The retina, the light-sensitive film at the back of the eye, is lined and nourished by a network of fine blood vessels.  These vessels can be damaged by the high blood sugar levels that occur in diabetes; as a consequence, they may leak blood, causing the retina to swell and sometimes for new, even leakier vessels to grow.  Left untreated, diabetic eye disease or retinopathy can ultimately cause blindness.  If you are diabetic, in many cases your optometrist will recommend that you be examined annually rather than every two years.  Control of underlying diabetes is very important in containing diabetic eye disease, although sometimes specialist treatment such as laser surgery to the retina by an ophthalmologist is also required.