Diabetes mellitus (DM) causes abnormal changes in the blood sugar (glucose) that your body ordinarily converts into energy to fuel different bodily functions. Uncontrolled diabetes allows unusually high levels of blood sugar (hyperglycemia) to accumulate in blood vessels, causing damage that hampers or alters blood flow to your body’s organs — including your eyes. Eye damage occurs when chronically high amounts of blood sugar begin to clog or damage blood vessels within the eye’s retina, which contains light-sensitive cells (photoreceptors) necessary for good vision.