Sensory impairment and all-cause mortality among the elderly adults in China: a population-based cohort study

Introduction

Most countries have been facing with the challenges of population aging due to the technological and medical development [1]. The proportion of population aged more than 65 years will rise from 12.4% in 2000 to 17.9% in 2050 in America, and from 13% in 2006 to 26% in 2050 in Australia [1, 2]. China is undoubtedly no exception. As reported, the elderly population in China will exceed 450 million in 2050, comprising more than 30% of the total population and nearly 25% of the world elderly population [3].

Although people are living longer and in better health at older ages than previously, the health status of the elderly people inevitable deteriorates and an increased number of the elderly people survives with more health problems, e.g., frailty, disability and noncommunicable diseases, which has been known as “failure or cost of success” [4]. As a type of age-related function loss, sensory impairment including vision impairment (VI), hearing impairment (HI), and dual sensory impairment (DSI) affect around 10% of the elderly population [5, 6], leading to partially or fully disconnect with surroundings [7, 8] and decreasing quality of life [9]. Previous studies have shown sensory impairment was associated with a variety of functional or clinical outcomes, including hip fracture, neurodegeneration, and cognitive impairment [5, 10–12].

The relationship between sensory impairment and all-cause mortality has also been explored in previous studies but with inconsistent results [13–16]. Besides, these studies were mainly focus on relatively younger (usually less than 80 years old) population, therefore, the aforementioned associations were still unclear among the oldest-old population including octogenarians (person with 80-89 years old), nonagenarians (person with 90-99 years old), or centenarians (person with more than 100 years old). Furthermore, previous studies usually only used the baseline sensory status to explore its influence on mortality risk, without accounting for the change of exposure status after enrollment, which could lead to misclassification of exposure status and inevitably cause bias [6].

Therefore, to tackle with the abovementioned knowledge gap, we hereby, using data from the Chinese Longitudinal Healthy Longevity Survey (CLHLS), investigated the association between sensory impairment and all-cause mortality among the elderly adults.

Results

The baseline characteristics of participants stratified by sensory impairment was shown in Table 1. Compared with participants without sensory impairment, participants with sensory impairment (HI, VI or DSI) tend to be older, more female, living in rural areas and town, and illiteracy. Those participants also have a higher percentage of ADL disability, physical performance disability, cognitive impairment, and social activity disengagement, but lower chronic disease score.

The risk of all-cause mortality in participants with sensory impairment were shown in Table 2. Consistent results were observed from model 1, model 2 to model 3 which fully adjusted for the potential confounders. Compared with participants without sensory impairment, participants with VI, HI or DSI had a 20% (HR=1.20, 95%CI: 1.15-1.24), 26% (HR=1.26, 95% CI: 1.21-1.31) and 46% (HR=1.46, 95% CI: 1.41–1.52) increased risk of all-cause mortality, respectively. Similar associations were observed in subgroup analyses based on gender and entry age. A significant interaction between exposure and age (P<0.0001 for interaction), rather than gender (P=0.0911 for interaction) was observed. Compared with octogenarians, nonagenarians, and centenarians, stronger association were observed among participants with an entry age less than 80 years old.

In sensitivity analysis, we observed participants with hearing aids had an increased risk of all-cause mortality (HR=1.06, 95% CI: 1.03-1.09) (Supplementary Table 1). We also assessed the risk of all-cause mortality among participants with newly incident sensory impairment (Table 3) and observed similar but stronger associations. The risk of all-cause mortality increased 22% (HR=1.22, 95% CI: 1.14-1.29), 39% (HR=1.39, 95% CI: 1.29-1.49) and 80% (HR=1.80, 95% CI: 1.80-1.91) among participants with VI, HI or DSI, respectively.

Discussion

In this large population-based cohort study, we found that sensory impairment was associated with higher risk of all-cause mortality among the elderly population in China. Compared with participants without sensory impairment, those with VI, HI and DSI had a 20%, 26% and 46% increased risk of all-cause mortality, respectively. The associations were robust in subgroup analyses and sensitivity analyses.

