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.

Diabetes generally is classified as two types:
Type 1 diabetes. Insulin is a natural hormone that helps regulate the levels of blood sugar needed to help "feed" your body. When you are diagnosed with type 1 diabetes, you are considered insulin-dependent because you will need injections or other medications to supply the insulin your body is unable to produce on its own. When you don't produce enough of your own insulin, your blood sugar is unregulated and levels are too high.

Type 2 diabetes. When you are diagnosed with type 2 diabetes, you generally are considered non-insulin-dependent or insulin-resistant. With this type of diabetes, you produce enough insulin but your body is unable to make proper use of it. Your body then compensates by producing even more insulin, which can cause an accompanying abnormal increase in blood sugar levels.

With both types of diabetes, abnormal spikes in blood sugar increase your risk of diabetic retinopathy.

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.