In diabetes, the retinal blood vessels can develop tiny leaks. Blood and fluid seep from the retinal blood vessels, and fatty material, called exudate, deposits in the retina. This causes swelling of the retina, and it is called nonproliferative diabetic retinopathy (NPDR). When this occurs in the central part of the retina (the macula), vision will be reduced or blurred. Leakage elsewhere in the retina will usually have no effect on vision.

A patient with a wet, swollen macula, or with exudate in the macula, will experience some loss of vision, including blurring, distortion, or darkening. If one eye is affected, the other eye is frequently affected also, though the problem may not be equally severe in both eyes. If the diabetic retinopathy has affected each macula severely, central vision may be lost from each eye. But even if the ability to see detail has been lost from both eyes, the person with severe NPDR will usually be able to get along fairly well by learning to use the area just outside of the macula to see some detail. This ability to look slightly off center usually improves with time, though the eyesight will never be as good as it was before the macula was damaged by the leakage of blood vessels. So patients who have NPDR will usually be able to see well enough to take care of themselves and continue those activities that do not require detail vision.