When Alzheimer's disease actually starts is often not clear, but it now appears that it may be preceded by rapid cognitive decline for up to six years before it becomes evident, a new study suggests.
This accelerated deterioration in memory and other mental function is not seen in people who do not develop Alzheimer's disease, the researchers said.
"Alzheimer's disease has a much longer course and affects substantially more people than generally recognized," said lead researcher Robert S. Wilson, a senior neuropsychologist at the Rush Alzheimer's Disease Center in Chicago.
"The results 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," he said.
For the study, published in the March issue of Archives of Neurology, Wilson's team evaluated information on 2,071 older adults without dementia who took part in two separate studies, including 1,511 who had no signs of cognitive impairment.
The participants were tested on specific cognitive functions such as working memory, perceptual speed and visuo-spatial ability.
During 16 years of follow-up, 462 people developed Alzheimer's disease.
"We found that dementia in Alzheimer's disease is preceded by an average of five to six years of rapid cognitive delince," Wilson said.