Age-related hearing loss and cognitive impairment may arise from the same underlying processes of neurodegeneration, but equally there is evidence for each of these forms of impairment to contribute to the other. Today's paper is an example of this sort of analysis, in which the researchers employed data on hearing aid use in older patients as a way to investigate this relationship. They find that hearing loss appears to contribute to cognitive impairment.
Modifiable risk factors for dementia include untreated mid-life hearing loss, and it has been estimated that 8% of dementia cases globally are attributable to this factor. Proposed mechanisms underlying the relationship between hearing loss and the development of dementia include (i) common underlying pathology (probably vascular), (ii) impoverished input affecting brain structure and function, (iii) cognitive resources overoccupied in listening unavailable for higher functions and (iv) interaction between auditory function and dementia pathology. These mechanisms are not mutually exclusive.
Health records data from 380,794 Veterans who obtained hearing aids from the US Veterans Affairs healthcare system were analysed. Consistent with previous findings, we found that patients over 60 years of age without cognitive impairment at the time of hearing-aid fitting, who remained persistent hearing-aid users, had 27% reduced odds of receiving a dementia diagnosis 3.5-5 years after hearing-aid fitting than patients who did not persist in hearing-aid use. Our odds ratio (OR) of 0.73 is broadly in line with the hazard ratio of 0.82 found in a large sample of adults over 66 years of age with diagnosed hearing loss. The adjusted OR for incident dementia was 0.73 for persistent (versus non-persistent) hearing-aid users. The adjusted OR for hearing-aid use persistence was 0.46 in those with pre-existing dementia (versus those remaining free of mild cognitive impairment and dementia).
This suggests that of the four possible mechanisms linking hearing loss and dementia, the first (common pathology) is not dominant, since hearing-aid treatment cannot affect that pathology. Further probing of candidate mechanisms would at the very least require data on duration of hearing loss, which was not available in our dataset. This study provides (to the authors' knowledge) the first quantitative evidence that the diagnosis of dementia is associated with subsequent lower persistence of hearing-aid use. This may be due to reduced abilities to perform instrumental activities, or diverse other mechanisms, including memory problems, reduced motivation to engage in social interaction, and carers prioritising other aspects of care.