Patients who suffer from type 1 and type 2 diabetes often have elevated blood glucose levels, which can damage ocular blood vessels, especially those around the retina, causing them to become weaker, circulate improperly, inflame, or grow abnormally. This is diabetic retinopathy, of which there are two types: nonproliferative and proliferative.
In nonproliferative retinopathy, which progresses from mild to moderate to severe if left untreated, the blood vessels degrade and begin to swell. This can make your vision blurry, cause you to see spots, or make seeing at night more difficult, but many patients experience little to no symptoms during this stage of the disease.
As the condition of the blood vessels worsens, you may develop proliferative retinopathy, in which your body attempts to remedy the situation by growing new blood vessels. However, these blood vessels do not develop properly, so they easily break and leak blood, obstructing your vision. Diabetic retinopathy can cause vision loss in both its nonproliferative and proliferative forms if leaking or swollen blood vessels damage the macula. This condition is referred to as macular edema.