Glaucoma is caused by high pressure inside the eye damaging the optic nerve
A new type of medicated eye drop may be able to reverse symptoms of glaucoma, an Italian study has suggested.
Glaucoma, the world's leading cause of blindness, is caused by pressure inside the eye - intraocular pressure - which damages cells in the optic nerve.
The study of rats and human patients found drops containing a nerve growth factor may stop these cells dying, and actually improve vision.
It is reported in Proceedings of the National Academy of Sciences.
It is estimated that 77m people have glaucoma around the world.
The build up of intraocular pressure can sometimes be controlled through other techniques.
But once pressure has started to damage optic nerve cells, called retinal ganglion cells (RGCs), it has proved impossible to recover that lost function.
And often people with glaucoma do not seek expert help until the condition is already relatively advanced.
The researchers, from the University of Rome, treated rats with symptoms of glaucoma with eye drops containing nerve growth factor.
The animals who were given the eye drops showed decreased levels of RGC death compared with those who did not receive the treatment.
The researchers went on to test the drops in three human patients whose intraocular pressure had started to be controlled, but who still showed signs of progressive deterioration in their vision.
In two of the patients vision improved, while in the other it was stabilised.
The improvements lasted up to 18 months after the eye drops were applied.
Nerve growth factor appears to trigger chemical changes within cells that prevent them from dying in response to damage.
It might also enable cells whose function had begun to be damaged to bounce back.
And it might boost the capacity of healthy RGCs to form new connections within the optic nerve, to compensate for any damage that had already taken place.
However, nerve growth factor cannot rescue RGCs that have already died - in common with brain tissue, the optic nerve cannot regenerate.
Lead researcher Dr Stefano Bonini said: "Although neuroprotection in glaucoma has already been attempted with several compounds, this is the first time that an improvement in visual function is observed in patients with advanced optic nerve damage."
David Wright, chief executive of the International Glaucoma Association, warned against drawing firm conclusions from such a small study - but said the results were encouraging.
He said: "There have been many false dawns in the search for neuro-protective agents for the treatment of glaucoma and it is a feature of research on other compounds that early promise does not always translate into clinical effectiveness when larger studies are undertaken.
"However, this research would seem to indicate both effectiveness and acceptability as to the means of drug delivery.
"If these early indications are carried through to wider trials and there are no other problems, then this has the potential to open a completely new method of treating glaucoma and of preventing unnecessary visual loss in the future."
Professor Peng Khaw is director of the National Institute of Health Research Biomedical Research Centre at Moorfields Eye Hospital and UCL Institute of Ophthalmology.
He said many other substances had achieved good results in animals, only to fail in humans.
However, he said, if the researchers could perfect an easy-to-use eye drop formulation that would represent a huge step forward.