If you have diabetes mellitus, your body does not use and store sugar properly. In diabetes, high blood sugar levels can damage the blood vessels in the retina, the nerve layer at the back of the eye that senses light and helps send images to the brain. Damage to the retinal vessels is called diabetic retinopathy. There are two types: non-proliferative diabetic retinopathy (NPDR) and proliferative diabetic retinopathy (PDR).