Nonproliferative retinopathy is caused by decreased blood flow within the retina and is marked by a number of additional changes. These include venous beading, which causes veins to resemble a string of beads caused by localized areas of dilation and constriction in the vessels.
Intraretinal microvascular abnormalities (IRMA) refer to abnormal, spaghetti-shaped blood vessels that run between the normal vessels in the retina; multiple areas of these vessels predict progression to proliferative retinopathy.
Another change is the development of cotton wool spots—areas of swollen nerve tissue that appear like white clouds on the retina, and are the result of cells dying due to blocked blood capillaries.
Proliferative retinopathy, the most advanced stage of diabetic retinopathy, is marked by the spontaneous growth of new blood vessels, which are more fragile than the normal retinal blood vessels and are more prone to breaking and bleeding. These vessels are accompanied by fibrous scarring and this fibrovascular tissue grows out of the retina itself and into the vitreous humor (the gel-like substance that fills the eyeball). Blindness can occur if these vessels bleed (vitreous hemorrhage) or pull on the retina, causing a retinal detachment. Similar fibrovascular tissue may develop in the front of the eye on the colored iris. This, too, can cause blindness due to neovascular glaucoma.