Association of Sensory Impairments With Cognitive Decline and Depression Among Older Adults in China

Key Points Question Are visual and/or hearing impairments associated with cognitive decline and depression? Findings In this cross-sectional study of 18 038 participants aged 45 years or older in China, visual impairment was associated with worse performance in episodic memory and global cognition as well as with worse depression symptoms. Hearing impairment was also associated with poorer performance in episodic memory, mental intactness, and global cognition. Meaning In this study, visual and/or hearing impairments were associated with higher risks of cognitive decline and depression among middle-aged and older adults in China.


Introduction
The risk of visual and hearing impairments increases with age, and some emerging literature considers them risk factors for Alzheimer disease and related dementia. 1 By 2050, it is estimated that more than 900 million people in the world will be living with clinically meaningful hearing loss. 2 It is also estimated that approximately 2.2 billion people in the world experience some form of vision impairment, including blindness and moderate to severe vision impairment. 3 Visual and hearing impairments are chronic conditions that may increase the risk of cognitive decline and depressive symptoms. [4][5][6] Although the prevalence of visual impairment increases with age, 7 some vision impairments can be corrected via wearing glasses or surgery. 8 Clinical evidence suggests that cataract surgery resulting in improved vision is associated with improved cognitive performance and increased gray matter volume in the cortex. 9 Similarly, hearing impairment is increasingly common among older adults. Despite evidence showing that hearing loss increases the risk of cognitive impairment and that it is somewhat modifiable, in most cases, hearing loss remains untreated. 10,11 It has been shown that older adults with peripheral hearing loss have a 24% higher risk of mild cognitive impairment than those without. 12 The number of older adults with visual and/or hearing loss is growing worldwide, and China, with a rapidly aging population, is no exception. Compared with Europe and the United States, China has a substantially higher prevalence of sensory impairments, and research regarding the association of sensory impairments with cognitive decline and depression is lacking. 13 Using a nationally representative sample of adults aged 45 years and older, we aimed to explore the association of sensory impairments with cognitive function and depression in China. We hypothesized that sensory impairments are strongly associated with cognitive decline and depressive symptoms.

Data and Study Sample
The   what season of the year it was; and redraw a picture of 2 overlapped pentagons. Their scores from the questions were aggregated into a single mental intactness score, which ranged from 0 to 11. 22 The global cognitive function was the summation of the episodic memory and mental intactness scores. For all these measures, higher scores indicate better cognitive function.
Furthermore, CHARLS uses the 10-item Center for Epidemiological Studies-Depression scale (CES-D 10) to assess depression symptoms. 25 As a brief screening tool for depressive symptoms, the CES-D 10 was validated among middle-aged and older respondents in China. 26,27 The CES-D 10 includes 10 items with 4 answers on a 4-scale metric, ranging from rarely or none of the time (<1 day), some days (1-2 days), occasionally (3-4 days), or most of the time (5-7 days). Each item was scored from 0 to 3, with a total possible score ranging from 0 to 30 (Cronbach α = 0.815). 25 A higher score reflects more severe depressive symptoms. Previous studies suggested a cutoff point greater than or equal to 10 to differentiate between those with and without depression. 15,28

Sensory Function
The presence of sensory impairment was identified by self-reported assessment of visual and hearing functions. Similar to the Survey of Health, Aging and Retirement in Europe, CHARLS collected selfreported data on visual functions using 2 questions: "Is your eyesight for seeing things at a distance excellent (1), very good (2), good (3), fair (4), or poor (5)?" and "How good is your eyesight for seeing things up close, like reading ordinary newspaper print? Would you say your eyesight for seeing things up close is excellent (1), very good (2), good (3), fair (4), or poor (5)?" Data on hearing functions were also collected using the question: "Is your hearing excellent (1), very good (2), good (3), fair (4), or poor (5)?" We identified respondents as having visual or hearing impairment if they reported fair or poor vision (for either long distance or near vision) or hearing. We then categorized these measures as follows: no impairment, visual impairment only, hearing impairment only, and dual sensory impairment (both visual and hearing). excellent, very good, good, fair, and poor), activities of daily living, and presence of certain chronic diseases. Regarding the self-reported general health status, excellent, very good, and good were grouped under good, whereas fair and poor were grouped under poor. The ADLs in CHARLS are measured using a 6-item summary assessed with an ADL scale that includes eating, dressing, transferring, bathing, using the toilet, and continence. 29 The respondents were considered independent if they were able to complete all 6 activities without difficulty. Certain comorbidities and chronic disease conditions were captured based on self-reported chronic conditions, including hypertension, diabetes, dyslipidemia, heart problems, stroke, kidney diseases, asthma, lung diseases, arthritis, liver diseases, or stomach diseases.

