https://orcid.org/0000-0002-9081-2349
Abstract
highly prevalagent hearing and vision sensory impairments among older people may contribute to the risk of cognitive decline and pathological impairments including dementia.
This study aims to determine whether single and dual sensory impairment (hearing and/or vision) are independently associated with cognitive decline among older adults and to describe cognitive trajectories according to their impairment pattern.
we used data from totals of 13,123, 11,417 and 21,265 respondents aged 50+ at baseline from the Health and Retirement Study (HRS), the English Longitudinal Study of Ageing (ELSA) and the Survey of Health, Ageing and Retirement in Europe (SHARE), respectively. We performed growth curve analysis to identify cognitive trajectories, and a joint model was used to deal with attrition problems in longitudinal ageing surveys.
respondents with a single sensory impairment had lower episodic memory score than those without sensory impairment in HRS (β = −0.15, P < 0.001), ELSA (β = −0.14, P < 0.001) and SHARE (β = −0.26, P < 0.001). The analysis further shows that older adults with dual sensory impairment in HRS (β = −0.25, P < 0.001), ELSA (β = −0.35, P < 0.001) and SHARE (β = −0.68, P < 0.001) remembered fewer words compared with those with no sensory impairment. The stronger associations between sensory impairment and lower episodic memory levels were found in the joint model which accounted for attrition.
hearing and/or vision impairments are a marker for the risk of cognitive decline that could inform preventative interventions to maximise cognitive health and longevity. Further studies are needed to investigate how sensory markers could inform strategies to improve cognitive ageing.
Introduction
Maintaining cognitive function in later life has become a public health priority as the burden imposed by dementia in the ageing population has increased more rapidly than that of most other diseases [1]. In the past 10 years, the disability-adjusted life-years (DALYs) for Alzheimer’s disease and other dementias have grown by one-third, from 17,905 DALYs per 1,000 population in 2005 to 23,779 DALYs per 1,000 population in 2015 [2]. Understanding cognitive change in later life is thus crucial, as cognitive decline is a hallmark of dementia [3].
Hearing and vision sensory impairments among older people may contribute to the risk of cognitive decline and pathological impairments including dementia. Numerous cross-sectional studies, but only a limited number of longitudinal studies have demonstrated an association between sensory impairment and cognitive decline [4–6]. A study using six waves of the Berlin Ageing Studies reported moderate size correlations between visual/hearing acuity and sensory decline [7]. An Australian longitudinal study found that the significant association between sensory impairment and cognitive declines diminished after adjusting for age and other potential confounding factors [8].
In this study, our aim is to identify how sensory impairment relates to trajectories of cognitive decline among older adults, accounting for attrition in our models. Longitudinal studies, especially in the field of ageing, suffer from attrition as their respondents tend to selectively dropout because of either death or worsening health function [9]. Unless those dropouts can be assumed to be ‘missing at random’, ignoring them can result in a bias in the analysis. Describing and understanding trajectories of cognitive decline and how these trajectories relate to sensory impairment may offer insight into the dynamics of cognitive decline and identify opportunities for intervention to maximise cognitive function and longevity in older age.
Data and measures
Data
This study used three international surveys of ageing: the Health and Retirement Study (HRS), the English Longitudinal Study of Ageing (ELSA) and the Survey of Health, Ageing and Retirement in Europe (SHARE). These three surveys provide information on the personal, socio-economic and health circumstances of individuals aged 50+ [10–12]. The baseline interviews of HRS were conducted with community-dwelling adults in the USA in 1992. Respondents who entered a nursing home after the baseline interview are retained in the sample and were interviewed if possible in the following waves. The first wave of ELSA data was collected in 2002, while the SHARE study was started in 2004. So far, there are twelve, seven and five waves available for HRS, ELSA and SHARE, respectively. In this study, we use data from a similar time range: 2002–14 for HRS and ELSA, and 2004–14 for SHARE. We restrict our attention to the nine countries with complete data in SHARE: Austria, Belgium, Denmark, France, Germany, Italy, Spain, Sweden and Switzerland.
Dependent variable: episodic memory score
We used the measure of cognitive function available in all the three surveys, namely, episodic memory. Episodic memory represents general cognition [13] and is more age-sensitive than other episodic measures [14]. Salthouse et al. [13] concluded that memory and cognitive control variables appear to have a common mechanism. In all surveys, the interviewer reads a list of 10 common nouns to the respondent and then asks the respondent to recall as many words as possible from the list in any order twice: immediately after the respondent heard the complete list (immediate recall) and at the end of the cognitive function module (delayed recall). The immediate recall represents the ability of respondents to learn or store new information, whereas delayed recall is the ability to recall that information after a period of distraction from that information. Including only one test does not represent the memory function particularly well. Prior study showed evidence of a high correlation between immediate and delayed recall (r = 0.70–0.75) in the first wave of ELSA [15]. We calculated the raw scores as the total of the number of correct words of immediate recall and delayed recall, with a maximum score of 20 [16, 17].
