Over time, high blood-glucose levels from diabetes can slowly damage tiny blood vessels on the retina. Diabetic retinopathy is the term for this damage to the retina.
First, the retina’s blood vessels swell. As damage worsens, the blood vessels become blocked and cut off the retina’s oxygen supply. In response, new, weak blood vessels grow on the retina and the surface of the vitreous gel. These blood vessels break easily and leak blood into the vitreous gel. The leaking blood keeps light from reaching the retina. When that happens, you may see floating spots or almost total darkness. Sometimes the blood clears by itself, but surgery may be needed.
In time, the swollen, weak blood vessels can form scar tissue and pull the retina away from the back of the eye. If the retina pulls away (detached retina), you may see floating spots or flashing lights. You may feel as if a curtain has been pulled over part of your vision. A detached retina can cause vision loss or blindness if not treated quickly. See an ophthalmologist right away if you have these symptoms.
Source: National Institutes of Health