The landmark United Kingdom Prospective Diabetes Study showed similar results for people with Type 2 diabetes. If you can keep your HbA1c results around or under 6.0%, you will greatly lower the odds of having serious eye complications from diabetes.
Why and how does high blood glucose damage the eyes? The “stock” answer given by medical professionals is that high blood glucose levels damage the smallest, most fragile blood vessels throughout the body, and the eyes have lots of small blood vessels. While this explanation is not incorrect, it is somewhat simplistic. The fuller answer is quite a bit more complex. Essentially, high blood glucose has two harmful effects in the eyes:
- It impairs the ability of small blood vessels to precisely control the volume of blood traveling through them. The result is excessive blood flow that mechanically injures those vessels, making them leaky. To visualize this, imagine a garden hose trying to accommodate the water flow from a fully opened fire hydrant.
- The breakdown (metabolism) of excess glucose by the body leads to an increase in the formation of harmful biochemicals (called glucose metabolites). These biochemicals make blood vessels in the retina leaky, promote the formation of abnormal, new blood vessels, and bind (“stick”) to the proteins that make up body tissues (including parts of the eyes), causing those proteins to function abnormally. Some of the harmful by-products of glucose metabolism include advanced glycosylation endproducts and protein kinase C.
Of course, the clear benefits of “tight” blood glucose control have to be balanced against the risks of severe hypoglycemia (low blood glucose). Tight control is best achieved with close consultation with members of your diabetes care team, careful attention to diet (especially portion control), regular exercise, medicine if necessary, and blood glucose monitoring on a regular basis.
Living with diabetes is admittedly often like walking a tightrope between high and low blood glucose levels. There is great variability in each person’s response to specific foods, activities, and treatments. The point is that the better you can learn to recognize and react to your unique blood glucose patterns, the closer to normal you can keep your blood glucose levels, and the healthier your eyes will be.
Blood pressure control
Keep your blood pressure well controlled through diet, regular physical activity, and medicine, if needed. The United Kingdom Prospective Diabetes Study showed that for Type 2 diabetes, controlling high blood pressure lowers the risk of retinopathy and worsening of existing retinopathy even more than tight blood glucose control. A 10/5 mm Hg reduction in blood pressure (for example, from 150/90 mm Hg to 140/85 mm Hg) reduced the risk of severe vision loss by nearly 50%. (In addition, this same blood pressure reduction lowered stroke risk by 44% and death by 32%!)
Elevated blood pressure increases blood flow into the eye, accelerating diabetic retinopathy. It also increases the turbulence of blood flow to the retina and optic nerve, which increases the risk of abnormal blood clotting (as happens in ischemic optic neuropathy and retinal vascular occlusion).
Recent research on the dynamics of blood flow to the eyes (called ocular hemodynamics) has given new insight into the importance of blood pressure control. The results of this work can help eye doctors gauge the risk of severe vision loss from diabetic retinopathy for individual patients. Research has demonstrated (using a technique called laser Doppler velocimetry) that at the onset of retinopathy, the volume of blood flow into the retina increases dramatically. As mentioned earlier, this is believed to be caused by high blood glucose, which impairs the ability of retinal vessels to constrict and precisely regulate blood flow. The pressure generated by this blood flow, known as retinal perfusion pressure (RPP), injures the walls of the smallest blood vessels (making them leaky) and redirects blood away from the smallest vessels (capillaries) into larger vessels. This results in destruction of the capillaries and inadequate blood circulation within retinal tissues (a phenomenon known as capillary nonperfusion). This destruction, in turn, leads to the release of biochemical messengers that promote the growth of new but abnormal blood vessels that bleed profusely and can lead to retinal detachment (as happens in proliferative diabetic retinopathy).