Main independent variable: sensory function
Sensory impairment was measured using self-reported hearing and vision quality. In all three surveys, hearing data was collected using the following self-reported measure of overall hearing function: ‘Is your hearing [using a hearing aid as usual] excellent (1), very good (2), good (3), fair (4) or poor (5)?’. In HRS and ELSA, self-reported vision quality was collected in all seven waves using the question: ‘Is your eyesight [using glasses or corrective lens as usual] excellent (1), very good (2), good (3), fair (4) or poor (5)?’. In SHARE, we used the two self-reported measures of visual function that are present in all waves: distance eyesight and reading eyesight. For each question, responses were recoded into two categories by combining the responses excellent, very good and good into one and collapsing fair and poor vision into a second category. We defined sensory impairment as having fair and poor hearing and/or vision [18]. As SHARE has two questions to measure vision quality, we classified its respondents as having poor vision if they had poor distance eyesight and/or reading eyesight. We then categorised the results as follows: no impairment, single (vision or hearing) sensory impairment and dual sensory impairment (impairment in both senses).
Co-variates
We included demographic (age and gender), socio-economic information, health behaviour and the presence of chronic diseases as determinants of cognitive function. As with other health functions, cognitive abilities are shaped by social determinants of health [19], including education, marital status and wealth. We categorised respondents’ education into less than high school as reference, high school and college. Marital status was classified as married and not married. We used quintiles of income by country each year to measure wealth, using the poorest quintile as the reference [16]. The social capital index is obtained in two steps. Firstly, we sum the activities in the month leading up to the day that the respondent was interviewed, such as performing voluntary work; helping friends, neighbours and relatives; and taking part in a community organisation. Secondly, we standardise those summation scores.
Cognitive function is affected by health behaviour (smoking status, drinking behaviour and physical activities) and health status [20]. In terms of smoking status, we categorised respondents as current smokers, past smokers and non-smokers. We classified respondents as drinking regularly if they consumed alcohol ≥5 days/week [16] and as having vigorous physical activities if they did those activities at least once a week. Functional status was measured using the Activity Daily Living (ADL) scale. The ADL scale used five items of self-performance in HRS (dressing, walking across the room, bathing, getting in or out of bed and eating). The self-performance of toilet use was added in SHARE and ELSA. We used the measures of the presence of depression (measured by Center for Epidemiologic Studies Depression (CESD) in HRS and ELSA and Euro-D in SHARE) and chronic diseases (the sum of several chronic diseases: diabetes, stroke, lung diseases and cancer) to capture the health status of the respondents.
Statistical analysis
We compared the characteristics of respondents in the first wave of HRS, ELSA and SHARE separately according to the presence of sensory impairment using Kruskal–Wallis one-way analysis of variance for numerical variables and ordinal chi-square tests for categorical variables. We then performed multilevel growth curve models separately for each survey to predict the level of cognition in Wave 1 and subsequent changes in cognition over further waves, all dependent on the Wave 1 age cohort. The multilevel models in this report consist of repeated observations nested within individuals. We included demographic and socio-economic determinants, lifestyle factors and the presence of chronic diseases as the co-variates.
To deal with biases due to dropout, we used a joint model where the random effects influence both episodic memory and attrition, and given these, episodic memory and attrition are independent. The joint model in this study had two parts: the growth curve model and the survival model (with sex, age polynomial of degree three and the random intercepts from the growth curve model) [16]. We compared the results of sensory impairment model with that of the joint model to assess the robustness of the sensory impairment model to attrition [21]. The statistical analyses were conducted using Stata 14.0 and Latent Gold 5.1.
Results
The characteristics of respondents in the first wave of the HRS (2002), ELSA (2002) and SHARE (2004) studies are presented in Table 1. On average, respondents in the first wave of HRS, ELSA and SHARE were able to memorise 10, 9.4 and 7.9, respectively. The descriptive analysis of respondents by the presence of sensory impairment was presented in Appendix A–C, available at Age and Ageing online. The proportion of respondents with dual sensory impairment was highest among SHARE respondents (7.7%), followed by HRS (7%) and ELSA (5.8%). Respondents with dual sensory impairment were likely to perform less well on the cognitive tests, to have lower income, to be older, less educated, have more ADL dependencies, and be less socially engaged than those with no or single impairment.
