Stability of the preclinical episodic memory deficit in Alzheimer's disease

We sought to determine the course of the preclinical episodic memory defici t in Alzheimer's disease. Using data from a population-based study, we comp ared persons who developed Alzheimer's disease (n = 15) with persons who we re non-demented (n = 105) 6 and 3 years prior to the diagnosis of dementia. Participants were tested on tasks assessing episodic memory (free recall a nd recognition of words) and short-term memory (digit span). The incident A lzheimer's disease cases performed more poorly than their non-demented coun terparts both 3 and 6 years before diagnosis on recall and recognition. The re were no group differences in either forward or backward digit span, The selective impairment of episodic memory before the diagnosis of Alzheimer's disease is consistent with the view that early changes in the hippocampal complex play an important role in the memory deficit in preclinical Alzheim er's disease. On both preclinical measurement occasions, recall and recogni tion made independent contributions to group classification in logistic reg ression analyses. However, there was no evidence for accelerated decline of episodic memory in the incident Alzheimer's disease group from 6 to 3 year s before diagnosis. These results indicate that Alzheimer's disease is char acterized by a long preclinical period during which episodic memory deficit s are detectable. The magnitude of these deficits appears to be quite stabl e, at least up to 3 years before diagnosis. This may reflect the fact that those biological events that eventually result in clinically diagnosed Alzh eimer's disease (e.g. the appearance of amyloid plaques and neurofibrillary tangles) accumulate at a relatively slow rate.