Mar 16, 2011

When Does Alzheimer's Disease Start?

This accelerated deterioration in memory and other mental function is not seen in people who do not develop Alzheimer's disease.

Alzheimer's Reading Room

The diagnosis of Alzheimer's disease (AD) and mild cognitive impairment (MCI), is preceded by 5 to 7 years of progressively accelerating decline in memory and thinking skills, according to a study in the March issue of Archives of Neurology.

Objective. To characterize the course of cognitive decline during the prodromal phase of Alzheimer disease.

Design. Longitudinal cohort study with up to 16 years of observation.

Participants. Older persons from 2 studies underwent annual clinical evaluations that included cognitive function testing and clinical classification of mild cognitive impairment, dementia, and Alzheimer disease. At baseline, there were 2071 individuals without dementia and 1511 without cognitive impairment.

Results. During follow-up, 462 persons developed Alzheimer disease (20 with dementia solely due to another condition were excluded). Five to six years before diagnosis, the rate of global cognitive decline accelerated more than 15-fold. The acceleration in cognitive decline occurred slightly earlier for semantic memory (76 months before diagnosis) and working memory (75 months) than other cognitive functions. Mild cognitive impairment was also preceded by years of cognitive decline that began earlier (80 months before diagnosis) and proceeded more rapidly (annual loss of 0.102 unit) in the amnestic than in the nonamnestic (62 months, 0.072 unit) subtype.

Conclusion. Dementia due to Alzheimer disease is preceded by about 5 to 6 years of accelerated decline in multiple cognitive functions. By contrast, little decline is evident in persons who do not develop Alzheimer disease.

"Alzheimer's disease has a much longer course and affects substantially more people than generally recognized.

The results of the study provide further evidence of the magnitude of the public health problem posed by Alzheimer's disease and related disorders and underscore the importance of developing strategies to delay its onset," according to lead researcher Robert S. Wilson, a senior neuropsychologist at the Rush Alzheimer's Disease Center in Chicago.

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For this study, Cognitive Decline in Prodromal Alzheimer Disease and Mild Cognitive Impairment, Wilson's team evaluated information on 2,071 older adults without dementia who took part in two separate studies. This includes the Religious Orders Study, which began in 1994, and the Rush Memory and Aging Project, which began in 1997. All participants were followed up annually with cognitive function tests for up to 16 years.

The participants were tested on specific cognitive functions such as working memory, perceptual speed and visuo-spatial ability.

During follow-up, 462 individuals developed Alzheimer's disease. Among these, the rate of cognitive decline "increased sharply" about 5 to 6 years before diagnosis, the investigators report. "After Alzheimer's was diagnosed, there was a further increase of about one-third in annual rate of global cognitive decline," the authors note.

According to Dr. Wilson added, "these results indicate that Alzheimer's disease has a much longer symptomatic course, and we think that this may be preceded by a period of similar length during which pathologic changes are accumulating in the brain but not yet producing symptoms".

By contrast, this accelerated deterioration in memory and other mental function is not seen in people who do not develop Alzheimer's disease.

This suggests that cognitive decline "may not be an inevitable consequence of old age."

The study also suggests that getting tested early and often is an imperative that everyone should consider.

Source Archives of Neurology

Original content Bob DeMarco, the Alzheimer's Reading Room