Statistical Analysis
The respondents' characteristics were compared across categories of sensory impairments (visual impairment, hearing impairment, and dual sensory impairment) using an analysis of variance for numerical variables and ordinal χ 2 tests for discrete variables. Continuous variables were summarized using means and SDs. Categorical variables were reported using numbers and percentages. Multiple

Results
The characteristics of the respondents in the 2015 wave of the CHARLS were shown in Unadjusted mean rates of visual, hearing, and dual sensory impairment were clustered by age and sex (Figure 2). The direction and magnitude of sensory impairments increased by age for both men and women. Both sensory impairments were more pronounced among women compared with men in younger age categories. For example, among those younger than 70 years, women had substantially higher rates of both hearing and visual impairments than men (hearing impairment:  In

Discussion
To our knowledge, this is the first study examining the association of sensory impairments with cognitive function and depression among adults aged 45 years or older in China. Using nationally representative data, this study had 2 major findings. First, self-reported visual, hearing, and dual sensory impairment were highly prevalent conditions among adults in China. Second, both single and dual sensory impairment were associated with poor cognitive performance and depression.
The prevalence of sensory impairments in China was much higher than in Western Europe and the United States. 30 The Beaver Dam Offspring Study 31 found that visual and hearing impairments among respondents aged 21 to 84 years were 14.2% and 7.8% in Beaver Dam, Wisconsin. Another cross-sectional study in Europe 32 found even lower prevalence of visual (10.2%) and hearing (13.5%) impairments among people aged 50 years and older. The higher prevalence of sensory impairments in China may be because of the low use of eyeglasses, hearing aids, or any assistive devices. 13 This could be explained by traditional views regarding assistive devices driven by misinformation (ie, wearing glasses can lead to further loss of eyesight 33 ), financial constraints, 34 and lack of knowledge about hearing aids. 35 Furthermore, the differences between men and women in the prevalence of both single and dual sensory impairment were considerable, which begs for attention. In agreement with prior research, our results revealed that more women were living with sensory impairments than men. 13,36 Social and cultural differences may expose women to a greater risk of illnesses, while social, cultural, and economic differences may reduce access to services for women. 37 Cognition is a complex system that involves multiple domains related to episodic memory, executive function, mental intactness, and working memory. [38][39][40] Our results indicate that respondents with single or dual impairments had poorer performance in cognitive function and more severe depression. Interestingly, when we adjusted for both visual and hearing impairment, the association between visual impairment and cognitive function disappeared, while the association between hearing impairment and cognitive function stayed intact. This might suggest that hearing impairment is more closely associated with cognitive function than visual impairment. The association between hearing impairment and cognitive decline is supported in the literature. Hearing impairment has been also linked to more severe changes in the temporal lobe structures and brain functions. 41,42 Findings from previous research on the association between sensory impairments and cognitive decline reported conflicting results. The English Longitudinal Study of Aging found that visual and hearing impairments were associated with subsequent cognitive difficulties in older age. 43  There are several plausible explanations for the association of sensory impairments with cognitive decline and depression. The first is the cognitive load on perception hypothesis: cognitive decline can impose a harmful cognitive load on perceptual functioning. The second is the sensory deprivation hypothesis: perceptual impairment may lead to more permanent cognitive decline because of neuroplastic changes, depression, and social isolation. The third is the information degradation hypothesis: decreased perceptual information can lead to cognitive decline among older adults. The fourth is the common cause hypothesis: a common cause results in both age-related cognitive decline and sensory impairments in the process of neural degeneration.
Our findings agree with the sensory impairment deprivation hypothesis and the common cause hypothesis. The respondents with sensory impairments in the present study had a worse ADL functional status and more severe depression. Previous studies 13,51 revealed that people with sensory impairment had lower ADLs and experienced more depression and social isolation. The sensory impairment deprivation hypothesis indicates that depression and social isolation may mediate the association between sensory impairments and cognitive decline. 52,53 Research suggests that visual impairment and dual sensory impairment are associated with depression because of increased social isolation and loneliness. 54,55 Less frequent social interactions may result in cognitive decline in older age. 56 The common cause hypothesis, which states that sensory impairments as well as cognitive decline and depression may be caused by a common pathological process, such as vascular disease, should also be considered. For example, β-amyloid pathology might damage both sensory and cognitive abilities. 57 These hypotheses are not mutually exclusive, and the present cross-sectional study does not allow for a conclusive distinction between them. More studies are needed to further investigate these hypotheses.

Limitations
This study has limitations. First, visual and hearing sensations were assessed though self-reported measures, without any comparative objective sensory data. Second, the cross-sectional nature of the design meant we could determine the mechanistic basis of the observed association of sensory impairments with cognitive decline and depressive symptoms, so the findings should be interpreted with caution. Third, the study did not investigate whether assistive devices (eg, hearing aids, glasses, or portable magnifiers) could potentially mitigate cognitive function and depressive symptoms.

Conclusions
In this study, the association of sensory impairments with cognitive function and depression were explored using a nationally representative sample in China. The findings suggest that sensory impairments are associated with a higher risk of cognitive decline and depression. The association between hearing impairment and cognitive function among older adults is of paramount importance.
There is a dearth of epidemiological studies on this topic in China. Considering China's rapidly aging population and high prevalence of sensory impairment, this is a salient topic with important clinical and public health implications.