Sample characteristics at baseline by survey
| HRS | ELSA | SHARE | |
|---|---|---|---|
| N = 13,123 | N = 11,417 | N = 21,265 | |
| Mean episodic memory | 10 (3.4) | 9.4 (3.5) | 7.9 (3.6) |
| Mean age | 67.8 (10.5) | 64.8 (10) | 64.8 (10.1) |
| Female, % | 58.2 | 54.4 | 54.5 |
| Married/cohab, % | 68.1 | 67 | 72.2 |
| Education | |||
| Less than high school, % | 19.4 | 34.5 | 50.8 |
| High school, % | 33.9 | 17.8 | 30.6 |
| Some college, % | 46.6 | 47.5 | 18.4 |
| Employed, % | 38.1 | 35.9 | 27.2 |
| Smoking behaviour | |||
| Non-smoker, % | 41.9 | 33.7 | 28.3 |
| Past smoker, % | 44.2 | 48.5 | 17.8 |
| Current smoker, % | 13.7 | 17.7 | 53.7 |
| Drink daily or almost daily, % | 12.3 | 28.2 | 26.3 |
| Mean depression scorea | 1.3 (1.8) | 1.5 (1.9) | 0.2 (0.4) |
| Mean ADL (SD) | 0.24 (0.73) | 0.4 (0.9) | 0.2 (0.7) |
| Vigorous exercise, % | 24.6 | 27.5 | 47.2 |
| Number of co-morbidities | 1.1 (1.1) | 0.6 (0.8) | 0.6 (0.8) |
| HRS | ELSA | SHARE | |
|---|---|---|---|
| N = 13,123 | N = 11,417 | N = 21,265 | |
| Mean episodic memory | 10 (3.4) | 9.4 (3.5) | 7.9 (3.6) |
| Mean age | 67.8 (10.5) | 64.8 (10) | 64.8 (10.1) |
| Female, % | 58.2 | 54.4 | 54.5 |
| Married/cohab, % | 68.1 | 67 | 72.2 |
| Education | |||
| Less than high school, % | 19.4 | 34.5 | 50.8 |
| High school, % | 33.9 | 17.8 | 30.6 |
| Some college, % | 46.6 | 47.5 | 18.4 |
| Employed, % | 38.1 | 35.9 | 27.2 |
| Smoking behaviour | |||
| Non-smoker, % | 41.9 | 33.7 | 28.3 |
| Past smoker, % | 44.2 | 48.5 | 17.8 |
| Current smoker, % | 13.7 | 17.7 | 53.7 |
| Drink daily or almost daily, % | 12.3 | 28.2 | 26.3 |
| Mean depression scorea | 1.3 (1.8) | 1.5 (1.9) | 0.2 (0.4) |
| Mean ADL (SD) | 0.24 (0.73) | 0.4 (0.9) | 0.2 (0.7) |
| Vigorous exercise, % | 24.6 | 27.5 | 47.2 |
| Number of co-morbidities | 1.1 (1.1) | 0.6 (0.8) | 0.6 (0.8) |
aDepression scores are CESD in HRS and ELSA and Euro-D in SHARE.
Sample characteristics at baseline by survey
| HRS | ELSA | SHARE | |
|---|---|---|---|
| N = 13,123 | N = 11,417 | N = 21,265 | |
| Mean episodic memory | 10 (3.4) | 9.4 (3.5) | 7.9 (3.6) |
| Mean age | 67.8 (10.5) | 64.8 (10) | 64.8 (10.1) |
| Female, % | 58.2 | 54.4 | 54.5 |
| Married/cohab, % | 68.1 | 67 | 72.2 |
| Education | |||
| Less than high school, % | 19.4 | 34.5 | 50.8 |
| High school, % | 33.9 | 17.8 | 30.6 |
| Some college, % | 46.6 | 47.5 | 18.4 |
| Employed, % | 38.1 | 35.9 | 27.2 |
| Smoking behaviour | |||
| Non-smoker, % | 41.9 | 33.7 | 28.3 |
| Past smoker, % | 44.2 | 48.5 | 17.8 |
| Current smoker, % | 13.7 | 17.7 | 53.7 |
| Drink daily or almost daily, % | 12.3 | 28.2 | 26.3 |
| Mean depression scorea | 1.3 (1.8) | 1.5 (1.9) | 0.2 (0.4) |
| Mean ADL (SD) | 0.24 (0.73) | 0.4 (0.9) | 0.2 (0.7) |
| Vigorous exercise, % | 24.6 | 27.5 | 47.2 |
| Number of co-morbidities | 1.1 (1.1) | 0.6 (0.8) | 0.6 (0.8) |
| HRS | ELSA | SHARE | |
|---|---|---|---|
| N = 13,123 | N = 11,417 | N = 21,265 | |
| Mean episodic memory | 10 (3.4) | 9.4 (3.5) | 7.9 (3.6) |
| Mean age | 67.8 (10.5) | 64.8 (10) | 64.8 (10.1) |
| Female, % | 58.2 | 54.4 | 54.5 |
| Married/cohab, % | 68.1 | 67 | 72.2 |
| Education | |||
| Less than high school, % | 19.4 | 34.5 | 50.8 |
| High school, % | 33.9 | 17.8 | 30.6 |
| Some college, % | 46.6 | 47.5 | 18.4 |
| Employed, % | 38.1 | 35.9 | 27.2 |
| Smoking behaviour | |||
| Non-smoker, % | 41.9 | 33.7 | 28.3 |
| Past smoker, % | 44.2 | 48.5 | 17.8 |
| Current smoker, % | 13.7 | 17.7 | 53.7 |
| Drink daily or almost daily, % | 12.3 | 28.2 | 26.3 |
| Mean depression scorea | 1.3 (1.8) | 1.5 (1.9) | 0.2 (0.4) |
| Mean ADL (SD) | 0.24 (0.73) | 0.4 (0.9) | 0.2 (0.7) |
| Vigorous exercise, % | 24.6 | 27.5 | 47.2 |
| Number of co-morbidities | 1.1 (1.1) | 0.6 (0.8) | 0.6 (0.8) |
aDepression scores are CESD in HRS and ELSA and Euro-D in SHARE.
