Global population aging has made it necessary for health and social systems to further enhance quality of life by adding healthy years to people’s lives. However, the prevalence of age-related health problems is becoming an important public health concern together with the population aging. One of the most important health care issues facing today’s elderly population is cognitive impairment and its implications [1]. Cognitive impairment is a complex syndrome that would undermine activities of daily living and quality of life [2]. Indeed, the prevalence of disability in basic activities of daily living (ADLs), such as feeding, dressing, and bathing, increases with advancing age as well [3]. In 2018, the World Health Organization proposed to build a supportive environment to prevent decline in physical and cognitive function of the elderly and to further enhance the quality of their prolonged lives. Hence, understanding the factors that influence ADL disability and cognitive functions of older adults will contribute to future social policy and long-term care service planning.
An increasing number of studies indicated the relationship between cognitive function and ADL disability is complex and potentially bidirectional. On one hand, there were research findings suggest that cognitive dysfunction is a major risk factor associated with ADL disability [4-7]. On the other hand, it was observed that older individuals with activities of daily living (ADL) limitations were more likely to be diagnosed with dementia [7-9]. A meta-analysis indicated that it could be possible that ADL disability and changes in cognitive function may, in part, share a common pathological basis [10]. However, previous studies have largely overlooked the bidirectional relationship of cognitive function and ADL disability. Thus, examining the bidirectional relationship between cognitive function and late-life disability in basic ADLs would contribute to the further delineating of their relationship. Furthermore, in literature testing relationship between cognition function and ADL disability, no study has taken into consideration of ADL hierarchy. This is not only a methodological limitation, but a theoretical limitation as the change of activities of daily living is hierarchical instead of continuous. If it was treated as a continuous, researchers would be likely to attain the weakened consequences. Therefore, this study aimed to provide clearer clues on the relationship between the elderly’s cognitive function and the loss hierarchy of activities of daily living, which may expand the accumulated knowledge and help professionals determine future healthcare needs.
The influence of cognitive function on ADL disability
Findings from cross-sectional studies across samples of older people with and without dementia have shown the association between the decline of cognitive function and ADL disability [4, 5]. Especially in the middle and later dementia stages, the association between cognitive function and functional ability in ADLs becomes stronger [11].
Furthermore, results from longitudinal research have demonstrated that cognitive impairment may precede ADL disability among older individuals [12]. In a 52-month study of functional decline in nursing home residents, researchers found that severe dementia predicted poorer ADL functioning [6]. Similarly, in a community-based longitudinal study, the main findings demonstrated that cognitive score predicted subsequent ADL disability in both non-demented older adults and patients with Alzheimer’s disease [13].
The biopsychosocial model of the disablement process (Verbrugge & Jette, 1994) may help to describe how cognition impairment leads to ADL limitations. According to this model, ADL disability is hypothesized to be an outcome based on physical and/or cognitive constraints as well as environmental barriers and/or psychological factors. For example, if elderly people are unable to bathe themselves independently (an ADL), it may be because of severe arthritis limiting hand movement (a physical impairment) or because of cognitive deficits restricting their ability to perform this task in sequence. Therefore, researchers suggested that regular screening of cognitive status is important to anticipate the potential onset of disability and delay this process where possible [4].
Although the cross-sectional studies mentioned above have shown that cognitive function is associated with older people’s ADL function, and evidence from longitudinal research indicated that cognitive function was an important predictor of the elderly’s subsequent limitations in ADLs, it remains unclear whether this holds true for different stages of the ADL function loss process. As demonstrated in many studies, there is a hierarchical structure to the loss of basic activities of daily living [14-16].
The effects of ADL disability on cognitive function
There are many cross-sectional studies observing the typical association of physical function and dementia that adopt ADL status as a diagnostic indicator of dementia. Specifically, older individuals with ADL limitations were more likely to be diagnosed with dementia [7-9].
Although longitudinal studies have provided some clues for understanding the dynamics of the relationship between physical and cognitive function of the elderly, there is a lack of detailed information on the effects of ADL disability on cognitive function among Chinese older adults. A few studies indicated that individuals without dementia or preclinical dementia were first experiencing frailty, such as gait and balance impairment, being underweight, and weaker grip strength [17, 18] or having limitations in the more complex instrumental activities of daily living (IADL) [19]. One possible explanation for this is that there is some specific pathology impacting physical function that exacerbates dementia progression. For instance, cognitive impairment may share neurologic pathology with physical performance tasks [20]. Only limited research found that ADL disability was a significant predictor of dementia onset, after controlling for baseline cognitive function [21].
