Age-Related Hearing Loss: A Loss of Voltage, Not Hair Cells

ABSTRACT

One of the most significant results from animal and human studies of age-related hearing loss involves degeneration of the lateral wall (stria vascularis and spiral ligament) of the cochlea, which is responsible for generating electrochemical gradients and regulating ion homeostasis. Accompanying this degeneration is a decrease in the endocochlear potential (EP), which is an 80- to 100-mV dc resting potential located in scala media. Reductions in the magnitude of the EP affect the function of the cochlear amplifier, which provides a gain of ~20 dB at low frequencies, increasing to ~60 dB or greater as frequency increases from ~1 to 8 kHz. Indeed, age-related threshold shifts observed in animals and humans can be accounted for almost totally by age-related degeneration of the stria vascularis/spiral ligament with attendant reductions in the EP and in the gain of the cochlear amplifier. Although considerable support exists for the involvement of strial vasculature in degeneration of the stria vascularis and the associated declines in EP and cochlear amplifier, the question of what constitutes the initial injury remains unclear. It is tempting to speculate that atrophy of strial marginal or intermediate cells, or both, occurs secondarily to vascular insufficiency resulting from capillary necrosis; however, the reverse could also be true. In gerbil and human, age-related degeneration of the auditory nerve may be the second most distinctive feature of age-related hearing loss. Strikingly absent in gerbil data and in some human data are age-related losses of outer and inner hair cells, except in the most basal and apical regions of the cochlea. These results and others suggest that age-related hearing loss should be viewed as a vascular, metabolic, neural hearing loss rather than a sensory hearing loss.

KEYWORDS

Age-related hearing loss - presbyacusis

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