To avoid confounding relationships and to arrive at net associations, results from the growth curve and joint models separately for each survey are presented in Table 2. It shows that sensory impairment has a significant negative relationship to cognitive ability. Focusing on the growth curve model, respondents with single and dual impairments performed less well than those with none. This negative association was considerably greater after attrition was taken into account in the joint model. For example, the episodic memory levels of HRS respondents with single and dual sensory impairments were lower by −0.15 and −0.25 words before accounting for attrition and those associations were larger in the joint model (by −0.56 and −1.14 words). The same findings are confirmed in ELSA and SHARE.
Growth curve and joint models predicting episodic memory scores
| HRS | ELSA | SHARE | ||||
|---|---|---|---|---|---|---|
| Growth curve | Joint | Growth curve | Joint | Growth curve | Joint | |
| Age | 0.4 (0.01)* | −0.13 (0.00)* | 0.42 (0.01)* | −0.12 (0.00)* | 0.33 (0.01)* | −0.13 (0.00)* |
| Age2 | −0.00 (0.00)* | 0.00 (0.00)* | −0.00 (0.00)* | 0.00 (0.00)* | −0.3 (0.00)* | 0.02 (0.00)* |
| Sensory function, ref: No impairment | ||||||
| Single impairment | −0.15 (0.02)* | −0.56 (0.01)* | −0.14(0.02)* | −0.55 (0.02)* | −0.26 (0.01)* | −0.58 (0.01)* |
| Dual impairment | −0.25 (0.04)* | −1.14 (0.02)* | −0.35 (0.05)* | −1.3 (0.03)* | −0.68 (0.03)* | −1.61 (0.02)* |
| Female | 1.21 (0.03)* | 1.05 (0.01)* | 0.79 (0.04)* | 0.66 (0.01)* | 0.97 (0.02)* | 0.74 (0.01)* |
| Married | −0.02 (0.03) | −0.07 (0.02)* | −0.01 (0.03) | −0.03 (0.02) | 0.08 (0.02)* | 0.05 (0.01)* |
| Education, ref: Less than high school | ||||||
| High school | 0.78 (0.04)* | 0.65 (0.03)* | 1.58 (0.04)* | 1.27 (0.03)* | 1.67 (0.02)* | 1.52 (0.02)* |
| Some college | 1.58 (0.04)* | 1.3 (0.03)* | 1.94 (0.05)* | 1.54 (0.03)* | 2.5 (0.02)* | 2.26 (0.02)* |
| Wealth, ref: 1st quartile (poorest) | ||||||
| 2nd Quartile | 0.22 (0.03)* | 0.28 (0.03)* | 0.07 (0.04) | 0.24 (0.04)* | 0.16 (0.02)* | 0.21 (0.02)* |
| 3rd Quartile | 0.35 (0.03)* | 0.44 (0.03)* | 0.23 (0.04)* | 0.49 (0.04)* | 0.2 (0.02)* | 0.32 (0.02)* |
| 4th Quartile | 0.46 (0.03)* | 0.59 (0.03)* | 0.33 (0.04)* | 0.7 (0.04)* | 0.27 (0.02)* | 0.41 (0.02)* |
| 5th Quartile (richest) | 0.59 (0.03)* | 0.76 (0.03)* | 0.4 (0.04)* | 0.99 (0.04)* | 0.35 (0.02)* | 0.51 (0.02)* |
| Employed | 0.28 (0.02)* | 0.22 (0.02)* | 0.13 (0.03)* | −0.02 (0.03) | 0.35 (0.02)* | 0.4 (0.02)* |
| Social capital | 0.2 (0.01)* | 0.32 (0.01)* | 0.28 (0.01)* | 0.24 (0.01)* | 0.37 (0.02)* | 0.47 (0.00)* |
| Smoking, ref: Non-smoker | ||||||
| Current smoker | −0.12 (0.04)* | −0.46 (0.02)* | −0.06 (0.05) | −0.5 (0.02)* | 0.39 (0.02)* | 0.2 (0.01)* |
| Past smoker | −0.01 (0.03) | 0.00 (0.01) | 0.00 (0.04) | 0.02 (0.01) | 0.34 (0.02)* | 0.45 (0.01)* |
| Drink daily or almost daily | 0.12 (0.03)* | 0.21 (0.03)* | 0.07 (0.03) | 0.2 (0.03)* | −0.2 (0.02)* | −0.19 (0.02)* |
| Vigorous physical activity | 0.15 (0.02)* | 0.1 (0.02)* | 0.14 (0.02)* | 0.23 (0.02)* | 0.31 (0.01)* | 0.39 (0.01)* |
| ADL | −0.23 (0.01)* | −0.26 (0.01)* | −0.08 (0.01)* | −0.12 (0.01)* | −0.25 (0.01)* | −0.38 (0.01)* |