The aforementioned findings have provided valuable clues for understanding how physical function relates to cognitive function among elder people. However, there is limited direct investigation examining the association between ADLs and cognitive impairment among older adults without dementia [22]. Although there were some studies indicating the association between IADLs (which require higher cognitive ability than ADLs) and cognitive impairment [10, 19], it could be possible that such relationship existed because essentially the pathology that eventually causes reduction in the cognitive abilities to complete IADLs may also cause the decline of cognitive status. A limited study which indicated that ADL was a risk predictor for dementia [21] failed to demonstrate whether different domains of ADL function loss carried comparable levels of risk to cognitive status because it only included ADL evaluations as a whole score in its research design. Thus, further studies need to be conducted to examine how changes in ADL loss predict cognition function. Demonstrating the relationship between cognitive function and changed need for ADL assistance is needed to help determine future healthcare needs.
Hierarchy in functional decline of ADLs
In order to capture the different domains of ADL function loss, previous studies have shown that the physical function loss of ADL was hierarchical in nature [14-16, 23] and can be formed as a pattern which would be helpful for monitoring ADL disability progression across episodes of care and predicting long-term care use among older adults who match the hierarchical loss pattern [15, 24, 25].
In some original research, it was indicated that among the ADL categories measured, participants on average lost the ability to bathe independently before losing their ability to dress, use the toilet, and transfer, continence, and feeding [26]. In terms of older individuals with dementia, Giebel et al.’s 2015 study found that bathing and dressing impairments were present in earlier stages, whereas impairment in toileting, ambulation, and feeding was spared until the later stages [16]. Similarly, Fields et al. 2010 study indicated that when using a caregiver-report measure, problems in bathing and grooming appeared first, whereas eating was the last to be affected [14]. Comparative analysis was also conducted to examine the ADL loss hierarchy among older Americans and Chinese, which identified that that bathing is the first activity that both older Americans and Chinese have difficulty with, while eating is the last activity. There are differences in the rank order for toileting (ranked more challenging in the Chinese sample) and dressing (ranked more challenging in the U.S. sample) [15]. However, in some medical studies, a common conclusion has been reached that older adults tend to lose ability in activities that require lower extremity strength (e.g., walking) earlier than activities that require upper extremity strength (e.g., eating) [27, 28].
Although the reviewed literature indicated that the loss of ability to perform activities of daily living tasks demonstrated hierarchical structures, the hierarchy in functional decline demonstrated mixed patterns across societies due to the different populations studied and different measurement adopted. Researchers had highlighted potential impacts of demographic factors and subpopulations on the changes of ADL function [29]. For instance, the loss of activities of daily living was assumed to demonstrated different patterns between the oldest-old group (above 80 years) and other older age groups (60-79 years) [3]. Given the facts that the Chinese population is experiencing rapid aging, ADL disability among the oldest-old group causes tremendous economic, family and social burden, information on the loss hierarchy in basic ADL disability and its relationship with cognitive function is critically relevant for policymakers. Therefore, in order to provide more appropriate evidence on context-sensitive policy and service design, there is a need to explore the ADL loss hierarchy among older adults especially among the oldest-old in Chinese communities.
Based on this overview, three research gaps were identified: (a) the stability of the relationship between cognitive function and ADL disability over time has not been ascertained, although there is general consensus that the loss of ADL is a process and demonstrated as hierarchical structure; (b) no research has explored the bidirectional relationship between ADL changes and measurable cognitive changes although previous research has shown that decline in ADL functions and cognitive functions are interrelated and influence one another; (c) no local evidences have existed by exploring the relationship between the hierarchy of ADL function loss and cognitive function among Chinese older adults, especially the oldest-old who have more daily caring needs.
The present study tried to fill the identified research gaps highlighted above by determining the bidirectional relationship between cognitive function and the loss hierarchy of ADLs among older adults aged 75 above in China. By examining the bidirectional relationships between cognitive function and the loss hierarchy of ADL, the cognitive status of the baseline would be controlled, therefore providing clearer clues for understanding the stability of the relationship between cognitive functions and ADL disability over time. Moreover, by taking the structure of ADL loss hierarchy into consideration, it would help us to better understand the role that cognitive function may play in the stages of the ADL loss process. This would be useful at a practice level by determining eligibility for care services and predicting long-term care use among older adults.