A drug may be able to prevent the onset of glaucoma - a leading cause of blindness - scientists have said.

They tested a new way of treating the condition, a group of eye illnesses that damage the optic nerve.

Once vision is lost, the loss is permanent, so doctors seek to slow down and prevent its progress.

At the moment, doctors seek to reduce pressure in the eye - which increases as the condition progresses - to stop the nerve becoming damaged.

However, this cannot be achieved in all cases, so the new treatment uses a drug to reduce the level of a chemical that causes nerve damage in the eye.

Chemical damage

Professor Dr Arthur Neufeld, of the Washington University School of Medicine, led the study.

He said: "Having seen reports on nerve damage caused by excessive nitric oxide, we decided to look for evidence of high levels of nitric oxide in human eyes with glaucoma.

"Using sophisticated staining techniques, we detected an enzyme called inducible nitric oxide synthase in the optic nerve head tissue of patients with glaucoma."

This enzyme can produce excessive amounts of nitric oxide, and Professor Neufeld and colleagues said its presence was evidence that nitric oxide might be involved with the cell damage seen in glaucoma.

Increased levels

They found that both rats and humans with increased eye pressure had higher levels of the enzyme.

Next they fed rats a drug called aminoguanidine in their water, which they hoped would reduce production of nitric oxide.

They found that those who received the drug suffered less eye damage than those who did not, even though the eye pressure remained the same.

The researchers hope the drug may be used to treat patients whose glaucoma does not respond to pressure-lowering drugs or surgery, and those patients whose glaucoma is not associated with raised eye pressure.

The study was published in the proceedings of the (US) National Academy of Sciences.