The retina, the delicate, light-sensitive membrane lining the back of the eyeball, may be damaged by diabetes. This is called retinopathy. In the first stage of retinal damage, called nonproliferative retinopathy, the tiny capillaries that feed the retinal tissue balloon out into pouches, called microaneurysms. This is a direct result of damage to the endothelial cells that line the vessels of the retina. Capillaries leak, and many close off, resulting in a decreased flow of blood to the retina called ischemia.

Ischemia causes the formation of abnormal new vessels. These new vessels grow in an irregular pattern and can cause bleeding into the eye, swelling of the macula (a critical area of the retina necessary for sight), and scarring, which can cause the retina to detach. The presence of these abnormal blood vessels is called proliferative retinopathy. Diabetes is the leading cause of blindness in the US in people under the age of 65, with 12,000-24,000 new cases each year.

Diabetes also increases the risk of developing cataracts and glaucoma. Cataracts, the clouding of the eye's lens, develop at an earlier age in people with diabetes. Glaucoma is an increase in fluid pressure inside the eye that leads to optic nerve damage. Someone with diabetes is twice as likely to get glaucoma as someone who doesn't have diabetes.