Discussion
Cognitively normal participants who developed DAT had worse physical performance at baseline than those who did not develop DAT. Impaired physical performance was associated with shorter time to diagnosis of DAT. These findings support the hypothesis that impairments in physical function can precede the cognitive symptoms of DAT.
Currently, the tools being used and studied for the detection of preclinical Alzheimer's disease (AD) focus on cognitive measures and biological measures using cerebrospinal fluid and neuroimaging. These tools may be difficult to use or access for many primary care physicians, who will be the first contact for individuals with DAT. A physical performance measure may be a more practical tool for primary care physicians and offer a warning of possible deleterious cognitive outcomes.
Several reports have found that frailty is a significant predictor of cognitive decline and that greater physical impairment may cause more-rapid decline in cognitively impaired older adults, but there is no consensus on the definition of frailty, which may include physical measurement only or a more-global assessment that includes comorbidities, disabilities, and subjective complaints. The current study focused on physical performance rather than the more-subjective construct of frailty. Performance-based measures are more reproducible and have greater sensitivity to change, making them more reliable for clinical use than self-report measures.
Previous studies support a strong relationship between cognitive and physical performance and show that declines in one sphere may predict future decline in the other. In a cohort of 823 older adults followed longitudinally, baseline frailty increased the risk of AD (HR = 2.44), and the annual rate of change in frailty was associated with the development of dementia (HR = 3.3). In a study of 367 persons with AD, lower cognitive performance predicted decline in physical performance, whereas another study noted that poor physical function and poor muscle strength often coexist with cognitive impairment. Likewise, improving physical activity may provide protective effects against the onset of dementia.
A recent randomized trial reported that a 6-month program of physical activity improved cognition in older adults with subjective memory complaints. This group of individuals, although reported as not having dementia, had a mean global CDR of 1, suggesting that more than subjective memory complaints were present. The current study reports predictors of future DAT in older adults without dementia characterized by informant interviews. Likewise, a number of studies used composite measures of frailty, including subjective complaints of fatigue, body composition, and components of the Unified Parkinson Disease Rating scale for their physical assessments. Although these measurements capture aspects of physical limitations, it is unclear whether they measure frailty (a multidimensional construct that represents age-dependent changes in physical reserve), extrapyramidal signs, or somatic complaints.
This study supports and confirms prior reports and uses a clinically well-characterized sample of older adults who were cognitively healthy at enrollment. Results reported here are based on the PPT, a validated, performance-based measure of physical function, and show that individuals with impaired physical performance (PPT score < 28) develop DAT sooner than individuals with better physical performance. Although this study included a large sample of older adults and a representative sample of African Americans (8.2%), the participants were well educated and relatively healthy. Therefore, the population is not representative of all community-dwelling older adults. Although participants with higher PPT scores being slower to develop DAT may suggest that exercise and physical activity can delay DAT, this cannot be confirmed because the study did not include data regarding physical activity and physical fitness. This report supports further investigation regarding the use of performance-based measures of physical function to identify older adults at risk for DAT.