MONDAY, March 14, 2011 (HealthDay News) -- 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 decline," Wilson said. In fact, the rate of cognitive decline in these participants accelerated more than 15-fold, the researchers noted.
"We also found that the precursor of Alzheimer's disease, mild cognitive impairment, is preceded by about six years of increased cognitive decline," he said. "By contrast, little cognitive decline was evident in people who did not develop these conditions."
Greg M. Cole, associate director for research at the Geriatric Research Clinical Center at the Greater Los Angles Veterans Medical Center and a professor at the University of California Los Angeles, said that the study "is important because researchers will have to test emerging prevention strategies in people with this type of accelerating early decline."
How well a treatment slows an early rate of decline may be useful in predicting prevention methods that work, he said.
"If we ever get there, it would be a great thing because the cost of prevention trials depends on the number of subjects and the time in the trial," he said. And, because most people who take part in trials do not develop Alzheimer's disease, there is a lot of wasted time and money, Cole said. Clinical trials can cost $40 million or more, even ones that fail, he noted.
Being able to predict which people were more likely to develop Alzheimer's would allow more bang for the buck. "If you cut that cost to, say, $5 million, you could do eight trials for the same cost and have eight times the probability of success," Cole said.
"With 450,000 new Alzheimer's disease cases every year and growing, something like this has to be done to try to find something that works and optimize it," he said.
The Alzheimer's Association has more on Alzheimer's disease.