Early signs of Alzheimer's can be detected in apparently healthy people with a combination of brain scans and spinal fluid testing, a study has shown.
Experts believe early diagnosis could lead to much more effective pre-emptive treatment.
The research involved 105 people in their 70s and 80s who appeared outwardly free of dementia. First, the group was split into individuals with high and low levels of cerebrospinal fluid (CSF) amyloid, which is typically reduced in Alzheimer's patients.
Magnetic resonance imaging scans were then conducted over 12 months to measure rates of brain shrinkage.
The results showed that the brains of people with low CSF amyloid levels, 38pc of the group, shrank twice as quickly as those with higher levels.
Study leader Dr Jonathan Schott, from London's Institute of Neurology, said: "We found that about one in three had a spinal fluid profile consistent with Alzheimer's disease.
"Using MRI scanning, we showed that these individuals also had increased brain shrinkage over the following year.
"The significance of these findings will only be clear with longer clinical follow-up but may suggest that these individuals are at increased risk of developing dementia.
"If so these results add to a growing body of work suggesting that Alzheimer's disease starts many years before the onset of symptoms." The findings are published online in the journal Annals of Neurology.
Rebecca Wood, of the Alzheimer's Research Trust which part-funded the study, said: "We are hamstrung by our inability to accurately detect Alzheimer's but these findings could prove to be pivotal."