Does hearing loss lead to dementia? A review of the literature

Recent studies have revealed a correlation between aging-related hearing loss and the likelihood of
developing Alzheimer Disease. However, it is not yet known if the correlation simply reflects the fact that
these two disorders share common risk factors or whether there is a causal link between them. The
answer to this question carries therapeutic implications. Unfortunately, it is not possible to study the
question of causality between aging-related hearing loss and dementia in human subjects. Here, we
evaluate the research surrounding induced-hearing loss in animal models on non-auditory cognition to
help infer if there is any causal evidence linking hearing loss and a more general dementia. We find
ample evidence that induction of hearing loss in animals produces cognitive decline, particularly hippocampal dysfunction. The data suggest that noise-exposure produces a toxic milieu in the hippocampus
consisting of a spike in glucocorticoid levels, elevations of mediators of oxidative stress and excitotoxicity, which as a consequence induce cessation of neurogenesis, synaptic loss and tau hyperphosphorylation. These data suggest that hearing loss can lead to pathological hallmarks similar to those
seen in Alzheimer’s Disease and other dementias. However, the rodent data do not establish that hearing
loss on its own can induce a progressive degenerative dementing illness. Therefore, we conclude that an
additional “hit”, such as aging, APOE genotype, microvascular disease or others, may be necessary to
trigger an ongoing degenerative process such as Alzheimer Disease.
© 2020