Discussion
VI is an important public health problem in China.33–38 Due to population growth and ageing, the prevalence of eye disease has steadily increased over the past decades.39 Data from the Global Burden of Diseases, Injuries and Risk Factors Study (GBD) 2019 show that the age-standardised prevalence of moderate and severe VI in China increased more rapidly than in any other G20 countries from 1990 to 2019.17 Our analysis using nationally-representative data revealed that people with VI had significantly elevated prevalence of self-reported chronic conditions. We also found that VI was significantly associated with higher weighted prevalence of poor health among those reporting chronic conditions.
We conducted a systematic literature search for published papers addressing VI and chronic conditions among Chinese populations in PubMed and Web of Science for English publications and China National Knowledge Infrastructure for Chinese publications on 20 January 2023. We used the search terms ‘‘vision disorders’’, ‘‘visual impairment’’, ‘‘vision impairment’’, ‘‘vision disorder’’, ‘‘vision loss’’, ‘‘loss of vision’’, “eye disease” and ‘‘eye disorder’’ to identify vision impairment. To identify chronic conditions, we use the search terms “chronic conditions”, “chronic diseases”, “multimorbidity”, “comorbidity”, “cardiovascular disease or CVD”, “cardiovascular metabolic disease, CMD” as well as each of 13 common chronic conditions included in our analysis. We limited our search to studies published in English in 2010 or later, using data on adults collected in China.
We found no studies examining the prevalence and effects of VI across a broad range of chronic conditions among older Chinese adults. We did encounter publications confirming the association between VI and a number of individual chronic condition in a dyadic analytic fashion. Such analyses have been limited to a few conditions, most notably cardiometabolic disease and its risk factors: diabetes,40 stroke,41 42 hypertension43 and dyslipidaemia.35 VI is also found to coexist frequently alongside hearing loss in elderly populations, and this dual sensory impairment is reported to be associated with negative impacts on function, cognitive abilities, mental health and well-being.12–14 Although rigorous casual evidence is limited, a number of potential pathways have been hypothesised to explain the observed association between VI and specific conditions. For example, in the area of cognitive decline and dementia, some common causes (eg, neurodegenerative or microvascular disease) of both VI and poor cognition might explain the correlation.44 45 Several indirect pathways have also been proposed. Poor vision is known to increase cognitive load,46 which causes sensory stress and takes a negative impact on cognitive function that increases dementia risk. VIs may also be associated with risk factors for cognitive impairment, such as depression, social isolation, and lack of physical activity.47 48 Sensory deprivation hypothesis suggests that a prolonged lack of adequate sensory input may directly result in cognitive deterioration and result in direct alteration of brain structure and function.49
In addition, there is another body of literature specifically studying multimorbidity, the coexistence of two or more chronic conditions; however, vision has rarely been included in such analyses. A few studies have examined visual impairment in conjunction with other chronic diseases,4 5 but these analyses have focused on the cumulative number of conditions, including VI, rather than assessing VI as a risk factor for other chronic conditions.
Our analysis using CHARLS 2018 data is, according to the above review, the most up-to-date, nationally representative estimate of the prevalence and impact of VI across a broad range of chronic conditions among older Chinese adults. Our analysis extended on previous work by including a broad inventory of 13 chronic conditions highly prevalent among older Chinese adults. Our analysis closely mirrors that of a cross-sectional US population study from the National Health Interview Survey (2010–2014).50 Both studies used a similar self-reported definition of VI and included a similar list of 13 chronic conditions, controlling for similar confounders. In both studies, participants with VI were more likely to report chronic conditions than those without. Among those with chronic conditions, participants reporting VI were more likely also to report poor health status than were those without VI, although the magnitude of that associated was larger in the Chinses setting (adjusted ORs for the 13 conditions: 2.20 to 4.04 in China vs 1.66 to 2.70 in the USA)
The demographic characteristics, social-economic development, risk profiles of chronic diseases and the healthcare systems in the USA and China differ in many ways. Nevertheless, the patterns of the prevalence and impact of VI across a broad range of chronic conditions among older adults in China parallel those found in their counterparts in the USA. This reflects the global epidemiological transition in low and middle-income countries, as previously seen in high-income countries. China, like many other countries in the world, is undergoing rapid epidemiological transition and population ageing with a rising burden of NCDs. Consequently, the prevalence of multimorbidity is increasing.4 5 The coexisting of VI and other chronic conditions warrants further investigation and contextually relevant evidence to inform polices.
