The existence of cognitive plateaus in Alzheimer's disease

The objective of this study was to evaluate the existence of cognitive plateaus in some individuals during the course of Alzheimer's disease (AD).

Data came from the historical patient group collected via the Consortium to Establish a Registry for Alzheimer's Disease (CERAD, Duke University, 1988-1996). Data reduction was performed by using principal components analysis to derive a single cognitive measure (F1), followed by application of a novel plateau-searching algorithm to individual patient data, looking for stable periods of 3 years or longer. To evaluate the time dependence of F1, we fitted a linear mixed model to the group and to individual data points.

Twenty-two percent of AD subjects (54/243) and 98% of healthy control subjects (253/258) exhibited a plateau. Within the AD plateau group, the most common pattern was a single plateau (mean, 3.6 years; range, 3 to 7 years) that extended for the entire measurement period (28/54 subjects). Briefer plateau durations were seen at the beginning or end of the measurement period. Initial cognitive function (F1) was slightly higher in the plateau group, which was also slightly older and less well-educated. Men and women were equally represented.

In a patient sample predating the widespread use of cholinesterase inhibitors, we found that approximately one fifth of individuals with AD demonstrated periods of prolonged cognitive stability. This significant interindividual variability must be considered when providing prognostic information to families and when assessing individual patient responses to pharmacotherapy. We advise caution when assessing results of potentially disease-modifying agents at the individual patient level.