Longitudinal Association Between Self-Reported Sensory Impairments and Episodic Memory among Older Adults in China: A Prospective Cohort Study

Abstract

Sensory impairments, such as visual and hearing impairments, and cognitive decline are prevalent among mid-age and older adults in China. With 4-year longitudinal data from the China Health and Retirement Longitudinal Study, we assessed the association between self-reported sensory impairments and episodic memory. Multivariate linear mixed-effects models were used to estimate the association of baseline sensory impairment in 2011-2012 with cognitive decline at 2- and 4-year follow-up visits. Among the 13,097 participants, longitudinal associations were identified between having hearing loss (β = -0.14, 95% CI: -0.22, -0.05), having both poor hearing and vision (β = -0.14, 95% CI: -0.23, -0.04) and decline in immediate word recall over 4 years, compared to those without self-reported sensory impairment. In addition, these associations were more significant among those aged 60 and older and among women. Further research is needed to investigate these associations in the longer term, providing evidence to support interventions that can prevent or delay sensory impairments and preserve cognitive functions in older adults.

Keywords

visual impairment hearing loss dual sensory impairment cognitive function aging

Introduction

Sensory impairments, such as hearing loss and visual impairment, are common among middle-aged and older adults. In 2015, about 1.33 billion people globally had hearing loss, and 473 million people had disabling hearing loss. The prevalence was estimated to be 18.06% and 6.42% respectively, with adjustments for those who used hearing aids.¹ Apart from this, it was estimated that 36 million people were blind, and 217 million people had vision impairment worldwide.² In China, the estimated prevalence of all hearing loss among those aged 60 years and older was 58.9%³, and the prevalence of visual impairment ranged from 6.05% to 15.3% in adults aged 50 and over, among whom the prevalence of presenting blindness was between 0.66% and 5.35% across 9 provinces of China.

Sensory impairments have been documented to be associated with decline in cognitive function in later life.^4-8 Several pathways can explain this association over a long period of time. The risk of hearing loss, for example, increases with age. Age-related hearing loss is related to changes in both central and peripheral auditory systems.⁹ This kind of hearing loss was clinically characterized as having problems understanding words in background noise⁹, and thus could possibly cause late-life cognitive disorders, a precursor of Alzheimer’s disease and other related dementias.¹⁰ Another pathway relates to its impact on psychosocial stress, social isolation and incident depression¹¹, all of which appear to be linked with cognitive decline and dementia.⁹ If left undetected and untreated, the impact of hearing loss on patients, their significant others and the society would be substantial.^12,13

Visual impairment such as refractive disorders, cataracts, age-related macular degeneration, diabetic retinopathy and glaucoma are more common with age.¹⁴ Among older adults in China, cataract was the most prevalent visual impairment and the leading cause of blindness.¹⁵ Age-based changes in visual functioning is also associated with the central nervous system, which shares a common pathway with cognitive aging.^16-18 Disorder in visual and cognitive functions may reflect a physiological change in the brain. Similar to hearing loss, an alternative hypothesis is related to an individual’s lack of opportunity to engage in cognitive-stimulating social interactions due to vision impairment or blindness, thus increasing the risk of cognitive impairment.¹⁹

Due to a rising aging population worldwide, cognitive impairment also becomes a challenging problem. As people grow older, changes in cognition occur in healthy human aging²⁰, while maintaining cognitive function can delay the onset of mild cognitive impairment or dementia.²¹ According to Prince et al, the age-standardized prevalence of dementia for people aged ≥60 years was between 5% and 7% globally.²² China has one of the largest aging population in the world, with more than 9.5 million people having diagnosed dementia.²³ A systematic review reported that the prevalence of dementia in China was 4.4% in 2018.^23,24 Dementia causes an enormous health and economic burden not only to individual patients but also to society as a whole.²⁵ Among all the cognitive domains, episodic memory is more age-sensitive than other episodic measures and involves re-experiencing a past event that is specific in time and place.^20,26 Additionally, memory and cognitive control variables appear to have a common mechanism.²⁷ Given the high burden of sensory impairments and dementia among older people in China, research on identifying risk factors or early signs of cognitive decline has great public health importance.²⁸ For example, interventions such as hearing aid and cataract surgery should be given high priority if sensory impairments are proved to be significant risk factors among older adults in China²⁹, whereas most of previous studies did not use nationally representative samples or longitudinal cohorts. This study used data material from the CHARLS cohort to assess the longitudinal link between sensory impairments and decline in cognitive function as measured by episodic memory. While some of the determinants of cognitive decline might not be modifiable at this point (e.g., genetic risk factors), sensory impairments are often treatable conditions and therefore our investigation of the link between sensory impairment and cognitive function could inform the cognitive health interventions designed to improve cognitive health among the elderly.

