Scientists studied 128 people from families who had a high risk of inheriting genes that lead to Alzheimer's disease.
It is hoped the signs, detectable in spinal fluid and brain scans, will aid the understanding of how the disease progresses in order to produce new treatments for the more common form of the disease that is not inherited.
The 'timeline' of the disease was developed by a team at Washington University School of Medicine in St. Louis and is published in The New England Journal of Medicine.
The earliest of these changes, a drop in spinal fluid levels of the key ingredient of Alzheimer's brain plaques, can be detected 25 years before the anticipated age of onset.
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The plaques in the brain become visible on a brain scan 15 years before memory problems become apparent and other signs, increased levels of a protein called tau in the spinal fluid appears and the shrinkage of parts of the brain become evident at around the same time.
A reduction in the brain's use of glucose and slight memory problems in certain areas can be detected 10 years before full blown symptoms, they found.
Author Dr Randall Bateman, professor of Neurology at Washington University School of Medicine, said: "A series of changes begins in the brain decades before the symptoms of Alzheimer's disease are noticed by patients or families, and this cascade of events may provide a timeline for symptomatic onset.
"As we learn more about the origins of Alzheimer's to plan preventive treatments, this Alzheimer's timeline will be invaluable for successful drug trials."
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Dr Laurie Ryan, clinical trials program director at the National Institute on Aging, in America, said: "These exciting findings are the first to confirm what we have long suspected, that disease onset begins years before the first sign of cognitive decline or memory loss.
"And while participants are at risk for the rare, genetic form of the disease, insights gained from the study will greatly inform our understanding of late-onset Alzheimer's disease."
Professor Clive Ballard, director of research at the Alzheimer's Society, said: "This important research highlights that key changes in the brain, linked to the inherited form of Alzheimer’s disease, happen decades before symptoms show, which may have major implications for diagnosis and treatment in the future.
"There are also good indications that these findings could apply to people with non-hereditary Alzheimer’s disease, but we can’t yet be sure. Further research into this complex condition is needed to confirm a definite link."
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