Disability in Basic and Instrumental Activities of Daily Living is Associated with Faster Rate of Decline in Cognitive Function of Older Adults

COGNITIVE decline in old age is common and is strongly associated with increased risk of cognitive impairment, Alzheimer’s disease, dementia, and death (1–5). With the U.S. population getting older, this problem of decline in cognitive function is expected to increase in future, underscoring the need to identify substrates contributing to cognitive decline and strategies to reduce their impact. Physical disability is one such adverse health condition that may be related to cognitive decline in older persons through neurodegenerative processes. Some studies have shown a relationship between self-reported physical disability and cognitive function (6,7), whereas others do not (8,9). In addition to physical disability, common chronic health conditions, such as diabetes (10,11) and pulmonary disease (12–15), and general indicators of demographic and anthropometric variables, such as education (16–18) and body mass index (19,20) have been shown to be associated with late-life cognitive decline. The relationship between physical disability and cognitive function is not well established, and understanding this association is likely to require a thorough understanding of the temporal course of change in cognitive function with respect to the temporal change in physical disability. Previous research either used a set cutoff point to define cognitive decline or focused on the relationship between baseline physical disability and cognitive decline. However, physical disability is a dynamic process that can be defined in terms of transitions and progression toward more severe disability. In this article, we focus on one transition defined by the onset of physical disability and severity of physical disability at the time of reported onset. Many previous studies have been limited in their ability to study temporal change in cognitive function in terms of temporal changes in physical disability due the insufficient longitudinal data. This study examines change in cognitive function before and after onset of disability in older persons using data from the Chicago Health and Aging Project (21) to test the hypothesis that onset of physical disability is associated with an increased rate of cognitive decline. Participants were 6,678 older residents of a geographically defined neighborhood in Chicago. During a mean of 12.6 years of observation of participants not disabled at baseline, cognitive function was assessed 2–6 times at 3-year intervals and their difficulties with activities of daily living (ADL) and instrumental activities of daily living (IADL) were ascertained at the same time. We used mixed-effects change point regression models to test the hypothesis that cognitive decline accelerates after onset of ADL and IADL disability.