Materials and Methods

Study Population

The study cohort was obtained from the China Health and Retirement Longitudinal Study (CHARLS), a nationally representative and prospective observational study of adults aged ≥45 years and their spouses, as the early retirement age in China was 45 years.³⁰ For the large-scale investigation 150 county-level administrative unit in 28 provinces of China were randomly selected. Within each county-level administrative unit, 3 communities were randomly selected with a probability-proportional-to-size sampling technique. “Community” is the lowest level of government organization, which is an administrative village in rural areas and a neighborhood in urban areas. In each county-level unit, CHARLS sent 2 field interviewers to interview about 72 households located in 3 communities. The interviewers were trained at Peking University by CHARLS staff members, and the interview took place in respondents’ homes with the use of face-to-face computer-assisted personal interview technology.³⁰ CHARLS included assessments of social, economic, and health circumstances of the sampled community residents in 2011-2012.³¹ The baseline interview was conducted in 2011-2012 (Wave 1). Of the 13,548 participants who underwent cognitive assessment, 13,509 (99.7%) had complete measures on memory tests and sensory functions. The 39 people did not have complete measures on memory or sensory functions. We excluded 348 individuals who had a stroke and 64 individuals who had hearing aids at the baseline, considering that these participants might already have impaired cognitive function. Therefore, our analytic cohort consisted of 13,097 individuals. The followed-up rates were 87% in 2013-2014 (Wave 2) and 84% in 2015-2016 (Wave 3), as shown in Figure 1. De-identified data are available to the public. All participants provided written informed consents, and survey protocols were approved by the Ethics Review Board at Peking University³¹ (IRB approval number: IRB00001052-11015).

Measurements

Episodic memory

Based on the CHARLS User Guide, episodic memory was tested at baseline and at the 2 follow-up visits using validated instruments. We chose episodic memory as the study outcome because memory was more age-sensitive than other cognitive domains.²⁷ Previous studies have validated the cognitive test module used in CHARLS among older adults in China^32,33, memory function was assessed with 2 word-recall tests: 1). immediate word recall: each participant was asked to repeat as many as possible of the 10 Chinese nouns just read to him/her. 2) delayed word recall: each participant was asked to recall the same 10-word list 5 minutes later. Scores for each test reflected the number of correct answers, ranging from 0 to 10.

Vision impairment and hearing loss

Self-reported distance vision and hearing were each assessed in a separate questionnaire at baseline and at the 2 follow-up visits, as “excellent,” “very good,” “good,” “fair,” or “poor”. Both visual impairment and hearing loss were defined as self-reported being “poor”. Respondents with both visual impairment and hearing loss were classified as having dual sensory impairments. A previous study classified those who self-reported being “poor” or “fair” as having sensory problems³⁰, and if we had used the same definition as theirs the prevalence of hearing loss in our analysis sample would have increased to 54.1%. Yet, we argue that this definition failed to identify the most vulnerable group that may experience moderate to severe sensory impairment and cognitive decline and who may need be the target of public health interventions.³⁴ In a study based on the English Longitudinal Study of Ageing conducted among older adults in England, sensory impairment was also defined as poor self-reported vision or hearing.³⁵

