The relationship between cognition and mortality in a community sample of older adults

Abstract

Cognitive performance has previously been shown to be associated with time to death in a broad range of studies. There are multiple perspectives that have been used to better understand this relationship, including identifying the types of cognitive abilities that predict mortality, investigating potential mechanisms that might explain the relationship, examining whether level of ability or changes in ability are responsible for the association, assessing evidence of terminal decline and exploring patterns of late-life decline. It is also important to rule out the confounding of the relationship between cognition and time to death by other factors such as mental health. Five research articles investigated multiple aspects of the relationship between cognition and mortality using the Canberra Longitudinal Study cohort, a sample of 896 community-dweIIing Australians aged 70 and older. Findings suggested that fluid inteIIigence was a better predictor of mortality than crystaIIised inteIIigence. While socioeconomic status, health behaviours and health status accounted for some of the variance in the relationship, certain domains of cognitive ability including processing speed and global ability had an independent effect on mortality. Using unbiased estimates of cognitive change, it was found that initial cognitive performance was a better predictor of mortality, particularly cardiovascular mortality, than the rate of change in ability. However, there was evidence of terminal decline in the sample, with decline accelerating two-to four-fold, beginning 6-8 years prior to death across various abilities. Although education modified the time course of terminal decline, the effects were not consistent with the predictions of the cognitive reserve hypothesis. Age-related decline in ability could not be attributed to common biological constraints, as little evidence was found for late-life dedifferentiation of abilities after accounting for dementia. FinaIIy, depression and anxiety had no significant association with mortality after accounting for physical health, indicating that the cognition-mortality relationship did not result from confounding by mental health status. Overall, late-life cognitive performance appears to be related to mortality partly because it reflects life-long outcomes from a range of health behaviours, disease states, educational experience and socioeconomic background. In addition, pathological events including dementia and cardiovascular changes may contribute to the decline of cognitive performance that occurs in proximity to death. However, beyond these influences, there is an aspect of cognitive ability that is independently predictive of mortality. Show more