Oct 2020 The Hearing Journal

Hearing impairment is common among older adults, with an estimated 33 percent of individuals aged between 61 and 70 years having difficulties in hearing and the prevalence of hearing impairment increasing to more than 80 percent in those aged 85 years and above. Epidemiologic studies have found associations between hearing impairment and cognitive impairment and decline, as well as increased risk for dementia.  According to the most recent Lancet Commission report that examined 12 modifiable risk factors for dementia, such as education, hypertension, alcohol, obesity, smoking, depression, social isolation, physical activity, diabetes, air pollution, and traumatic brain injury, hearing loss was found to have the greatest impact on dementia risk among those assessed. Dementia, including its most common cause, Alzheimer's disease (AD), is a major public health challenge in many ageing societies worldwide, and identification of modifiable risk factors, such as hearing loss, is vitally important to developing risk reduction strategies that can delay and prevent the onset of dementia.

Dementia riskVisual impairment is another common problem among older adults and is also independently associated with dementia.  Although studies have predominantly focused on these sensory impairments individually, research is limited on the impact of concurrent hearing and visual impairment, or dual sensory impairment (DSI), on dementia risk. The prevalence of DSI in older adults ranges between 1.6 percent and 22.5 percent, depending on the sample and methods used to ascertain hearing and visual impairments; with an ageing population, the number of DSI cases is expected to increase over time.  Most, but not all, studies have shown that DSI is associated with greater cognitive decline and increased cognitive impairment.  To address this gap in knowledge regarding the potential impact of DSI on the development of dementia in older adults, we recently published a study that examined the association between DSI and incident dementia. In a cohort of older adults who were dementia-free when the study began, we found that DSI was associated with a significantly increased risk of all-cause dementia and AD compared with those with no impairments in hearing and vision.

Table 1Table 1: Associations Between Total Number of Sensory Impairments at Baseline and Risk of Incident All-cause Dementia, Alzheimer's Disease, and Vascular Dementia

Table 2Table 2: Associations Between Baseline Severity of DSI and Risk of All-cause Dementia

DEMENTIA DEVELOPMENT AND DSI

To conduct this prospective cohort study, 2,051 participants from the Ginkgo Evaluation of Memory (GEM) Study who were dementia-free at study baseline and aged 75 years and older were analysed. Briefly, the GEM Study was originally designed as a double-blind randomised controlled trial to determine the efficacy of Ginkgo biloba in preventing dementia among older adults. Participants were asked to self-report any problems with hearing or vision at the baseline visit of the GEM Study. Those who reported having hearing and vision problems were considered to have DSI. One of the major strengths of leveraging the GEM Study to investigate the research question was the rigorous ascertainment of dementia. Participants were assessed every six months for incident dementia, which was determined in a standard fashion as previously described.  This included taking into consideration the effects of hearing and visual problems at the time of dementia classification by the adjudication committee, which had a narrative from the examiner in each dementia case. All participants who were adjudicated as developing incident dementia based on criteria from the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition.  Designation of specific dementia subtypes was based on diagnostic criteria from the National Institute of Neurological and Communicative Disorders and Stroke, Alzheimer’s Disease and Related Disorders Association,  National Institute of Neurological Disorders and Stroke, Association Internationale pour la Recherche et l‘Enseignement en Neuroscience, and the AD Diagnostic and Treatment Centres.

Using Cox regression models, we estimated the risk of dementia associated with the number of sensory impairments (0, 1, or 2) by computing hazard ratios (HRs) and 95 percent confidence intervals (CIs). A group with no sensory impairment was used as the reference group. Models were adjusted for baseline covariates, including demographics and cardiovascular disease and dementia risk factors. Separate models were developed for all-cause dementia, AD only, and VaD (with or without AD). We also explored whether a dose-response relationship existed between DSI severity and the risk for all-cause dementia by constructing a summary score of impairments as the best approximation for DSI severity.

Among the 2,051 participants in our sample, 72 percent did not have self-reported hearing or visual impairment at baseline, whereas 23 percent had single sensory impairment in either hearing or vision and five percent had DSI with both hearing and visual impairment. In fully adjusted models, we found that DSI was associated with 86 percent increased risk for all-cause dementia (HR = 1.86; 95% CI = 1.25 – 2.76; p = 0.002) as compared with no sensory impairment (Table 1). DSI was also associated with 112 percent increased risk for AD (HR = 2.12; 95% CI = 1.34 – 3.36; p = 0.001) and a 1.22 greater risk for VaD (HR = 1.22; 95% CI = 0.52 – 2.89; p = 0.65). Among those with single sensory impairment, either in hearing or vision, we did not find any significantly increased risk of dementia. 

Evaluation of DSI severity in relation to dementia showed that the risk of all-cause dementia increased significantly with greater severity (Table 2). Compared with participants without DSI, those at the highest severity level were at a five-fold increased risk for dementia (HR = 5.09; 95% CI = 1.24 – 20.85; p = 0.02). Even those at the lowest severity level were at a 74 percent increased risk for dementia (HR = 1.74; 95% CI = 1.04 – 2.90; p = 0.03).

These findings suggest that older adults with DSI represent a high-risk population that could be a target for intervention prior to the onset of dementia. Several hypotheses can be advanced to explain the relationship between sensory and cognitive function. Greater impairment in hearing and vision may accelerate cognitive decline because of the association of sensory impairment with social isolation, depression, reduced physical and mental activities, and functional limitations. These impairments may also limit or stress the neural resources needed for optimal performance of cognitive tasks and can lead to changes in brain structure and function. Alternatively, common pathological processes, including vascular disease, inflammation, or some combination of these, may be responsible for the relationship between DSI and dementia.  Additional research is needed to determine the mechanisms underlying these associations. Future studies should also characterise the exact role of sensory impairments and whether treatments that improve sensory function, such as surgery or sensory aids, devices, and prostheses, can modify this risk. Because the public health burden of dementia is expected to increase over the next decades, evaluation of hearing and vision function in older adults may help identify patients at elevated risk of developing dementia.

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