Mild nonproliferative diabetic retinopathy is the earliest stage, where tiny bulges or swelling happen in the blood vessels in the retina. This swelling can cause fluid leakage into the retina, resulting in swelling at the back of the eye. This stage is often not detectable.
Moderate nonproliferative diabetic retinopathy happens when blood vessels build up and swell. The blood flow to the retina gets blocked, disrupting its working. Due to this build-up in the macula in the retina, one can experience blurry or foggy vision.
Severe nonproliferative diabetic retinopathy is the third stage, where even more blood vessels in the retina start to get blocked. The flood flow to the eyes decreases significantly and can lead to the formation of new blood vessels. These new blood vessels are fragile and can cause swelling in the retina. This can cause one to see dark spots and blurry vision. Sudden and permanent loss of vision can also result at this stage if there is excess leakage of vessels into the macula.
Proliferative diabetic retinopathy is the most advanced stage of this condition. The new blood vessels cause scar tissue inside the eyes. These tissues are dangerous and can result in retinal detachment. The detachment of the retina can cause permanent loss of vision that is irreversible if not treated immediately.