Patients with diabetes are more likely to develop eye problems such as cataracts and glaucoma, but the disease's affect on the retina is the main threat to vision. The effect of diabetes on the eye is called Diabetic Retinopathy. It is caused by changes in the blood vessels of the retina.

In some people with diabetic retinopathy, retinal blood vessels may swell and leak fluid. In other people, abnormal new blood vessels grow on the surface of the retina. These changes may result in vision loss or blindness.

This is the early stage of the disease, when damaged capillaries break and leak. Fluid buildup in the macula (called macula edema) causes blurred vision. (The macula is located in the centre of the retina and is the structure that renders fine, detailed vision.)

During this later stage, abnormal, fragile blood vessels grow in the retina and into the vitreous body (clear gel-like substance that fills the chamber between the lens and the retina). This process is called neovascularization. These fragile vessels are prone to rupturing and bleeding into the vitreous body, causing blurred vision and possibly temporary blindness. If scar tissue forms, it may pull the retina away from the back of the eye, which can result in permanent vision loss. Macular edema can also occur during this stage.