Our finding was consistent with results from previous studies [5, 16–18]. A six-year follow-up study in Italy showed that hearing deficit was associated with a significant increased mortality risk [17]. One study of 1754 adults aged 65 or older from Japan reported increased risk of mortality in those with DSI and cognitive impairment [5]. A large European study found that DSI was associated with increased mortality at nursing homes [18], and similar result was also observed in Australian older adults [16]. However, there are some studies failed to observe positive association as well [6, 13, 14, 19]. For example, one study conducted in Japan found the association between VI and mortality was attenuated and became non-significant after adjustment (HR 1.05; 95% CI, 0.83–1.32) [13]. Among the 1658 participants from National Health and Nutrition Examination Survey, HI and VI were not independently associated with all-cause mortality [14], and non-significant result was also observed in Epidemiology of Hearing Loss Study participants [19]. The conflicting findings are probably due to the heterogeneity in studied ethnicity, age distribution of enrolled population, study design, sample size, or definition criteria of VI or HI. Some studies objectively measured sensory impairment by using medical tools such as sound-treated booth or Early Treatment Diabetic Retinopathy Study charts [19], while others used subjective assessment by interviewers [5, 6] or self-report [14]. One study that used data from CLHLS was also explored the association between sensory impairment and all-cause mortality [6]. However, compared to our results, much lower effect estimates and wider 95% CIs were observed, which might be caused by smaller sample size, shorter follow-up time, and misclassification of exposure status (without considering the change of sensory impairment after enrollment).

We observed the effects of sensory impairment on mortality risk decreased with aging. Similar deceasing pattern was also found in other well-established mortality risk factors among younger adults, such as increased body mass index, hypertension, hypercholesterolemia, cancer and heart disease, which may not continue to pose a risk to the oldest-old adults [20]. Such deceasing pattern may be explained by the increased number of chronic diseases and function inabilities related to mortality risk with increasing age, which makes the effect of any single disease or inability on mortality less important [20].

The precise mechanisms regarding to the association between sensory impairment and all-cause mortality remain unclear. Cognitive impairment, reported to be associated with higher mortality [21], may be one of the underlying mechanisms. Sensory impairment may be associated with cognitive impairment through age-related neurological changes (e.g., neurodegeneration in the central nervous system, or neuronal atrophy due to decreased sensory input [22]), and vascular changes (e.g., atherosclerotic or microvascular changes) [23, 24]. A causal relationship between them was also suggested through increased cognitive load, sensory deprivation, depression, or social isolation [25]. Besides, the increased risk of all-cause mortality observed in persons with sensory impairment might be mediated by factors known to increase the risk of sensory impairment in the elderly adults (e.g., hypertension, diabetes, accident and injury) [16]. Furthermore, sensory impairment may be a proxy for frailty (e.g., handgrip strength, peak expiratory flow), which is also a strong predictor of mortality [26].

Our study strengthens and extends previous findings in three aspects. First, the present study based on a large population-based cohort with verified outcomes and abundant covariate information, which enable us to do informative subgroup analyses and sensitivity analyses. Second, taking into account the variation of sensory impairment status during the follow-up period could avoid immortal time bias and more precisely assess the association. Moreover, this nationwide representative sample of Chinese was mainly consisted of the oldest-old population, such as octogenarians, nonagenarians, and centenarians, where the influence of sensory impairment on all-cause mortality was rarely explored. As aging is an inevitable trend, it is critical to promote the health of the elderly through the study on the longevity [3].

However, several limitations of our study should not be ignored. First, misclassification of exposure status could be a concern. Although all interviewers were required to attend a series of standardized training sessions before commencing interviews, subjective assessment of sensory impairment could underestimate the real prevalence in comparison with objective assessment, as suggested in one study [27]. Besides, background noise and voice variations of interviewers during hearing function assessment could further misclassify exposure status. However, as such misclassifications are unlikely to be differential, they are likely to dilute the real association towards the null. Second, as cognitive impairment affects a person’s ability of correctly understanding, following instructions or interpreting communication [28]. Subjective evaluation precludes the possibility of distinguishing participants with sensory impairment between those with cognitive impairment or those with combined impairments in sensory and cognitive. Third, unlike randomized trails, our study is prone to residual confounding due to unmeasured or imprecisely measured confounders. However, the robust results observed in subgroup analyses and sensitivity analyses argue against that residual confounding is a big concern here. Fourth, as many of the participants die at home and cause of death of deceased participants was reported by family number, the association between sensory impairment and cause-specific mortality was unable to be investigated. Fifth, lacking of information on the reason of sensory impairment makes us unable to further explore whether the influence of sensory impairment on all-cause mortality differs among population with different underlying conditions that caused sensory impairment, such as congenital and acquired. Finally, because the majority of studied population was the oldest-old Chinese and subjective assessment leads to underestimation of sensory impairment among them, the generalization of our findings to other areas, ethnicities and assessment methods should be considered with caution.

In summary, our findings demonstrated that sensory impairment were associated with higher risk of mortality among the elderly adults in China, which indicates sensory impairment might be a signal for identifying the individuals at a higher risk of death. Healthcare providers therefore should be aware of the increased risk of mortality in those with sensory impairment in their routine clinical practices.

Materials and Methods

Supplementary Materials

Supplementary Figure 1

Supplementary Tables