| Depression score | −0.05 (0.00)* | −0.07 (0.00)* | −0.06 (0.00)* | −0.08 (0.00)* | −0.44 (0.01)* | −0.49 (0.02)* |
| Number of co-morbidities | −0.11 (0.01)* | −0.01 (0.01)* | −0.07 (0.01)* | −0.03 (0.01) | −0.06 (0.01)* | −0.02 (0.01)* |
| Constant | −1.58 (0.49)* | 17.07 (0.07)* | −2.47 (0.6)* | 17 (0.08)* | −1.68 (0.4)* | 15.2 (0.05)* |
| HRS | ELSA | SHARE | ||||
|---|---|---|---|---|---|---|
| Growth curve | Joint | Growth curve | Joint | Growth curve | Joint | |
| Age | 0.4 (0.01)* | −0.13 (0.00)* | 0.42 (0.01)* | −0.12 (0.00)* | 0.33 (0.01)* | −0.13 (0.00)* |
| Age2 | −0.00 (0.00)* | 0.00 (0.00)* | −0.00 (0.00)* | 0.00 (0.00)* | −0.3 (0.00)* | 0.02 (0.00)* |
| Sensory function, ref: No impairment | ||||||
| Single impairment | −0.15 (0.02)* | −0.56 (0.01)* | −0.14(0.02)* | −0.55 (0.02)* | −0.26 (0.01)* | −0.58 (0.01)* |
| Dual impairment | −0.25 (0.04)* | −1.14 (0.02)* | −0.35 (0.05)* | −1.3 (0.03)* | −0.68 (0.03)* | −1.61 (0.02)* |
| Female | 1.21 (0.03)* | 1.05 (0.01)* | 0.79 (0.04)* | 0.66 (0.01)* | 0.97 (0.02)* | 0.74 (0.01)* |
| Married | −0.02 (0.03) | −0.07 (0.02)* | −0.01 (0.03) | −0.03 (0.02) | 0.08 (0.02)* | 0.05 (0.01)* |
| Education, ref: Less than high school | ||||||
| High school | 0.78 (0.04)* | 0.65 (0.03)* | 1.58 (0.04)* | 1.27 (0.03)* | 1.67 (0.02)* | 1.52 (0.02)* |
| Some college | 1.58 (0.04)* | 1.3 (0.03)* | 1.94 (0.05)* | 1.54 (0.03)* | 2.5 (0.02)* | 2.26 (0.02)* |
| Wealth, ref: 1st quartile (poorest) | ||||||
| 2nd Quartile | 0.22 (0.03)* | 0.28 (0.03)* | 0.07 (0.04) | 0.24 (0.04)* | 0.16 (0.02)* | 0.21 (0.02)* |
| 3rd Quartile | 0.35 (0.03)* | 0.44 (0.03)* | 0.23 (0.04)* | 0.49 (0.04)* | 0.2 (0.02)* | 0.32 (0.02)* |
| 4th Quartile | 0.46 (0.03)* | 0.59 (0.03)* | 0.33 (0.04)* | 0.7 (0.04)* | 0.27 (0.02)* | 0.41 (0.02)* |
| 5th Quartile (richest) | 0.59 (0.03)* | 0.76 (0.03)* | 0.4 (0.04)* | 0.99 (0.04)* | 0.35 (0.02)* | 0.51 (0.02)* |
| Employed | 0.28 (0.02)* | 0.22 (0.02)* | 0.13 (0.03)* | −0.02 (0.03) | 0.35 (0.02)* | 0.4 (0.02)* |
| Social capital | 0.2 (0.01)* | 0.32 (0.01)* | 0.28 (0.01)* | 0.24 (0.01)* | 0.37 (0.02)* | 0.47 (0.00)* |
| Smoking, ref: Non-smoker | ||||||
| Current smoker | −0.12 (0.04)* | −0.46 (0.02)* | −0.06 (0.05) | −0.5 (0.02)* | 0.39 (0.02)* | 0.2 (0.01)* |
| Past smoker | −0.01 (0.03) | 0.00 (0.01) | 0.00 (0.04) | 0.02 (0.01) | 0.34 (0.02)* | 0.45 (0.01)* |
| Drink daily or almost daily | 0.12 (0.03)* | 0.21 (0.03)* | 0.07 (0.03) | 0.2 (0.03)* | −0.2 (0.02)* | −0.19 (0.02)* |
| Vigorous physical activity | 0.15 (0.02)* | 0.1 (0.02)* | 0.14 (0.02)* | 0.23 (0.02)* | 0.31 (0.01)* | 0.39 (0.01)* |
| ADL | −0.23 (0.01)* | −0.26 (0.01)* | −0.08 (0.01)* | −0.12 (0.01)* | −0.25 (0.01)* | −0.38 (0.01)* |
| Depression score | −0.05 (0.00)* | −0.07 (0.00)* | −0.06 (0.00)* | −0.08 (0.00)* | −0.44 (0.01)* | −0.49 (0.02)* |
| Number of co-morbidities | −0.11 (0.01)* | −0.01 (0.01)* | −0.07 (0.01)* | −0.03 (0.01) | −0.06 (0.01)* | −0.02 (0.01)* |
| Constant | −1.58 (0.49)* | 17.07 (0.07)* | −2.47 (0.6)* | 17 (0.08)* | −1.68 (0.4)* | 15.2 (0.05)* |
Note: Reported are co-efficients (standard errors). Sig.: *Significant at 1%.
