Normal retinal blood vessels are watertight and do not leak. In diabetes, the retinal blood vessels can become damaged and develop tiny leaks. This is called nonproliferative diabetic retinopathy (NPDR). Blood and fluid seep from the leaks in the damaged retinal blood vessels, and fatty material (called exudate) can deposit in the retina. This causes swelling of the retina. When leakage occurs and causes swelling in the central part of the retina (the macula), it is called macular edema, and vision will be reduced or blurred. Leakage elsewhere in the retina will usually have no effect on vision.
A patient with macular edema, or with exudate in the macula, will usually experience some loss of vision, including blurring, distortion, and 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 areas just outside of the macula to see more 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 patient who have severe NDPR will usually be able to see well enough to take care of themselves and continue those activities that do not require detail vision.