In this nationally representative study of 60- to 69-year-old adults, greater hearing loss was independently associated with poorer cognitive functioning using a nonverbal test of executive function and psychomotor speed. These results were robust to analyses accounting for multiple confounders and excluding participants with moderate or severe hearing loss. The magnitude of the reduction in cognitive performance associated with hearing loss is clinically significant with the reduction associated with a 25 dB hearing loss being equivalent to an age difference of 7 years. Hearing aid use was associated with higher cognitive scores, but these results were based on a small number of individuals using hearing aids.
Our results contribute to the literature examining the association between hearing loss and cognition. Our findings are consistent with prior research, demonstrating significant associations between greater hearing loss and poorer cognitive function on both verbal (21
) and nonverbal cognitive tests (23
) and in both cross-sectional and prospective studies (23
). In contrast, other studies have not found similar associations (31
). One key limitation across these prior studies is the variability in how hearing loss was measured and how audiometric data were analyzed (eg, choice of pure tone thresholds used to define hearing loss). Most studies utilized portable or screening audiometers (23
) or tested participants under varying environmental conditions (eg, home-based testing) (28
), whereas some did not adequately describe their audiometric testing protocol (24
). The effect of biased or imprecise assessments of hearing thresholds would likely decrease sensitivity to detect associations due to increased variance. These prior studies have also generally been conducted in nonrepresentative study populations in which the observed results may not be generalizable. Strengths of our current study are that our results are based on a nationally representative sample, and a standardized audiometric testing protocol using a definition of hearing loss adjudicated by the World Health Organization (16
) was applied to all individuals.
A number of mechanisms could explain the observed association between hearing loss and cognition. Poor verbal communication associated with hearing loss may confound cognitive testing or vice-versa there may be an overdiagnosis of hearing loss in individuals with subclinical cognitive impairment. Confounding by poor verbal communication is unlikely since the DSST does not rely heavily on the presentation of verbal information, and mild–moderate hearing loss minimally impairs face-to-face communication in quiet environments (ie, during cognitive testing) (33
) particularly in the setting of testing by experienced examiners who are accustomed to working with older adults. We also conducted a sensitivity analysis excluding individuals with moderate or severe hearing loss, and a previous study has demonstrated that artificially induced hearing loss (through the use of occlusive headphones) did not acutely affect the results of neurocognitive testing using both verbal and nonverbal cognitive tests (34
). An overdiagnosis of hearing loss in those with preexisting cognitive impairment is also a possibility but unlikely given that reliable audiometric thresholds have been obtained even in patients with early dementia (22
), and pure tone audiometry is routinely performed even in children as young as 4 years.
A shared neuropathologic etiology underlying both hearing loss and cognitive function may explain our results, but the neuropathologic mechanism is unknown. Pure tone audiometry is considered to be a measure of the auditory periphery because detection of pure tones relies on cochlear transduction and neuronal afferents to brainstem nuclei without requiring significant higher auditory cortical processing (35
). Neuropathology associated with Alzheimer’s disease has not been found in the peripheral auditory pathways (36
). The likelihood of another neurobiological process such as microvascular disease causing both hearing loss and dementia is a possibility, but known cardiovascular risk factors were adjusted for in our models.
Finally, hearing loss may be associated with cognitive decline through a causal pathway, possibly mediated by social isolation or cognitive load, or through a direct neurobiologic mechanism. Communication impairments caused by hearing loss can lead to social isolation in older adults (38
), and epidemiologic (40
) and neuroanatomic studies (42
) have demonstrated associations between poor social networks and cognitive decline and dementia. The effect of hearing loss on cognitive load is suggested by studies demonstrating that under conditions where auditory perception is difficult (ie, hearing loss), greater cognitive resources are dedicated to auditory perceptual processing to the detriment of other cognitive processes such as working memory (43
). Finally, previous animal studies have also demonstrated a possible direct neurobiological link between hearing loss and/or environmental enrichment (possibly analogous in humans to having access to auditory and environmental stimuli) with hippocampal neurogenesis and cognitive functioning (46
In the current study, self-reported hearing aid use was associated with higher scores on the DSST, but these results must be interpreted with caution because of the small number (n
= 13) of participants using hearing aids. The direction of the observed association also cannot be established in this cross-sectional study. For example, although hearing aids could plausibly improve cognitive functioning through decreased social isolation or reduced cognitive load, individuals with better cognitive function may also be more likely to obtain hearing aids. Ultimately, investigating causality between hearing aid use and improved cognitive functioning will require a randomized control trial. Interestingly, one small randomized study of hearing aids performed in older military veterans has been performed, and this study demonstrated improved cognition in veterans using hearing aids (48
). However, these results have not been subsequently studied or confirmed in larger and more representative cohorts.
A key limitation of our study is that our results are based on cross-sectional data rather than on longitudinal trajectories of hearing loss and cognitive function over time. Therefore, our estimates of the expected change in cognitive scores associated with hearing loss and age may be subject to bias by cohort effects or obscured by interindividual heterogeneity in participant characteristics. However, the restricted age range of our study population (60- to 69-year-olds) may help limit potential biases introduced by cohort effects. In addition, interindividual heterogeneity in participant characteristics would tend to bias any results toward the null hypothesis, whereas our results demonstrated a robust association between hearing loss and cognitive scores.
Residual confounding by other medical or environmental factors is also possible, but known risk factors for hearing loss and cognitive decline were adjusted for in our models.
If our results are confirmed longitudinally and in other independent studies, our findings potentially have significant implications for public health. Hearing loss is highly prevalent (49
), and hearing loss may be both potentially preventable and treatable with rehabilitative devices and strategies that remain grossly underutilized (50
). Further research into whether such interventions could impact cognition and dementia are critically needed.