how does blidness due to decaying of nerves in eyes occur?is there any treatment for this?

Does this question have anything to do with the novel?

Well, even if has nothing to do with the novel, (and the fact that I'm five years late), nerve decaying is easily prevented by stopping straining of the eye. Certain illnesses, such as glaucoma (where the final, untreated result is blindness), occur due to high pressure in the optic nerve. Adding up all the possible ways to avoid going blind via nerve decay would be:

-Healthy diet (generally low-carbs keep you safe)
-Regular exercise (not excessive, just a jog in the weekend might help)
-Use of protective eyewear during certain types of work (injuries to the eyes might damage the optic nerves leading to blindness)
-Regular checks at the ophtalmologist, taking into account the person's age and biological sex, since certain people are more propepense to falling ill to certain diseases.

Certain genetical issues, such as hypoplasia of the optic nerve, cannot be prevented (optical axons are not developed properly). Other diseases, such as ON (optic neuritis), are episodical and can be fixed rather easily (temporal blindness).

Above all, just keep a good diet. Currently, experts are working on ways to regenerate nerve tissue in ocular areas, probably taking a maximum of 30 years.

Hope it helps!