Covariates

Covariates such as demographic and socioeconomic status were collected at baseline. The variables included: age, gender (male or female), educational attainment (no formal education, partial primary school education, completed primary school, middle school, and high school or above), marital status (married, widowed, separated, divorced, or never married). Health behavior covariates included smoking (current, former or never) and alcohol consumption (current, former or never). Due to an emerging trend of chronic non-communicable diseases in China with cardiovascular diseases and stroke as the leading causes of death³⁶, health condition covariates included self-reported history of having the most prevalent chronic conditions among older adults that are related with cognitive function: high blood pressure, and diabetes.^37-39 We did not control for physical activity (PA) because most participants were not asked about their PA status.⁴⁰

Statistical Analysis

Study participants’ characteristics were analyzed and presented for the full sample and stratified by sensory impairment status. Differences among the groups were examined using analysis of variance (ANOVA) or chi-square tests. Alpha, or significance level, is set to 0.05. To aid in comparison of cognitive test results, z scores (difference between participant’s score and the mean, divided by the standard deviation at baseline) were created for both immediate and delayed word recall, and a global z-score was calculated as (difference between individual average of 2 item-specific z scores and baseline population average of 2 item-specific scores, divided by standard deviation of population average of 2 item-specific scores at baseline).

Multivariate linear mixed effects models were used to assess the association of sensory impairments with scores in episodic memory over time, with individual word recall scores and changes in word recall scores over time modeled as random effects. In other words, random effects were used to account for correlation across time for each individual. Two time-splines were created for Wave 2 and Wave 3 data. The repeated measures of sensory impairments enable the updating of sensory impairment status during the follow-up. All models were further adjusted for the following variable types: demographics (age, gender, education, marriage), health behaviors (smoke, drink) and health conditions (self-reported hypertension, and diabetes). To test differential associations across subgroups, the analyses were further stratified by age and gender. To assess potential bias due to participants’ vision, we conducted the following sensitivity analyses. First, we excluded 239 individuals who reported having undergone cataract surgery at baseline. Second, we excluded 1,526 individuals who reported wearing eyeglasses at baseline, though participants were not asked about what kinds of eyeglasses they wore. Therefore, we were not able to differentiate the impact of wearing different types of glasses on memory test results. In order to evaluate potential bias due to non-response and attrition, a sensitivity analysis was conducted using the same models with the complete sample of participants taking part in all 3 waves. All analyses were conducted in Stata 14.1 (StataCorp LP, College Station, TX, USA).

Results

The mean age of the study cohort was 58.5 ± 9.3 years. Among them, 48.7% were men, about a quarter were illiterate, close to 90% were either married or partnered, about 40% were either current or former smokers/alcohol drinkers, the prevalence of hypertension and diabetes were 26% and 6%, respectively (Table 1). Compared to those without self-reported sensory impairment (n = 9,475, 72.3%), participants with visual impairment (n = 2,022, 15.4%), hearing loss (n = 877, 6.7%) or dual sensory impairments (n = 723, 5.5%) were older, less educated, less likely to be married and had higher prevalence of hypertension or diabetes. In addition, participants who had visual impairment and dual sensory impairments were less likely to be male, or less likely to smoke, or drink alcohol (Table 1).

Table 1.

Baseline Characteristics of Participants by Sensory Impairment Status.

