A pioneering eye test could save the sight of millions by helping doctors tackle the biggest global cause of permanent blindness.
The test allows early detection of glaucoma so that treatment can start before vision begins to deteriorate.
It also has potential for diagnosing degenerative neurological conditions including Parkinson's, Alzheimer's and multiple sclerosis.
Glaucoma, caused by the death of retinal cells at the back of the eye, affects 60 million people around the world, a tenth of whom are completely blind.
The new technique, called Darc (detection of apoptosing retinal cells) uses a special fluorescent marker which attaches to cell proteins when injected into patients.
Unhealthy cells appear as white fluorescent spots during an eye examination that employs equipment routinely kept at hospitals.
"For the first time, we have been able to show individual cell death and detect the earliest signs of glaucoma," University College London's Institute of Ophthalmology professor Francesca Cordeiro, who led the research, said.
"Detecting glaucoma early is vital as symptoms are not always obvious.
"Although detection has been improving, most patients have lost a third of vision by the time they are diagnosed.
"While we cannot cure the disease, our test means treatment can start before symptoms begin."
Patients could be diagnosed 10 years earlier than was previously possible, Chief investigator Professor Philip Bloom, from the Western Eye Hospital, part of Imperial College Healthcare NHS Trust, said.
The scientists believe the test may have wider applications because the nerves that help us see are essentially an extension of the brain.
Programmed cell death, or apoptosis, at the back of the eye could provide an early indication of neurodegenerative disease.
Results of an early clinical trial of the test appear in the latest issue of the journal Brain.
