Having one of the LHON genes does NOT mean that someone is bound to lose their eyesight. In my own case I am affected by LHON but my brother and cousins are not.
We do not yet know what triggers the eyesight loss. It is likely that many things can act as a trigger if they help to reduce the energy supply to the cells of the retina and optic nerve.
Some studies have suggested that one or more of the following things may be the extra factor involved:
