A plateau in pre-Alzheimer memory decline: evidence for compensatory mechanisms?

To compare logistic and bilogistic models to describe the pattern of cognitive decline in the preclinical phase of Alzheimer disease (AD).

We conducted mixed effects modeling of Mayo Cognitive Factors Scores to determine the longitudinal pattern of cognitive decline in the period 10 years prior to and 5 years following a clinical diagnosis of AD. Our analysis included 199 people that eventually received a diagnosis of clinically probable AD. Participants had at least two neuropsychological evaluations including one before the evaluation at which they received the AD diagnosis.

A bilogistic model, including terms for a plateau in the course of cognitive decline, better fit longitudinal memory scores than a simple logistic model. On average the plateau began about 4 years prior to the clinical diagnosis of AD and ended with a decline that probably contributed to the clinical diagnosis of AD. A similar plateau was not evident in four other cognitive domains.

The current findings may support proposed compensatory hypotheses involving redundant memory systems, up-regulation of neurotransmitters, or recruitment of other neural networks.