Diabetic retinopathy begins as a mild disease. During the early stage of the disease, the small blood vessels in the retina become weaker and develop small bulges called microaneurysms. These microaneurysms are the earliest signs of retinopathy and may appear a few years after the onset of diabetes. They may also burst and cause tiny blood spots (hemorrhages) on the retina but they do not usually cause symptoms or affect vision.

As retinopathy progresses, fluid and protein leak from the damaged blood vessels and cause the retina to swell. This may cause mild to severe vision loss, depending on which parts of the retina are affected. If the center of the retina (macula) is affected, vision loss can be severe. Swelling and distortion of the macula (macular edema), which results from a buildup of fluid, is the most common complication of retinopathy.

In some people retinopathy progresses over the course of several years. In these cases, reduced blood flow to the retina stimulates the growth (proliferation) of fragile new blood vessels on the surface of the retina. This is called proliferative diabetic retinopathy. As the new blood vessels multiply, one or more complications may develop and damage the person's vision. These complications can include:

• The formation of scar tissue that pulls on the retina, which may lead to retinal detachment.
• Bleeding inside the eye (preretinal or vitreous hemorrhage).
• The growth of new blood vessels on the surface of the iris (rubeosis iridis), which eventually leads to a form of severe glaucoma called neovascular glaucoma.

Any of these later complications may cause severe, permanent vision loss.