Elevated blood sugar levels in the context of diabetes can cause progressive damage to the retinal blood vessels. This damage causes the blood vessels to either leak or block off.
If the vessel leaks, the released fluid causes the retina to swell and vision is reduced. If the vessel blocks off, the reduced blood supply to the retina causes the growth of new, abnormal blood vessels that can bleed. The blood released into the vitreous (the jelly filling the back of the eye) profoundly reduces vision. Subsequent fibrosis can then cause the retina to detach resulting in blindness.
As the early changes from diabetes in the eye have no symptoms, every person with a diagnosis of diabetes should be monitored as early treatment can prevent ongoing damage.