Mild cognitive impairment in different functional domains and incident Alzheimer's disease

Background: Little is known about factors that predict transition from mild cognitive impairment to Alzheimer's disease (AD). Objective: To examine the relation of impairment in different cognitive systems to risk of developing AD in persons with mild cognitive impairment. Methods: Participants are 218 older Catholic clergy members from the Religious Orders Study. At baseline, they met criteria for mild cognitive impairment based on a uniform clinical evaluation that included detailed cognitive testing. Evaluations were repeated annually for up to 10 years. Analyses were controlled for age, sex, and education. Results: Eighty two persons (37.6%) developed AD. In separate analyses, episodic memory, semantic memory, working memory, and perceptual speed, but not visuospatial ability, were associated with risk of AD, but when analysed together only episodic memory and perceptual speed were associated with AD incidence, with the effect for episodic memory especially strong. Overall, those with impaired episodic memory were more than twice as likely to develop AD as those with impairment in other cognitive domains (relative risk (RR) = 2.45; 95% confidence interval (CI): 1.53 to 3.92), and they experienced more rapid cognitive decline. Lower episodic memory performance was associated with increased risk of AD throughout the observation period, whereas impairment in other cognitive domains was primarily associated with risk during the following year but not thereafter. Conclusion: Among persons with mild cognitive impairment, episodic memory impairment is associated with a substantial and persistent elevation in risk of developing AD compared to impairment in other cognitive systems.