	Total	Normal	VI only	HI only	Both VI and HI	P
	N = 13097	N = 9475	N = 2022	N = 877	N = 723	P
Age (year)						<0.001
45-54	4909 (37.5%)	4018 (42.4%)	534 (26.4%)	246 (28.1%)	111 (15.4%)
55-64	4981 (38.0%)	3526 (37.2%)	854 (42.2%)	337 (38.4%)	264 (36.5%)
65-74	2358 (18.0%)	1485 (15.7%)	454 (22.5%)	201 (22.9%)	218 (30.2%)
≥75	849 (6.5%)	446 (4.7%)	180 (8.9%)	93 (10.6%)	130 (18.0%)
Male	6373 (48.7%)	4810 (50.8%)	761 (37.6%)	481 (54.8%)	321 (44.4%)	<0.001
Educational Level						<0.001
Illiterate	3243 (24.8%)	1959 (20.7%)	694 (34.3%)	257 (29.3%)	333 (46.1%)
Part of Primary School	2381 (18.2%)	1617 (17.1%)	432 (21.4%)	163 (18.6%)	169 (23.4%)
Primary School	2866 (21.9%)	2113 (22.3%)	412 (20.4%)	228 (26.0%)	113 (15.6%)
Middle School	2856 (21.8%)	2308 (24.4%)	331 (16.4%)	146 (16.6%)	71 (9.8%)
High School or Above	1750 (13.4%)	1477 (15.6%)	153 (7.6%)	83 (9.5%)	37 (5.1%)
Married or partnered	11562 (88.3%)	8502 (89.7%)	1728 (85.5%)	758 (86.4%)	574 (79.4%)	<0.001
Smoke						<0.001
Current	4095 (31.3%)	3059 (32.3%)	537 (26.6%)	301 (34.3%)	198 (27.4%)
Former	1168 (8.9%)	810 (8.5%)	169 (8.4%)	98 (11.2%)	91 (12.6%)
Never	7833 (59.8%)	5605 (59.2%)	1316 (65.1%)	478 (54.5%)	434 (60.0%)
Alcohol use						<0.001
Current	4400 (33.6%)	3387 (35.8%)	535 (26.5%)	280 (31.9%)	198 (27.4%)
Former	1065 (8.1%)	699 (7.4%)	194 (9.6%)	88 (10.0%)	84 (11.6%)
Never	7628 (58.3%)	5386 (56.9%)	1293 (63.9%)	509 (58.0%)	440 (60.9%)
Hypertension	3400 (26.1%)	2316 (24.5%)	608 (30.2%)	240 (27.6%)	236 (32.8%)	<0.001
Diabetes	817 (6.3%)	513 (5.5%)	192 (9.6%)	58 (6.7%)	54 (7.5%)	<0.001

Note: ^a P-values were calculated from ANOVA or chi-squared tests. P < 0.05 was considered to be statistically significant.

VI=Visual impairment; HL=Hearing loss

The following results showed the relationships between sensory impairments and episodic word recall. Figure 2 illustrates the difference in baseline memory scores and decline of memory scores over time by sensory impairment status. At baseline, adjusting for demographic characteristics, health behaviors and health conditions, having visual impairment only, having hearing loss only and having dual sensory impairments were significantly associated with poorer baseline memory test scores (Table 2). On average, out of 10 words read to the participants, participants with normal vision and hearing were able to remember 4.2 words immediately (immediate word recall) and 3.3 words 5 minutes later (delayed word recall). The number of words was 3.7 and 2.7 respectively for participants with vision problems only, 3.7 and 2.7 for participants with hearing problems only, and 3.2 and 2.2 for those with dual impairments (β=-0.17, 95% CI: -0.24, -0.11; β=-0.17, 95% CI: -0.24, -0.10). Dual sensory impairments were associated with poorest global word recall (β=-0.18, 95% CI: -0.24, -0.11), compared to those without sensory impairment. Over the 2 years of follow-up period, having visual impairment only, having hearing loss only and having dual sensory impairments were associated with subsequent conative trajectories in all 3 word-recall outcomes. However, none of the coefficients was statistically significant.

Table 2.

Association Between Sensory Impairments and Episodic Word Recall (WR).

	Immediate WR	Delayed WR	Global WR
Cross-sectional difference
Normal	Reference
VI only	-0.10	-0.08	-0.09
	[-0.14,-0.05] ^a	[-0.12,-0.04] ^a	[-0.13,-0.05] ^a
HI only	-0.07	-0.10	-0.09
	[-0.13,-0.01] ^c	[-0.16,-0.04] ^b	[-0.15,-0.03] ^b
Both VI and HI	-0.17	-0.17	-0.18
	[-0.24,-0.11] ^a	[-0.24,-0.10] ^a	[-0.24,-0.11] ^a
Longitudinal Difference at 2 Years
Normal	Reference
VI only	-0.00	-0.05	-0.03
	[-0.07,0.06]	[-0.11,0.01]	[-0.09,0.03]
HI only	-0.04	-0.07	-0.06
	[-0.12,0.05]	[-0.16,0.01]	[-0.15,0.02]
Both VI and HI	-0.07	-0.08	-0.07
	[-0.16,0.03]	[-0.17,0.01]	[-0.16,0.02]
Longitudinal Difference at 4 Years
Normal	Reference
VI only	-0.02	-0.04	-0.03
	[-0.08,0.04]	[-0.10,0.03]	[-0.09,0.03]
HI only	-0.14	-0.07	-0.11
	[-0.22,-0.05] ^b	[-0.16,0.02]	[-0.20,-0.03] ^b
Both VI and HI	-0.14	-0.05	-0.10
	[-0.23,-0.04] ^b	[-0.14,0.05]	[-0.19,-0.00] ^c
N	33708	33666	33537

