Alzheimer's could one day be diagnosed by a simple eye test.

The disease is thought to kill cells in the eyes at the same time as it kills cells in the brain. This means it may be possible to spot dementia via an easy and inexpensive test at an optician’s.

Alzheimer’s and other forms of dementia affect more than 800,000 Britons and the number is expected to double in a generation as the population ages. Techniques used in diagnosis, including memory tests and brain scans, are far from perfect and definitive proof only comes on examining the brain after death.

Simplifying and speeding up diagnosis could mean that many more people get the help they need. US researchers capitalised on the fact that the retina, the light-sensitive ‘film’ at the back of the eye, is a direct extension of the brain. It is also the only part of the brain that is readily accessible.

In tests on mice, layers of the retina were up to 49 per cent thinner in animals genetically-engineered to develop Alzheimer’s, compared with healthy creatures of the same age. Researcher Scott Turner, of Georgetown University in Washington DC, said: ‘The retina is an extension of the brain, so it makes sense to see if the same pathologic processes found in an Alzheimer’s brain are also found in the eye.’

It is thought that when brain cells die in Alzheimer’s, leading to shrinking of the organ, they also die in the eye, leading to thinning of the retina.Dr Turner told the Society for Neuroscience’s annual conference in San Diego, California, that the research is still at an early stage and he still has to prove that the same thing happens in people.

If it does, it may be possible to use equipment already used to detect other eye conditions to spot Alzheimer’s. The eye scan could also be given to track how well a drug is working – or even to predict who is going to develop the disease in years to come.

Dr Turner said: ‘Changes in the brain are found ten to 20 years before the onset of dementia. Retinal thinning may similarly predict impending Alzheimer’s disease but this is speculation so far.’

Those given early warning could change their diet or do more exercise in an attempt to keep their brain healthy for as long as possible.

Dr Laura Phipps, of Alzheimer’s Research UK, said: ‘There is increasing evidence linking a loss of retinal cells to Alzheimer’s disease, and it’s positive to see this line of research being followed up.

‘This early-stage study, which is yet to be published in full, was carried out in mice, and further research will be necessary to determine whether changes in the retina found here are also found in people with Alzheimer’s.

‘Diagnosing Alzheimer’s with accuracy can be a difficult task, which is why it’s vital to continue investing in research to improve diagnosis methods.’ Dr Clare Walton, of the Alzheimer’s Society, said: ‘It would be great if we could simply look into someone’s eyes to see if they had Alzheimer’s but unfortunately spotting the disease is a lot more complicated than that.

‘This study builds on existing evidence linking the loss of cells in the eyes to dementia but, whilst a link could provide a useful and non-invasive aid to diagnosis, it’s important to stress that this study looked at mice and therefore the results may not be the same in people.

‘Much more research needed in humans so that we can spot Alzheimer’s early and stop the disease in its tracks.’