Simple eye and smell tests could be used to spot dementia years before sufferers experience memory symptoms, new research suggests.
Researchers at Moorfields Eye Hospital and the UCL Institute of Ophthalmology found a link between poor cognitive ability - a "clear warning sign" of the early stages of Alzheimer's - and the thickness of people's retinal nerves.
In a trial of more than 33,000 participants who had tests on memory, reaction time and reasoning, eye scans showed the nerve fibre layer was "significantly thinner" among those who performed poorly on cognitive tests.
The findings, presented at the Alzheimer's Association International Conference in Toronto, Canada, could be key as doctors believe diagnosing the condition early is "essential" if treatments are to be found that will give people who develop the condition the best chance.
Dr Clare Walton, research manager at Alzheimer's Society, said: "Changes in the brain associated with dementia can begin several years before any memory symptoms appear. This research suggests that some of these changes happen in the retina of the eye too, which could give us a relatively easy, non-invasive way to spot them early.
"Eye tests are fairly common for older people, so there is great potential to incorporate additional tests into their regular check-up."
While the tests could help with early intervention, it is not expected to be a primary way to diagnose the condition, she added.
Dr Simon Ridley, director of research at Alzheimer's Research UK, said the tests could help identify people at risk of memory decline, but more research was needed.
He said: "While this study did not look at changes in people with dementia, it does show that thinning of a layer of cells in the retina is associated with reduced cognitive performance. Further work is required to see whether thinning of the retinal nerve fibre layer is predictive of cognitive decline and dementia, to assess better whether nerve cell loss in the eye could be a potential early marker of the condition."
Other findings presented at the conference suggest smell tests could help predict cognitive decline and detect early-stage Alzheimer's disease.
Researchers from Columbia University Medical Centre used a 40-item "scratch and sniff" test on 397 adults with an average age of 80.
Some 50 participants had developed dementia four years on, and researchers found low test scores were "significantly associated" with dementia and Alzheimer's disease.
Dr Rosa Sancho of Alzheimer's Research UK, said: "This research suggests a potentially simple and cost-effective way to identify those at risk of memory decline, but while an odour detection test could bolster current diagnostic approaches like brain scanning and pen and paper tests, this test is not yet able to reliably predict who will develop dementia in future.
"Further studies using larger numbers of people are required to unravel how odour identification is affected by the brain changes in dementia and the potential that could hold for identifying those at risk of the condition."