Note: VI=Visual impairment; HL=Hearing loss. 95% confidence intervals were reported in brackets. Model adjusted for age, gender, education, marriage status, smoking, alcohol consumption, hypertension, diabetes.

^a P < 0.001

^b P < 0.01

^c P < 0.05

Over the 4 years of follow-up period, having hearing loss only and having dual sensory impairments were significantly associated with the decline in immediate word recall (having hearing loss only [β=-0.14, 95% CI: -0.22, -0.05]; having dual sensory impairments [β=-0.14, 95% CI: -0.23, -0.04]), and with the decline in global word recall: (having hearing loss only [β=-0.11, 95% CI: -0.20, -0.03]; having dual sensory impairments [β=-0.10, 95% CI: -0.19, -0.00]).

The analysis stratified by age (<60 years and ≥60 years) and gender showed that the associations between sensory impairments and greater decline in memory scores were stronger among the older age group. For example, having dual sensory impairments was significantly associated with the decline in immediate word recall [β=-0.17, 95% CI: -0.29, -0.05] only among those aged 60 and over (Table 3) and among women (Table 4). Having dual sensory impairments was significantly associated with a larger decline in immediate word recall [β=-0.17, 95% CI: -0.30, -0.04] only among women. The results were largely unchanged after we removed 239 participants who had undergone cataract surgery and excluded 1,526 individuals who reported wearing glasses from the sample (Results are shown in Appendix). The impact of sensory impairments on cognition did not change in the sensitivity analysis with the complete sample (Data not shown).

Table 3.

Association Between Sensory Impairments and Episodic Word Recall (WR), Stratified by Age.

	45 <= Age < 60			Age >= 60
	Immediate WR	Delayed WR	Global WR	Immediate WR	Delayed WR	Global WR
Cross-sectional Difference
Normal	Reference			Reference
VI only	-0.14	-0.14	-0.15	-0.08	-0.05	-0.07
	[-0.20,-0.08] ^a	[-0.20,-0.08] ^a	[-0.21,-0.09] ^a	[-0.14,-0.02] ^c	[-0.11,0.01]	[-0.12,-0.01] ^c
HI only	-0.08	-0.14	-0.12	-0.08	-0.08	-0.09
	[-0.17,0.01]	[-0.23,-0.05] ^b	[-0.20,-0.03] ^b	[-0.17,0.00]	[-0.16,0.01]	[-0.17,-0.00] ^c
Both VI and HI	-0.24	-0.22	-0.24	-0.16	-0.16	-0.17
	[-0.35,-0.13] ^a	[-0.34,-0.10] ^a	[-0.35,-0.13] ^a	[-0.24,-0.07] ^a	[-0.25,-0.08] ^a	[-0.25,-0.09] ^a
Longitudinal Difference at 2 Years
Normal	Reference			Reference
VI only	0.07	0.04	0.06	-0.05	-0.11	-0.09
	[-0.02,0.15]	[-0.05,0.13]	[-0.03,0.14]	[-0.14,0.04]	[-0.20,-0.02] ^c	[-0.18,-0.01] ^c
HI only	-0.01	0.01	-0.01	-0.05	-0.12	-0.09
	[-0.14,0.11]	[-0.12,0.13]	[-0.13,0.12]	[-0.17,0.07]	[-0.24,-0.00] ^c	[-0.21,0.03]
Both VI and HI	0.05	0.03	0.05	-0.11	-0.11	-0.11
	[-0.10,0.21]	[-0.13,0.19]	[-0.11,0.20]	[-0.23,0.00]	[-0.23,0.01]	[-0.22,0.00]
Longitudinal Difference at 4 Years
Normal	Reference			Reference
VI only	0.03	0.01	0.02	-0.01	-0.03	-0.03
	[-0.05,0.11]	[-0.08,0.10]	[-0.06,0.10]	[-0.11,0.08]	[-0.12,0.06]	[-0.11,0.06]
HI only	-0.05	0.06	-0.00	-0.14	-0.14	-0.15
	[-0.18,0.07]	[-0.07,0.19]	[-0.13,0.13]	[-0.27,-0.02] ^c	[-0.26,-0.02] ^c	[-0.28,-0.03] ^c
Both VI and HI	0.03	0.06	0.05	-0.17	-0.05	-0.11
	[-0.12,0.19]	[-0.10,0.22]	[-0.10,0.20]	[-0.29,-0.05] ^b	[-0.17,0.07]	[-0.23,0.01]
N	19930	19891	19842	13778	13775	13695

