Scientists are developing a new test that could detect glaucoma ten years before symptoms appear - and potentially save the sight of millions of people.

The technology, which detects dying nerve cells in the eye, has the potential to identify other neurological conditions at an early stage, such as Alzheimer's and Parkinson's.

Glaucoma is the leading cause of preventable blindness in the world and affects 500,000 in Britain, though experts warn many more could be undiagnosed.

The test, which is being trialled for the first time in humans at the Western Eye Hospital in London, uses a type of fluorescent compound that makes the back of the eye 'glow' if glaucoma is present.

Glaucoma is caused when drainage tubes in the eye become blocked and fluid starts to build up in the eyeball. This leads to high pressure, causing damage to the optic nerve (which carries signals from the eye to the brain) and irreversible blindness.

However, the condition is largely symptomless and can progress for more than ten years before patients become aware of sight problems, such as a loss in peripheral vision.

The research, which is a collaboration between University College London, the Wellcome Trust and Imperial College Healthcare NHS Trust, will involve 28 patients - two-thirds will have glaucoma or similar diseases and one-third will be healthy controls.

Patients will receive one dose of the fluorescent dye via a vein in their arm. Their vision will be monitored for eight weeks.

The team, led by consultant ophthalmologists Professor Philip Bloom and Francesca Cordeiro, are developing the dye as an eye drop, so the test can be done at a High Street optician.