What Causes Diabetic Retinopathy?
There are two forms of diabetes: type 1 and type 2. In people with type 1 diabetes, mild abnormalities in the retina begin to appear an average of seven years after the diabetes begins, but damage that threatens vision usually does not develop until much later. In people with type 2 diabetes—the more common type—retinopathy may be present at the time of diagnosis or relatively soon afterward. This is because the onset of type 2 diabetes is gradual, and changes in the retina may have taken place before diabetes is diagnosed.
Almost all individuals with type 1 diabetes will eventually develop retinopathy, compared with 70% of those with type 2 diabetes. People with type 2 diabetes are also less likely to develop more advanced retinopathy than those with type 1. Currently, an estimated one in 600 people age 18 and older with type 1 diabetes have retinopathy severe enough to threaten their vision.
In the early, or nonproliferative, stages of diabetic retinopathy, blood vessels in the retina develop weak spots that bulge outward (microaneurysms) and may leak fluid and blood into the surrounding retinal tissue. These initial abnormalities usually cause no visual symptoms, and in many people the disease progresses no further. However, microaneurysms can lead to macular edema.
Swelling around the macula (macular edema) caused by the leakage and accumulation of fluid can occur in people with diabetes. The swelling alters the position of the retina and causes blurred vision. Loss of vision is more pronounced when the center of the macula is affected.
This is the most dangerous form of diabetic retinopathy, characterized by neovascularization—the growth of new blood vessels onto the back surface of the vitreous humor. Acute loss of vision can occur when new blood vessels rupture and bleed into the vitreous humor or when these blood vessels lead to traction on the retina, causing it to detach from the back of the eye (retinal detachment).
Experts do not yet know exactly how high blood glucose levels cause diabetic retinopathy. One possibility involves a protein known as vascular endothelial growth factor (VEGF), which promotes the growth of new blood vessels in the eye and is secreted into the eye in response to the damage caused by diabetes.
Studies also suggest that elevated levels of cholesterol and triglycerides as well as high blood pressure can increase the risk of diabetic retinopathy. These conditions are more common in people with diabetes than in the general population.
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