8Presbycusis — the ageing cochlea
Presbycusis — age-related hearing loss — is the quiet giant of this chapter. It is the commonest cause of hearing loss overall, the reason the global burden rises as populations age, and an increasingly important route to cochlear implantation as candidacy expands to older adults. But 'presbycusis' is not one disease; Schuknecht's temporal-bone work resolved it into several distinct cochlear pathologies, each with its own audiogram and its own consequences for speech understanding. The distinction matters here because one of them — the neural subtype, a loss of spiral-ganglion neurons — strikes at exactly the cells the implant relies on, and explains why some older recipients do less well than their audiogram would predict.
TThe commonest cause
With age the cochlea accumulates damage from a lifetime of metabolic wear, noise, vascular change and genetic susceptibility, producing the gradual, symmetrical, high-frequency-dominant loss we call presbycusis. It is the single largest contributor to the global hearing-loss burden, and because untreated hearing loss is the leading modifiable risk factor for dementia (Chapter 5), treating it — increasingly with implants in severe cases — has importance well beyond audiology.
CSchuknecht's four subtypes
Schuknecht and Gacek correlated the audiograms of older ears with their temporal-bone pathology and described four principal patterns. The sensory type is hair-cell loss at the base, giving a steep high-frequency drop. The neural type is loss of spiral-ganglion neurons, with a downsloping loss and disproportionately poor word recognition(“phonemic regression”). The strial or metabolic type is atrophy of the stria vascularis, giving a characteristically flatloss with good word scores. And the cochlear-conductivetype reflects stiffening of the basilar membrane, giving a gently downsloping loss.[1993]
CWhy the neural subtype matters most
Real presbycusis is usually a mixture, but the scheme earns its keep by isolating the neural component. Where the loss is driven by spiral-ganglion depletion, the audiogram understates the disability — speech sounds are detected but not resolved — and the substrate the implant must drive is already thinned. This is the cochlear correlate of the variability seen in elderly recipients: the more neural the presbycusis, the less the implant has to work with.
CImplanting the older ear
None of this argues against implanting older adults — they benefit greatly, and the dementia link makes the case stronger. It argues for realistic counselling: outcomes in the elderly are good but more variable, partly because of the neural component of their presbycusis and partly because central processing and cognition also shape what the brain makes of the new signal (Chapter 3). The audiogram sets candidacy; the type of presbycusis helps set expectations.
Which presbycusis subtype likely explains the poor word scores, and why does it matter for implantation?
In Schuknecht's scheme, which presbycusis subtype gives disproportionately poor word recognition?
Why does the neural subtype matter most for implant counselling?