^b P < 0.01

^c P < 0.05

Table 4.

Association Between Sensory Impairments and Episodic Word Recall (WR), Stratified by Gender.

	Female			Male
	Immediate WR	Delayed WR	Global WR	Immediate WR	Delayed WR	Global WR
Cross-sectional Difference
Normal	Reference			Reference
VI only	-0.09	-0.07	-0.08	-0.10	-0.09	-0.10
	[-0.15,-0.04] ^b	[-0.13,-0.01] ^c	[-0.14,-0.03] ^b	[-0.17,-0.03] ^b	[-0.16,-0.03] ^b	[-0.16,-0.03] ^b
HI only	-0.09	-0.10	-0.10	-0.06	-0.09	-0.08
	[-0.19,-0.00] ^c	[-0.20,-0.01] ^c	[-0.19,-0.01] ^c	[-0.14,0.03]	[-0.18,-0.01] ^c	[-0.16,0.00]
Both VI and HI	-0.15	-0.15	-0.16	-0.21	-0.19	-0.21
	[-0.24,-0.05] ^b	[-0.24,-0.06] ^b	[-0.25,-0.07] ^b	[-0.30,-0.11] ^a	[-0.29,-0.09] ^a	[-0.30,-0.11] ^a
Longitudinal Difference at 2 Years
Normal	Reference			Reference
VI only	0.02	-0.05	-0.02	-0.03	-0.04	-0.05
	[-0.06,0.10]	[-0.13,0.03]	[-0.10,0.06]	[-0.13,0.06]	[-0.14,0.06]	[-0.14,0.05]
HI only	-0.02	-0.08	-0.06	-0.05	-0.08	-0.06
	[-0.15,0.11]	[-0.21,0.06]	[-0.18,0.07]	[-0.17,0.06]	[-0.19,0.04]	[-0.18,0.05]
Both VI and HI	-0.08	-0.07	-0.08	-0.05	-0.09	-0.06
	[-0.20,0.05]	[-0.20,0.06]	[-0.20,0.05]	[-0.19,0.09]	[-0.23,0.05]	[-0.19,0.08]
Longitudinal Difference at 4 Years
Normal	Reference			Reference
VI only	-0.04	-0.04	-0.05	-0.01	-0.04	-0.03
	[-0.13,0.04]	[-0.13,0.04]	[-0.13,0.03]	[-0.11,0.08]	[-0.13,0.06]	[-0.12,0.06]
HI only	-0.15	-0.02	-0.09	-0.12	-0.11	-0.13
	[-0.29,-0.01] ^c	[-0.16,0.11]	[-0.22,0.04]	[-0.24,-0.00] ^b	[-0.23,0.01]	[-0.24,-0.01] ^b
Both VI and HI	-0.17	-0.04	-0.10	-0.11	-0.07	-0.10
	[-0.30,-0.04] ^b	[-0.17,0.09]	[-0.23,0.02]	[-0.25,0.03]	[-0.22,0.07]	[-0.24,0.04]
N	17344	17315	17243	16364	16351	16294

