The number of dementia patients in the UK is predicted to double
A failing memory may not be the first mental problem to signal the onset of Alzheimer's disease, research suggests.
A University of Kansas study found a decline in other thinking and learning skills may be a warning sign years before diagnosis.
The spatial skills, such as reading a map or completing a jigsaw, seem to be the first to falter.
The Archives of Neurology study may help doctors to identify Alzheimer's at an earlier stage.
This is key as current drugs are more likely to have a significant impact if given when the disease is still in its early stages.
The latest study is based on 444 people, of whom 134 developed dementia.
In each case the participants underwent a battery of mental skills tests, and this allowed the researchers to construct computer models showing how and if these skills declined prior to a diagnosis of dementia.
The results showed that visuospatial skills - required to perceive the distance between objects - began to decline sharply three years before diagnosis.
An overall decline in mental abilities was seen in the following year.
But a sharp decline in memory skills was not registered until one year before clinical diagnosis.
The researchers said the findings suggested that focusing on failing memory as a way to detect Alzheimer's was flawed, as it was unlikely to pick up on the earliest manifestations of the disease.
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said: "Early intervention will be crucial to the effectiveness of the Alzheimer's treatments of the future, so methods of improving detection will become increasingly important.
"This study suggests that diagnostic tests could be broadened to pick up the disease earlier."
Dr Susanne Sorensen, head of research at the Alzheimer's Society, said the study underlined that changes in the brain could start to occur years before Alzheimer's became apparent.
"More research is now needed to better understand how we can improve early diagnosis.
"With more funding for dementia research we hope treatments can be developed which target dementia in the early stages."
It is estimated that 700,000 people in the UK have dementia, with levels expected to soar in coming years as the population ages.