by alteration of the temporal coding of acoustic signals in auditory fibers with consequent reducction of auditory perceptions [1–3]
Diagnosis relies on decrease of speech perception beyond that expected for the hearing loss, absence or profound abnormality of auditory brainstem responses, and normal outer hair cell activities
compound action potential (CAP) results from the weighted sum of the extracellular components of the action potentials generated by individual auditory nerve fibers in response to acoustic stimulation, with the main contribution coming from fibers showing high characteristic frequency and short latency of activation localized to the basal portion of the cochlea [40]
Summary
Auditory neuropathy (AN) is a disorder characterized by alteration of the temporal coding of acoustic signals in auditory fibers with consequent reducction of auditory perceptions [1–3]. The ECochG potentials evoked in response to acoustic stimuli result from the superimposition of three components, two originating from receptor elements, the cochlear microphonic (CM) and summating potential (SP); and the other, the compound action potential (CAP), arising from auditory nerve fibers ([36] for a review). CAP results from the weighted sum of the extracellular components of the action potentials generated by individual auditory nerve fibers in response to acoustic stimulation, with the main contribution coming from fibers showing high characteristic frequency and short latency of activation localized to the basal portion of the cochlea [40]. Bourien et al [41] have shown that the CAP mostly reflects the contribution of high- and medium-SR (spontaneous rate) fibers, while there is little to no contribution of low-SR fibers
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