^b P < 0.01

^c P < 0.05

Discussion

Results from the present analysis showed that in a large cohort of middle-aged and older adults in China, over the 4 years of follow-up period, having self-reported hearing loss and having both hearing loss and visual impairment were associated with decline in immediate and global word recall, compared to those without self-reported sensory impairment. In addition, these associations were larger among those who aged 60 and older and among women, both known to be at elevated risk for Alzheimer’s disease.⁴¹

Existing studies have demonstrated the cross-sectional associations between visual and hearing impairments and cognitive impairment and dementia.^6,30,42 A meta-analysis based on 12 high-income countries (e.g., United States, Canada, etc.) identified a small but significant association between hearing loss and a variety of cognitive domains, including memory, attention, processing speed and reasoning.⁴² A recent study of 2 nationally representative samples of population aged 60 years and above in the US found that visual dysfunction, based on self-reported and objective measures, was associated with 1.9 to 2.8-fold higher odds of cognitive dysfunction or dementia.⁶ Findings from low- and middle-income countries were limited, possibly due to lack of data resources.

Our study contributes to the literature on the longitudinal relationship between sensory impairment, particularly those with hearing loss or those with both visual and hearing impairments, and episodic memory. One recent prospective cohort study found that dual sensory impairments were associated with cognitive decline among older adults of all genders in the UK⁴³, whereas a US-based prospective study found that vision impairment and combined vision and hearing impairment at baseline were associated with cognitive decline only among older women.⁴⁴ A 2016 report did not observe a significant association, potentially due to a low statistical power, as only 100 (3%) of the participants had dual impairments at baseline.⁴⁵ Our study, with a larger proportion of dually impaired participants (765 in a sample of 13,097 persons), found dual sensory impairments or hearing loss alone to be associated with decline in immediate word recall measured over 4 years and this association was stronger among women and older adults, signaling the importance of enhancing screening and treatment among these demographic groups.

We did not find a significant association between self-reported visual-only impairment and memory scores. A study based on the Health and Retirement Study in the US identified that poor vision was associated with an increased risk of cognitive impairment or Alzheimer’s disease if vision was untreated and the persons affected haven´t seen a specialist.⁴⁶ The null finding may be due to the relatively short follow-up of 4 years in our study, as opposed to 6 years or more in some prior studies.⁴³ Another possible explanation was that incidence of cognitive decline increases among adults aged 65 years and older,⁴³ suggesting that the study population in our analysis may have been too young to fully identify these associations. Also, other risk factors such as smoking were known to be associated with age-related macular degeneration and could have toxic effects on the retina, thus may amplify the association between visual impairment and cognitive decline,⁴⁷ whereas two thirds of the CHARLS participants with visual-only impairment had never smoked. Further analysis could assess this association among smokers. It is not impossible, though, that the increasing use of visual care might have mitigated the impact of visual impairment on cognitive function.^48,49 Future studies with longer follow-up periods and older participants are warranted to assess the long-term change in cognitive function following visual impairment, particularly among women and older adults.

A number of potential mechanisms have been hypothesized to explain the observed link between sensory impairments and poor memory scores. Some common causes (e.g., vascular disease) of both sensory impairments and poor cognition might explain the correlation between the 2 and why the correlation is stronger among demographic groups at higher risk of Alzheimer’s disease (in our study, older adults and women). For example, hearing and cognition may rely on shared neurocognitive resources and therefore a degeneration of these resources could lead to both hearing loss and cognitive decline.⁵⁰ It may be thinkable, that among people with hearing loss an increase in cognitive load also causes sensory stress, which takes a negative impact on test performance. Sensory impairments may be associated with risk factors for cognitive impairment, such as depression, social isolation, and lack of physical activity.⁵¹ Findings by Cosh et al⁵² suggest that older adults with hearing loss were more likely to develop or to experience anxiety disorders and depression, a risk factor for cognitive impairment. Theories like the sensory deprivation hypothesis suggest that a prolonged lack of adequate sensory input may directly result in cognitive deterioration and change brain structure and function.⁵³ And, finally, certain psychotropic drugs may also lead to sensory problems and cognitive decline.⁵⁴