Growth curve and joint models predicting episodic memory scores
| HRS | ELSA | SHARE | ||||
|---|---|---|---|---|---|---|
| Growth curve | Joint | Growth curve | Joint | Growth curve | Joint | |
| Age | 0.4 (0.01)* | −0.13 (0.00)* | 0.42 (0.01)* | −0.12 (0.00)* | 0.33 (0.01)* | −0.13 (0.00)* |
| Age2 | −0.00 (0.00)* | 0.00 (0.00)* | −0.00 (0.00)* | 0.00 (0.00)* | −0.3 (0.00)* | 0.02 (0.00)* |
| Sensory function, ref: No impairment | ||||||
| Single impairment | −0.15 (0.02)* | −0.56 (0.01)* | −0.14(0.02)* | −0.55 (0.02)* | −0.26 (0.01)* | −0.58 (0.01)* |
| Dual impairment | −0.25 (0.04)* | −1.14 (0.02)* | −0.35 (0.05)* | −1.3 (0.03)* | −0.68 (0.03)* | −1.61 (0.02)* |
| Female | 1.21 (0.03)* | 1.05 (0.01)* | 0.79 (0.04)* | 0.66 (0.01)* | 0.97 (0.02)* | 0.74 (0.01)* |
| Married | −0.02 (0.03) | −0.07 (0.02)* | −0.01 (0.03) | −0.03 (0.02) | 0.08 (0.02)* | 0.05 (0.01)* |
| Education, ref: Less than high school | ||||||
| High school | 0.78 (0.04)* | 0.65 (0.03)* | 1.58 (0.04)* | 1.27 (0.03)* | 1.67 (0.02)* | 1.52 (0.02)* |
| Some college | 1.58 (0.04)* | 1.3 (0.03)* | 1.94 (0.05)* | 1.54 (0.03)* | 2.5 (0.02)* | 2.26 (0.02)* |
| Wealth, ref: 1st quartile (poorest) | ||||||
| 2nd Quartile | 0.22 (0.03)* | 0.28 (0.03)* | 0.07 (0.04) | 0.24 (0.04)* | 0.16 (0.02)* | 0.21 (0.02)* |
| 3rd Quartile | 0.35 (0.03)* | 0.44 (0.03)* | 0.23 (0.04)* | 0.49 (0.04)* | 0.2 (0.02)* | 0.32 (0.02)* |
| 4th Quartile | 0.46 (0.03)* | 0.59 (0.03)* | 0.33 (0.04)* | 0.7 (0.04)* | 0.27 (0.02)* | 0.41 (0.02)* |
| 5th Quartile (richest) | 0.59 (0.03)* | 0.76 (0.03)* | 0.4 (0.04)* | 0.99 (0.04)* | 0.35 (0.02)* | 0.51 (0.02)* |
| Employed | 0.28 (0.02)* | 0.22 (0.02)* | 0.13 (0.03)* | −0.02 (0.03) | 0.35 (0.02)* | 0.4 (0.02)* |
| Social capital | 0.2 (0.01)* | 0.32 (0.01)* | 0.28 (0.01)* | 0.24 (0.01)* | 0.37 (0.02)* | 0.47 (0.00)* |
| Smoking, ref: Non-smoker | ||||||
| Current smoker | −0.12 (0.04)* | −0.46 (0.02)* | −0.06 (0.05) | −0.5 (0.02)* | 0.39 (0.02)* | 0.2 (0.01)* |
| Past smoker | −0.01 (0.03) | 0.00 (0.01) | 0.00 (0.04) | 0.02 (0.01) | 0.34 (0.02)* | 0.45 (0.01)* |
| Drink daily or almost daily | 0.12 (0.03)* | 0.21 (0.03)* | 0.07 (0.03) | 0.2 (0.03)* | −0.2 (0.02)* | −0.19 (0.02)* |
| Vigorous physical activity | 0.15 (0.02)* | 0.1 (0.02)* | 0.14 (0.02)* | 0.23 (0.02)* | 0.31 (0.01)* | 0.39 (0.01)* |
| ADL | −0.23 (0.01)* | −0.26 (0.01)* | −0.08 (0.01)* | −0.12 (0.01)* | −0.25 (0.01)* | −0.38 (0.01)* |
| Depression score | −0.05 (0.00)* | −0.07 (0.00)* | −0.06 (0.00)* | −0.08 (0.00)* | −0.44 (0.01)* | −0.49 (0.02)* |
| Number of co-morbidities | −0.11 (0.01)* | −0.01 (0.01)* | −0.07 (0.01)* | −0.03 (0.01) | −0.06 (0.01)* | −0.02 (0.01)* |
| Constant | −1.58 (0.49)* | 17.07 (0.07)* | −2.47 (0.6)* | 17 (0.08)* | −1.68 (0.4)* | 15.2 (0.05)* |
| HRS | ELSA | SHARE | ||||
|---|---|---|---|---|---|---|
| Growth curve | Joint | Growth curve | Joint | Growth curve | Joint | |
| Age | 0.4 (0.01)* | −0.13 (0.00)* | 0.42 (0.01)* | −0.12 (0.00)* | 0.33 (0.01)* | −0.13 (0.00)* |
| Age2 | −0.00 (0.00)* | 0.00 (0.00)* | −0.00 (0.00)* | 0.00 (0.00)* | −0.3 (0.00)* | 0.02 (0.00)* |
