When it is in its early stage, it can be cured with both internal and ophthalmic treatment. But when retinopathy becomes worse, present medical science has no cure for it.
When retinopathy advances to a later stage, bleeding occurs in the eyes(retinal
bleeding), in the end you lose your eyesight.
In Japan the main reason for losing one's eyesight is diabetic retinopathy.
Beside retinopathy, cataract sometimes can occur, if you do not care about your blood sugar level. This is a disease where the lens, which is comparatively closer to the surface of the eyes than the retina, becomes unclean.
So, if you are diagnosed with diabetes, even if you do not feel any symptoms concerning your eyes, you should be examined at ophthalmology at least once a year to prevent these serious secondary disease. The most effective way to do this is by keeping your blood sugar level stable.