Sensory impairments among older Chinese adults was often perceived as an inevitable aspect of aging and therefore remained underdiagnosed and undertreated.⁵⁵ In the United States, strategies to promote treatment of visual impairment and hearing loss, such as cataract surgery and hearing aids, have been suggested to support healthy aging and improve population health.^56,57 Our findings may suggest that screening for both hearing loss and visual impairment in China can be useful to identify those at enhanced risk of cognitive impairment. Despite the high prevalence of both cognition and sensory impairments among the aging Chinese population, there is only little to no public awareness concerning this association. This might also be a reason leading into late presentation and ineffective interventions. Experts highly recommend sensory impairment screening at the primary care setting and population-based screening tools that delineate the different impairments.⁵⁸ These procedures might help to overcome the strong emphasis on specialty treatment in many health care systems.⁵⁹

Study Limitations and Conclusions

Strengths of the present study included the longitudinal design and a large sample size, assessing the link between sensory impairments and episodic memory among a nationally representative, community-dwelling population aged 45 years and older in China. Compared to previous literature, our study population was relatively young. This younger population is of considerable interest, because dementia often starts with cognitive decline and mild cognitive impairment at a younger age²², and early primary prevention will be most effective in preventing more severe damage. Finally, our study assessed the association between dual impairments and memory, which has not been adequately addressed in the prior literature, particularly for developing countries.

Despite these strengths, our study has several limitations. First, the CHARLS study did not include objective measures of visual impairment and hearing loss, and thus our analysis relied on self-reported information. Previous studies have suggested that self-reported eyesight and hearing scales demonstrate a high correlation with objective measurements^60,61, though some discrepancies may remain.⁶² It is common for older adults to claim that they have good hearing even when they would be considered to have clinically significant hearing impairment based on threshold testing.⁶³ For example, factors such as aging, cultural background, psychological and personological traits may be related to overestimation or underestimation of the actual presence of an impairment. Second, study participants were followed up for 4 years, which may not be sufficient to observe a significant difference in cognitive decline between those with and without sensory impairment. Longer-term studies are needed to examine the association between sensory impairment and cognitive decline in Chinese population. Third, hearing and visual impairment may affect the perceiving and encoding of the test stimuli. It is likely that the measurement of memory can be influenced by sensory alterations,⁶⁴ a measurement error that should be assessed in future studies based on laboratory measurements. Having learning difficulties or being illiterate may also influence the cognitive test results. However, recent publications using the CHARLS cognitive assessment have shown that these instruments were valid even among people who had experienced sensory impairments or learning difficulties.^30,33 Finally, clinically meaningful measures on the memory performance or overall cognitive function was not assessed in the current study.⁶⁵ In addition, we did not have information on mild cognitive impairment, dementia, depression or other causes of cognitive impairment among participants at baseline, and thus we were not able to assess the potential bias caused by the impact of having dementia on the episodic memory tests. Even though previous studies have reported these cognitive measures,^30,66 further research is needed to confirm those associations, and to assess the public health and clinical implications of those findings, such as to promote sensory impairment screening among older adults to help preserve their cognitive function.⁶⁷

Supplemental Material

Supplemental Material, sj-xlsx-1-jgp-10.1177_08919887211006467 - Longitudinal Association Between Self-Reported Sensory Impairments and Episodic Memory among Older Adults in China: A Prospective Cohort Study

Supplemental Material, sj-xlsx-1-jgp-10.1177_08919887211006467 for Longitudinal Association Between Self-Reported Sensory Impairments and Episodic Memory among Older Adults in China: A Prospective Cohort Study by Xiaochen Ma, Jingkai Wei, Nathan Congdon, Yan Li, Lu Shi and Donglan Zhang in Journal of Geriatric Psychiatry and Neurology

Footnotes

Authors’ Note

The analysis is based on publicly available and de-identified data. Ethical approval was not required.

Acknowledgments

This study was supported by China Medical Board (grant number: 17-267), a United States-based nonprofit organization that promotes health education and research in the medical universities of China and Southeast Asia.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by China Medical Board (grant number: 17-267).

ORCID iD

Donglan Zhang, PhD

Supplemental Material

Supplemental material for this article is available online.

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