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Introduction: Hearing and vision loss are highly prevalent in elderly adults, and thus frequently occur in conjunction with cognitive impairments. Studies have shown that hearing impairment is associated with a higher risk of dementia. However, evidence concerning the association between vision loss and dementia, as well as the co-occurrence of vision and hearing loss and dementia, has been inconclusive.
This analysis adds important evidence that contributes to the limited body of knowledge on the association between hearing and/or vision loss and dementia, demonstrating that hearing impairment only is significantly associated with a higher risk of dementia. Neither of the documented diagnoses objectively indicated that vision loss or the combination of visual and hearing impairment was significantly associated with dementia.
Several previous studies have evaluated the association between hearing loss and the risk of dementia. Loughrey et al. (2018) revealed, based on a meta-analysis of 20,264 subjects from different prospective cohort studies, that hearing impairment was significantly associated with dementia diseases (OR 1.28; CI 95% 1.02–1.59). This finding is in line with the results of the present analysis. Due to the larger sample size in our analysis, the confidence interval of our analysis was smaller and, thus, our results are more precise (Wei et al., 2017; Loughrey et al., 2018).
However, there is some evidence suggesting that vision loss is associated with an increased risk of developing dementia diseases. Based on a retrospective cohort study that included 7,685 patients, Davies-Kershaw et al. (2018) revealed that patients who rated their own vision as moderate were two times (HR 2.0, CI 95% 1.4–3.1) and those with very poor vision, which was comparable to being blind, four times (HR 4.0, CI 95% 2.6–6.1) more likely to have dementia compared to those without vision impairment. Furthermore, based on a population cohort of 7,736 initially healthy and non-dementia patients, Naël et al. (2019) revealed that moderate to severe near-vision impairment was also associated with an increased risk of dementia in the first 4 years of follow-up (HR 2.0, CI 95% 1.2–3.3), but not when patients were followed for more than 4 years. Furthermore, self-reported vision impairment was associated with an increased risk of dementia within the time frame of 4 years (HR 1.5, CI 95% 1.1–2.0), but this association was no longer significant after adjusting for important baseline covariates, such as cognitive impairment. Therefore, these results may suggest that vison loss could be associated with an increased risk in the short-term. However, results remain uncertain in the long run. Our findings showed that there was no significant association with an increased risk of dementia for any of the objective measures of vision loss. Neither refraction disorders nor severe visual impairment—including blindness—were found to be significantly associated with an increased risk of dementia diseases, which is contrary to some previous findings. The study of Davies-Kershaw et al. (2018) was based on a large nationwide sample of people aged 50 years and older. However, just 2.5% (n = 195) of patients in the sample were diagnosed with dementia diseases, which is well below the general population estimates for a group of the same age composition. Furthermore, the observed association was based on a much smaller sample size when compared to this analysis (Matthews et al., 2013, 2016).
Even though dementia diseases are, in fact, underdiagnosed, the low number of dementia cases in this study limits the generalizability of these findings. Furthermore, even though some studies confirmed the comparability and validity of subjective and objective measures of vision impairment (Whillans and Nazroo, 2014), the use of some listed diagnoses made by general practitioners that are related to vision loss (Davies-Kershaw et al., 2018) could be a further reason for the deviating results, especially since only a single question was used to assess the self-reported vision loss, whatever the cause of vision loss was. The information given in primary and secondary data sets could tremendously differ with respect to the identification of patients’ sensory impairments. Whereas hearing impairment is usually well-documented in general practitioners’ files, this does not apply for vision impairment, as demonstrated by this analysis. Therefore, further research is needed to evaluate if the secondary datasets are valid for the identification of sensory impairments, especially vision impairment. In addition, the ability to self-report vision loss probably varies according to cognitive capacities. This could be the reason for the inconsistent findings regarding a significant association between vision loss and the risk of dementia, which has been observed in previous studies. Therefore, further research is needed to clarify the validity of objective and subjective measures for vision loss, as well as explain the differences in their association with a higher risk of developing dementia diseases.
The results of our study demonstrated that only hearing loss is associated with a higher risk of dementia diseases and that the combination of both hearing and vision loss certainly increases sensory impairment, but not the risk of developing dementia diseases, thus demonstrating the importance of hearing ability as a protective factor against dementia patients. People with hearing loss have various neuronal changes. Hearing loss in older adults may result in an acceleration of aging because the nervous systems can alter synapses and neural anatomy (Martini et al., 2014). To compensate for the decreased auditory input that is caused by hearing loss, more listening effort is required through the additional recruitment of frontal areas (Campbell and Sharma, 2013). In hearing-impaired patients, an increase in cognitive load occurs. Consequently, cognitive processes such as memory and executive function are adversely affected (Boyle et al., 2008). Thus, hearing loss may alter the usual pattern of resource allocation in the brain, affecting neural reserves and cognitive performance (Lin et al., 2013), and leading to altered auditory processing. Hence, early onset of hearing loss in older adults may causally accelerate atrophy in the entire brain, which could lead to cognitive reserve depletion in the brain (Lin et al., 2014). The coexistence of hearing loss in patients living with dementia could cause adverse patient-related outcomes due to difficulties in participating in daily social activities and to breakdowns in communication between patients and caregivers and between patients and professionals. These challenges, in turn, can lead to social exclusion, increase stress and fatigue, and exacerbate neuropsychiatric behaviors, such as apathy, depression, and aggression (Slaughter et al., 2014; Palmer et al., 2017). All of these are related to a lower health-related quality of life.
Even though the prevalence of hearing loss in patients living with dementia is very high and the association between hearing loss and cognitive decline is well studied, the utilization of hearing aids is only moderate (Nirmalasari et al., 2017). Maharani et al. (2018) revealed that early recognition and treatment of hearing loss could have the potential to slow down cognitive decline and potentially delay the onset of dementia, and should, therefore, be a focus of healthcare providers and care management models. Therefore, hearing impairment must be identified early in older adults and in patients living with dementia, and the unmet need for effective hearing aids must be met. Collaborative care management approaches could be a potential tool for improving this situation, and thus help patients get access to such important aids, which can delay the progression of this degenerative disease. Therefore, several commissions—such as, for example, the Lancet Commission on Dementia Prevention, Intervention, and Care—highlight the importance of optimizing hearing in patients with dementia in order to improve the management of psychosis, agitation, and depression related to this condition (Livingston et al., 2017). According to the findings of previously published studies, as well as the current study, early recognition and treatment of hearing loss has the potential to delay the onset of dementia diseases and improve patients’ outcomes. It should, therefore, be a focal point for healthcare providers and care management models. However, further research is needed to evaluate the efficacy and cost-effectiveness of such hearing aids in patients where cognitive impairment or dementia and hearing loss coexist.
We conducted a case-control study of patients with and without a diagnosis of a dementia disease, using data from GP practices. The diagnoses listed in the files of the general practitioners are usually used for reimbursement. Therefore, the data may not be complete, especially not for vision impairment diagnoses, which are, in most cases, not documented in general practitioners’ files. Among the elderly, the prevalence of vision impairment is approximately 70%. In this study, diagnoses related to vision impairment of any type was only documented in 28% of cases, demonstrating that vision impairment was underdiagnosed and underrepresented in this data set. This limits the generalizability of the non-significant association found between dementia and vision impairment and should, therefore, be evaluated in further research, using other objective or subjective measures.
Furthermore, hearing impairment during the cognitive assessment or dementia screening procedures might anticipate the dementia diagnoses due to the fact that patients are unable to hear and, thus, to understand questions included in the assessment correctly, even though these patients might not be cognitively impaired. This could lead to false-positive dementia diagnoses and, thus, could support the association between hearing loss and dementia. In addition, our analyses were based on the initially documented dementia diagnoses. It should be noted that the basic process for recognizing cognitive impairment is completely different compared to the process used for reimbursement purposes. Therefore, the diagnoses used in the analyses are not verified for their correctness and accuracy. This also applies to the diagnosis of hearing and vision loss. In addition, dementia diseases are underdiagnosed by general practitioners. In Germany, only 40% of PwD are formally diagnosed with dementia. This diagnosis rate of dementia in German primary care is well within the range of the international data (20%–50%; Eichler et al., 2014). This analysis was based on diagnoses listed in the files of general practitioners. Diagnoses made by specialists such as neurologists or psychiatrists, who usually handle the differential diagnosis process, were not included in this analysis, which limits the generalizability of the results.
Despite these limitations, the strength of the present study was that the analysis was based on a total of 122,708 patients treated by their general practitioners. Therefore, the sample is adequate for answering the research questions, with considerable external validity.
Bernhard Michalowsky,1 Wolfgang Hoffmann,1,2 and Karel Kostev3
Study reported at the following link:
