Is vision impairment and blindness due to diabetic retinopathy caused by the damaging effects of high blood sugar levels, or is there a damaging chronic virus infection within the eye and local tissues? If so, in what percent of cases is the latter a major factor?

You are seeing now for yourself, the trap of making assumptions from an association of physiologic changes with pathologic consequences in cells and tissues. In this case, being subjected to frequent abnormally high glucose levels for prolonged periods is a characteristic of diabetes but not a cause of tissue injury as a major consequence. It is actually the fact that diabetes is a viral disorder and that virus can be present in many locations in the body in addition to the pancreas that provides the link in understanding vision impairment experienced by diabetics. There is a chronic virus present in the eyes of those individuals that causes local tissue injury just as has taken place in the endocrine tissues to interfere with metabolic regulation of glucose handling and the production of and response to insulin. Here again, it is universally the case that diabetic retinopathy results from local changes within the eye and the associated blood vessels and nerves due to presence of a chronic virus.