A Cognitive Turning Point in Development of Clinical Alzheimer's Disease Dementia and Mild Cognitive Impairment: A Biracial Population Study

Background: Cognitive changes during the preclinical phase of Alzheimer's disease (AD) dementia have been characterized among European Americans (EAs), but studies of preclinical changes among African Americans (AAs) are notably absent. Methods: Preclinical changes in cognition before the development of AD dementia and mild cognitive impairment over a period of 18 years were examined using change points in a biracial sample of 2,125 older adults. Results: Of 2,125 participants, 442 (21%) developed AD dementia and 661 (31%) developed mild cognitive impairment. A cognitive change point occurred between 4 and 5 years before the clinical diagnosis of AD dementia. Differences between AAs and EAs were observed: EAs had a higher starting level of composite cognitive function, and a change point occurred 4.3 years before AD dementia among AAs and 4.7 years among EAs. The slope of cognitive decline after the change point among those developing clinical AD dementia was significantly greater among EAs (0.233 units/y) than among AAs (0.171 units/y; p <.001). This difference in slope of cognitive decline persisted after diagnosis of AD dementia so that at the conclusion of observation the difference in average cognitive level was reversed. AAs without cognitive impairment had a lower average baseline of cognition than EAs, but the slopes of cognitive decline were similar. Conclusions: A prominent change to a steeper slope of cognitive decline occurs between 4 and 5 years prior to the diagnosis of AD dementia. The slope of cognitive decline after the change point is steeper among EAs than AAs.