In addition, the larger magnitude of the association between VI and poor health among people with chronic conditions in our study highlights the challenge China and many other LMICs are facing regarding management of NCDs. Hypertension is a case in point. Among the US adults with hypertension, 81.6% are aware, 73.1% are on treatment, and 48.6% have well-controlled blood pressure.51 By contrast, the corresponding rates are only 44.7%, 30.1%, and 7.2% in China.52 Many factors contribute to these differences, a crucial one being the suboptimal quality of primary healthcare (PHC) in China.53 Shortfalls in financing, service integration, education and training still exist in China that undermine its PHC system’s ability to deliver services for the prevention and management of chronic conditions in equitable fashion.53
Our results have profound public health implications for eye health services in China and other LMICs: the fact that VI is linked with the prevalence of a broad list of chronic conditions underscores the importance of prevention and treatment of VI as public health priorities. Due to traditional beliefs,14 older Chinese adults often view age-related eye disease as an inevitable part of ageing and may be reluctant to receive formal diagnosis or treatment. A recent cross-sectional study in rural areas across nine Chinese provinces shows that mild and moderate-to-severe visual impairment are mostly caused by uncorrected refractive error and cataract, both highly treatable.34 Our results suggest that public awareness should be raised and VI among the elderly treated more actively. The additional burden of poor health among people with chronic conditions suffering from VI highlights the need for more frequent vision assessment,54 particularly among those with hypertension and dyslipidaemia. As the global burden of NCDs increases, PHC is emerging as the focus of both prevention and life-long management of chronic disease.55 In the era of increasing disease and economic burden of NCDs and multimorbidity,4 18 19 56 health system must shift from a single disease focus to a person-centred approach.57 The GBD 2019 China Study17 shows that age-standardised prevalence of all common blinding eye diseases dropped over the past three decades in China, except for diabetic retinopathy. Their analysis found that the increase was attributed more to age-specific prevalence than to population ageing, largely due to the substantial changes in the lifestyle of Chinese people. Within the context of universal health coverage (UHC), it is of particular importance to integrate the prevention and control of chronic eye diseases into China’s national management of chronic diseases. As China advances its UHC in the Healthy China 2030 policy,58 this also illustrates the need for an Integrated People-centred Eye Care strategy59 in China.60
The causal relationship between VI and general health is complex, recently published reviews have summarised a number of potential pathways (illustrated in online supplemental figure S1)6 61: (1) direct pathways through the effect of VI on systemic health: increased risk of chronic disease, decreased functional status, frailty; (2) indirect pathways through activity and participation: reduced access to healthcare, increased injuries, limitations in physical activity, increased risk of social isolation, leading to depression and dementia; (3) shared common risk factors (individual-level traits, environmental and health system characteristics) including smoking, alcohol consumption, socioeconomic status and conditions with both ocular and systemic manifestations (eg, cardiovascular disease, diabetes, hypertension, stroke). However, most of the existing studies are conducted from, future studies based on representative data from LMICs are warranted to generate context-relevant evidence for countries in the developing world.
Strengths of the present study included the large, nationally representative sample, and the detailed information regarding demographic, socioeconomic and health-related behavioural characteristics in addition to self-reported data on vision, chronic conditions and health status collected in CHARLS. Compared with previous studies,12 14 40–43 the wider array of 13 chronic conditions allows us to explore more fully the impact of VI on the health of older Chinese adults.
Despite these strengths, our study has several limitations. First, the data we used were cross-sectional; we, therefore, cannot interpret the associations found as causal. Irrespective of their direction, these associations underscore the need for more attention to VI in the current health system. Second, our measurement of VI was non-objective. As a large-scale, nationally representative survey, it was only practical for CHARLS to acquire self-reported vision measures. Similar definition of self-reported poor vision was also used based on the English Longitudinal Study of Ageing62 as well as the National Health Interview Survey50 conducted among older adults in England and the USA. Although previous studies have shown a strong correlation between self-reported vision and objective measurements,63 some discrepancies may exist.64 Last but not least, we used participant-reported diagnosis by a physician for 12 of the 13 chronic illnesses studied (with the exception of depression). Despite China’s recent health system reform, which extended essential healthcare and insurance coverage to the entire population, high-quality care for prevention and management of chronic conditions is still less accessible in rural settings, particularly at the PHC level.53 As a result, the actual prevalence of the included chronic conditions might be higher than the current estimate, due to underdiagnosis. Hence, the actual association between VI and prevalence of chronic conditions might be even stronger than the current estimate in our analysis.
A higher prevalence of a broad range of chronic conditions is significantly associated with VI among older Chinese adults. Among those with chronic conditions, the burden of poor health is significantly greater among persons with VI compared with those without. These findings from a nationally representative cohort suggest the need for further attention among both policymakers and researchers to the coexistence of VI and chronic conditions, both to better understand the problem and to inform contextually relevant strategies to promote healthy ageing among older persons in China and other LMICs.
