One striking finding of this work is that the rate of age-related cognitive decline is significantly greater in older adults with a hearing loss than among their normal-hearing peers, even when controlling for other known risks (Laryngoscope Investig Otolaryngol. 2017). While the presence and magnitude of this effect vary across studies, overall results demonstrate that the negative impact of hearing loss extends well beyond quality of life issues. Understanding the nature of the relationship between hearing loss and cognition can potentially lead to the design of effective interventions to benefit an individual's well-being and reduce the disease burden on society.
Several general mechanisms have been proposed to account for a relationship between hearing and cognition (JAMA Intern Med. 2013;173:293). First, the basis could arise from a common cause, such as the general neural degeneration associated with aging that affects both hearing and cognitive function. Second, cascading consequences of the chronic sensory deprivation due to hearing loss may prevent the appropriate stimulation of higher central auditory processing structures, leading to a subsequent decline in cognitive status. The increased listening effort used to compensate for sensory deprivation may itself affect cognitive processing, resulting in suboptimal allocation of limited cognitive resources and reduced performance on other tasks. Finally, hearing loss may indirectly affect cognitive status through a concomitant reduction in the extent of social interaction and an increase in clinical depression.
ETHNICITY AND RACE FACTORS
A recent study by Golub and colleagues highlighted an important but less-explored aspect of the relationship between hearing loss and cognition in aging–the influence of ethnicity and race (J Am Geriatr Soc. 2017;65:1691). The researchers examined the prevalence of incident dementia and hearing loss in a large longitudinal sample of aging adults from an ethnically diverse neighborhood of New York City. Among 1,881 baseline participants (40% Hispanic, 31% black, and 29% white), 377 developed incident dementia during the average of 7.4 (+/- 4.6) years of follow-up visits. Overall, there was a 1.69 greater risk of incident dementia among participants with hearing loss than normal-hearing participants, even when controlling for other potential contributors such as cardiovascular risk and stroke. When the sample was stratified by ethnicity and race, however, hearing impairment increased the risk of incident dementia only for black participants (2.62, P <.01). The risk of dementia among Hispanics and whites with hearing loss was also greater than that for those with normal hearing (1.43 and 1.61, respectively), though this trend was not statistically significant. Differences among the three groups persisted even when factors such as education and income were considered in statistical modeling.
Golub, et al., observed that the specific reasons for the greater risk of dementia in older black adults with hearing loss are not known. The findings are generally consistent with previous research that demonstrated how ethnicity and race can be significant predictors of cognitive status in older adults (J Int Neuropsychol Soc. 2016;22:66). In our previous work, tests of spectral-temporal processing in which listeners were asked to discriminate changes in the phase of a spectral-ripple and compare brief (0.5 sec) spectro-temporal frequency patterns showed reduced performance among black participants compared with white participants (PLoS One. 2015;10:e0134330). Performance on these tests was significantly correlated with global cognition assessed using a battery of 12 neuropsychological tests, more specifically with scores on working memory tests. The authors suggested that differences in the response strategies used by white and black participants in tasks with a high degree of uncertainty may have contributed to the performance differences between the groups. Another study, however, found that black adults demonstrate a greater resilience to age-related hearing loss (J Gerontol A Biol Sci Med Sci. 2011; 66:582). Therefore, the relationship between hearing and cognition in black adults is more complex than a simple group-wise association.
Interpretation of the findings of Golub, et al., on racial and ethnic differences requires further caution for several reasons. First, the criterion used to determine the presence of hearing loss in the study was quite lax. The determination was based on the examiner's observations of a participant's hearing status, including whether the examiner needed to speak loudly or if the participant wore a hearing aid. As the authors acknowledged, this assessment approach may potentially confound the effects of hearing loss and dementia because the ability to repeat and follow the experimenter's instruction—part of the basis for hearing loss determination—also involves cognitive processing of linguistic information. In addition, the performance of participants with hearing loss on cognitive tests may in part reflect their inability to hear instructions well.
HEARING AIDS AND COGNITIVE DECLINE
The reliability of hearing assessment becomes a critical question as an increasing number of studies use examiner observations and patient reports to evaluate the relationship between hearing and cognition. For example, a study by Amieva and colleagues reported that older adults with self-reported hearing loss and who used hearing aids had the same rate of cognitive decline as their normal-hearing peers, while those who had hearing loss but did not use a hearing aid showed a significantly steeper rate of cognitive decline (J Am Geriatr Soc. 2015;63:2099). This finding, based on the analysis of 3,414 participants in a 25-year longitudinal study, is encouraging because it suggests that the negative impact of hearing loss on cognition may be a modifiable risk factor that can be effectively addressed through hearing aid use.
On the other hand, the determination of hearing loss in that study was made based on a single question (“Do you have hearing trouble?”) that was asked only once at the baseline assessment. Participants who were either unaware of their hearing loss or felt uncomfortable admitting it, would not be correctly categorized. Similarly, hearing aid use was also assessed only at baseline, thus reflecting neither the frequency nor consistency of hearing aid use over time. Notably, the study findings revealed that the steeper rate of cognitive decline among participants with hearing loss was no longer different from normal-hearing participants after controlling for psychosocial variables. Amieva, the lead author of the study, later observed that “it is highly unlikely that hearing aids have a direct effect on cognition,” and hypothesized that depression and social isolation associated with hearing loss may mediate the relationship (J Am Geriatr Soc. 2015).
IMPACT OF HEARING LOSS
Indeed, hearing loss affects many aspects of a person's life. In a recent study, Vas and colleagues examined the complaints of those affected by hearing loss and their communication partners (Trends Hear. 2017). Their analysis documented that in addition to complaints directly related to auditory function such as listening and communication, both hearing-impaired individuals and their communication partners presented with a large range of complaints related to social interactions and individual well-being.
Given that social engagement and self-appraisal are known factors in the cognitive decline of older adults, it is likely that these may also play a mediating role between hearing and cognition in older adults. Although there is yet a full understanding of the relationship between hearing and cognition, our current knowledge of the mediating factors can already inform the development of a wide range of effective interventions. Among these, hearing instruments as well as cognitively-focused and healthy lifestyle interventions may serve as effective tools to improve the social circumstances and overall well-being of people with hearing loss and those of their families.
Journal Club Highlight
Observed Hearing Loss and Incident Dementia in a Multiethnic Cohort
Golub, JS, et al. J Am Geriatr Soc. 2017; 65:1691.
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