Hidden hearing loss: Primary neural degeneration in the noise-damaged and aging cochlea

Abstract

In acquired sensorineural hearing loss, the hearing impairment arises mainly from damage to cochlear hair cells or the sensory fibers of the auditory nerve that innervate them. Hair cell loss or damage is well captured by the changes in the threshold audiogram, but the degree of neural damage is not. We have recently shown, in animal models of noise-damage and aging, and in autopsy specimens from aging humans, that the synapses connecting inner hair cells and auditory nerve fibers are the first to degenerate. This primary neural degeneration, or cochlear synaptopathy, leaves many surviving inner hair cells permanently disconnected from their sensory innervation, and many spiral ganglion cells surviving with only their central projections to the brainstem intact. This pathology represents a kind of ''hidden hearing loss.'' This review summarizes current speculations as to the functional consequences of this primary neural degeneration and the prospects for a therapeutic rescue based on local delivery of neuroptrophins to elicit neurite extension and synaptogenesis in the adult ear.