A test that can detect Alzheimer's up to 20 years before any symptoms show is being developed by British scientists.
The simple and inexpensive eye test could be part of routine examinations by high street opticians in as little as three years, allowing those in middle age to be screened.
Dementia experts said it had the power to revolutionise the treatment of Alzheimer's by making it possible for drugs to be given in the earliest stages.
The technique, being pioneered at University College London, could also speed up the development of medication capable of stopping the disease in its tracks, preventing people from ever showing symptoms.
Rebecca Wood, of the Alzheimer's Trust, said: 'These findings have the potential to transform the way we diagnose Alzheimer's, greatly enhancing efforts to develop new treatments.'
Alzheimer's and other forms of dementia blight the lives of 700,000 Britons and their families, and the number of cases is expected to double within a generation.
