Diabetes (diabetes mellitus) is a common disease in which blood-sugar levels are chronically too high. The disease has many related complications, and several eye diseases among them. The most common eye complication of diabetes is diabetic retinopathy, a leading cause of adult blindness.
Diabetic retinopathy occurs when high blood-sugar levels affect the functionality of blood vessels in the retina (light-sensing cells in the eye). In early phases of the disease, capillaries will leak blood or fluid. This can cause swelling in the retina (which may result in blurring of central vision), and it can leak into the vitreous humor (the fluid surrounding the retina) causing floaters or obscuring vision.
During the beginning stage serious vision damage is less likely, however, it can lead to a more advanced stage of the disease, called proliferative diabetic retinopathy. In this form, blood vessels in the retina actually close off. New blood vessels grow to make up for the lack of blood flow to the retina. The new blood vessels unfortunately are accompanied by scarring and more leakage. This can lead to serious vision loss and blindness.