| Sensory function, ref: No impairment | ||||||
| Single impairment | −0.15 (0.02)* | −0.56 (0.01)* | −0.14(0.02)* | −0.55 (0.02)* | −0.26 (0.01)* | −0.58 (0.01)* |
| Dual impairment | −0.25 (0.04)* | −1.14 (0.02)* | −0.35 (0.05)* | −1.3 (0.03)* | −0.68 (0.03)* | −1.61 (0.02)* |
| Female | 1.21 (0.03)* | 1.05 (0.01)* | 0.79 (0.04)* | 0.66 (0.01)* | 0.97 (0.02)* | 0.74 (0.01)* |
| Married | −0.02 (0.03) | −0.07 (0.02)* | −0.01 (0.03) | −0.03 (0.02) | 0.08 (0.02)* | 0.05 (0.01)* |
| Education, ref: Less than high school | ||||||
| High school | 0.78 (0.04)* | 0.65 (0.03)* | 1.58 (0.04)* | 1.27 (0.03)* | 1.67 (0.02)* | 1.52 (0.02)* |
| Some college | 1.58 (0.04)* | 1.3 (0.03)* | 1.94 (0.05)* | 1.54 (0.03)* | 2.5 (0.02)* | 2.26 (0.02)* |
| Wealth, ref: 1st quartile (poorest) | ||||||
| 2nd Quartile | 0.22 (0.03)* | 0.28 (0.03)* | 0.07 (0.04) | 0.24 (0.04)* | 0.16 (0.02)* | 0.21 (0.02)* |
| 3rd Quartile | 0.35 (0.03)* | 0.44 (0.03)* | 0.23 (0.04)* | 0.49 (0.04)* | 0.2 (0.02)* | 0.32 (0.02)* |
| 4th Quartile | 0.46 (0.03)* | 0.59 (0.03)* | 0.33 (0.04)* | 0.7 (0.04)* | 0.27 (0.02)* | 0.41 (0.02)* |
| 5th Quartile (richest) | 0.59 (0.03)* | 0.76 (0.03)* | 0.4 (0.04)* | 0.99 (0.04)* | 0.35 (0.02)* | 0.51 (0.02)* |
| Employed | 0.28 (0.02)* | 0.22 (0.02)* | 0.13 (0.03)* | −0.02 (0.03) | 0.35 (0.02)* | 0.4 (0.02)* |
| Social capital | 0.2 (0.01)* | 0.32 (0.01)* | 0.28 (0.01)* | 0.24 (0.01)* | 0.37 (0.02)* | 0.47 (0.00)* |
| Smoking, ref: Non-smoker | ||||||
| Current smoker | −0.12 (0.04)* | −0.46 (0.02)* | −0.06 (0.05) | −0.5 (0.02)* | 0.39 (0.02)* | 0.2 (0.01)* |
| Past smoker | −0.01 (0.03) | 0.00 (0.01) | 0.00 (0.04) | 0.02 (0.01) | 0.34 (0.02)* | 0.45 (0.01)* |
| Drink daily or almost daily | 0.12 (0.03)* | 0.21 (0.03)* | 0.07 (0.03) | 0.2 (0.03)* | −0.2 (0.02)* | −0.19 (0.02)* |
| Vigorous physical activity | 0.15 (0.02)* | 0.1 (0.02)* | 0.14 (0.02)* | 0.23 (0.02)* | 0.31 (0.01)* | 0.39 (0.01)* |
| ADL | −0.23 (0.01)* | −0.26 (0.01)* | −0.08 (0.01)* | −0.12 (0.01)* | −0.25 (0.01)* | −0.38 (0.01)* |
| Depression score | −0.05 (0.00)* | −0.07 (0.00)* | −0.06 (0.00)* | −0.08 (0.00)* | −0.44 (0.01)* | −0.49 (0.02)* |
| Number of co-morbidities | −0.11 (0.01)* | −0.01 (0.01)* | −0.07 (0.01)* | −0.03 (0.01) | −0.06 (0.01)* | −0.02 (0.01)* |
| Constant | −1.58 (0.49)* | 17.07 (0.07)* | −2.47 (0.6)* | 17 (0.08)* | −1.68 (0.4)* | 15.2 (0.05)* |
Note: Reported are co-efficients (standard errors). Sig.: *Significant at 1%.
Apart from age and sensory impairment co-efficients, several socio-demographic characteristics and other confounders showed stable significant associations with cognitive function in all three surveys. Being female, having attained a higher level of education, being employed, and being relatively wealthy were associated with better cognitive abilities. The social capital index and physical activities showed a positive and significant association with higher cognitive function. Functional status as measured by ADL and depression had a significant negative association with cognitive function in all surveys.
Figure 1 illustrates the predicted baseline episodic memory score and trajectory over time for respondents with different levels of sensory functions (hearing and visual). After controlling for the co-variates, in general, the cognitive trajectories in all surveys took on a curvilinear shape. Respondents with better hearing function were able to recall more words in all surveys. Similarly, the predicted value of episodic memory scores of respondents with better vision function is higher than those with poor function. The presence of sensory impairment had a negative correlation with cognitive trajectories. The cognitive trajectories of older adults with no sensory impairment followed curvilinear shapes, while those of older adults with dual sensory impairment showed a consistent trajectory of cognitive decline after the age of 50.
The predicted trajectories of summary cognitive scores of the HRS, ELSA and SHARE participants by the presence of sensory impairment.
Discussion
Our findings, from three longitudinal surveys of ageing, showed that both single and dual sensory impairments in older adults were independently associated with accelerated rates of decline in cognitive abilities. The association is stronger among those with dual sensory impairment. Our findings extend the discussion in the literature on the relationships between sensory impairment and cognitive function by estimating the trajectories of summary cognitive scores for older adults with different levels of sensory function using a large longitudinal multinational sample. We found that, in general, the trajectory of cognitive function took on a curvilinear shape. However, when we separated the cognitive trajectories according to the presence of sensory impairment, the cognition of respondents with sensory impairment declined faster than that of respondents with no sensory impairment. Crucially, the trajectories of respondents with dual sensory impairment took the shape of a more linear decline after the age of 50.
The patterns of association between hearing and visual sensory impairment with cognitive decline described in the present study partially support previous longitudinal studies [22, 23]. However, studies in Australia [24] and the Netherlands [25] reported that decline in hearing function was not associated with cognitive ability. A key limitation across these prior studies is that those studies did not account for attrition and lead to bias. The strengths of our study include the fact that it performed a joint model to deal with that limitation. Our analysis using the joint model shows stronger negative relationships between sensory impairment and levels of episodic memory.
Additionally, the 10-year follow-up duration in the ELSA data included in our study facilitates a fuller examination of trajectories of cognitive ageing. The literature has proposed several possible relationships: cognitive decline precedes sensory impairment through the reduction of the cognitive resources that are available for sensory perception (the ‘cognitive load on perception’ hypothesis), sensory impairment causes cognitive loss, possibly through the effects of sensory impairment on social isolation (the ‘sensory deprivation’ hypothesis), and the presence of third factors causes both declines (the ‘common-cause’ hypothesis) [26]. Our analysis showed that respondents with dual sensory impairment joined fewer social activities than those with single impairment and no impairment. It is possible that sensory impairments may impact cognitive trajectories via facilitating or limiting social activity as the magnitude of the association between sensory impairment and cognitive function in our study declines after we include the social capital index and other demographic co-variates. The common-cause hypothesis should also be considered. Our findings do not allow for a conclusive distinction between hypotheses, and the hypotheses are not mutually exclusive. It may be the case that all the possibilities described are valid to some extent. Further studies are needed to disentangle them. For instance, hypotheses that suggest an impact of sensory function on cognition may be tested by identifying changes in cognitive trajectories following sensory remediation (e.g. cataract surgery and provision of a hearing aid).
The use of self-reported measures of sensory function is a limitation of this study because self-report measures may under-estimate rates of impairment and do not provide estimation of the severity of the impairment. However, self-reported measures of sensory function are commonly used in epidemiological studies [27], and previous studies support the validity of self-reported measures of both vision impairment [18] and hearing impairment [28]. The second limitation of this study is that the episodic memory score does not define all the cognitive abilities of older adults and other abilities have different rates of decline with advancing age. However, this measure has been known to have a good validity and to relate to the everyday activities of older people [29]. The last limitation is the observational design of the study, which means that the relationship between sensory impairment and cognition may be affected by unmeasured predictors of cognitive ageing, such as social network, employment status and dietary intakes and the findings should be interpreted with caution.
In conclusion, those with sensory impairment are at a greater risk of developing cognitive impairment and may show a faster trajectory of cognitive decline that those without sensory impairment. A recent publication using ELSA found that respondents with the most advantaged trajectory of episodic memory had an odds ratio more than five times less than those with the most disadvantaged trajectory after allowing for established risk factors for dementia [30]. Further studies are needed to investigate how sensory markers could inform strategies to prevent cognitive decline. Strategies may include hearing and vision rehabilitative intervention in combination with healthy ageing interventions to promote social engagement, physical activity and positive health behaviours.
Older adults with single or dual sensory impairment (hearing and/or vision) were able to recall fewer words than those without sensory impairment.
The association between sensory impairment and lower episodic memory levels were stronger after the attrition considered in the joint model.
The cognitive trajectories of older adults with no sensory impairment followed curvilinear shapes, while those of older adults with single and dual sensory impairments showed more linear shapes.
Supplementary data
Supplementary data mentioned in the text are available to subscribers in Age and Ageing online.
Acknowledgements
The Sense-Cog WP1 group are detailed in Appendix D of the Supplementary data.
Conflict of interest
None.
Funding
This work was supported by SENSE-Cog project. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 668648.
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