Diabetic retinopathy is a disease of the retina which is the tissue lining the back of the eye that turns light images into visual signals sent to the brain.  The longer one is a diabetic, the more likely it is that he or she will have some degree of diabetic retinopathy.  Diabetic retinopathy affects the retina in two ways.
The first form of diabetic retinopathy is called proliferative diabetic retinopathy which means new unwanted blood vessels begin to grow (proliferate) in the eye.  These vessels are believed to grow in an attempt to supply blood to areas of the eye where the blood supply is limited by diabetic changes in the blood vessels.  The new vessels grow haphazardly and often extend into the clear vitreous jelly that fills the back of the eye.  These vessels leak fluid causing local swelling in the retina resulting in decreased vision.  Eventually, the vessels can bleed into the eye and cause traction on the retina leading to a detachment of the retina from the back of the eye. 
Proliferative Diabetic Retinopathy
The second form of diabetic retinopathy is called non-proliferative diabetic retinopathy as it is not a result of new vessels but rather it is due to damage of the patient’s own retinal blood vessels.  These vessels become weak and develop small outpouchings called aneurysms.  They leak blood and fluid into the surrounding retina and the resulting retinal swelling can decrease the vision. 
Nonproliferative Retinopathy
Diabetic Macular Edema – Nonproliferative Retinopathy
Ailments and Treatments
Both proliferative and nonproliferative retinopathy can be treated by laser surgery.  There are certain criteria developed during multiple national studies used to determine which individuals would benefit by having laser surgery.  In other words, all people with diabetic retinopathy do not need laser surgery; only those with a high risk of visual loss are candidates for treatment.  Even with treatment, the goal is primarily to prevent further visual loss.  It is difficult (but sometimes possible) to regain sight already lost before the laser surgery.  Unfortunately, a few patients can lose vision despite the laser treatments.  Recently, we have seen widespread use of corticosteroid and anti-growth factor injections being used as an adjuvant to laser treatments.
Studies have shown that good control of blood sugar and blood pressure can reduce the severity of the disease.  We generally recommend trying to keep the Hemoglobin A1C level under 7.0.  Even so, there are some patients under reasonably good control who still develop the retinopathy.
Typically diabetic retinopathy can progress a great deal before the patient notices any visual changes.  Furthermore, laser treatment is most effective when performed early in the disease process; therefore it is very important that all diabetics have a dilated eye exam every year by a professional familiar with the treatment of the disease.
Diabetics can develop eye movement abnormalities due to cranial nerve palsies (palsies are weaknesses).  The nerves are believed to have a decreased blood supply due to the diabetes and lose function.  Because they control the eye muscles, the eyes can no longer properly align themselves and the patient sees double images.  Fortunately, the large majority of these cranial nerve palsies resolve spontaneously over several weeks.
The optic nerve, which is the nerve that takes all of the visual information from the retina to the brain, can sometimes become swollen in diabetics.  The cause of this optic neuropathy is unclear but it may be due to insufficient blood supply.  Like the nerve palsies mentioned above, the majority of these optic neuropathies resolve on their own over several weeks.  Overall, this is an uncommon problem in diabetics.
The lens of the eye can be affected in two ways by diabetes.  First, wide swings in blood sugar cause the lens to shrink or swell.  This change in shape alters how it focuses light onto the retina and thus changes the patient’s refraction (that is their glasses prescription).  It is, therefore, usually best to wait until the blood sugar is under control before buying new glasses.  The second way diabetes affects the lens is by cataract formation.  A cataract is a clouding or discoloration of the clear lens rendering it difficult to see through.  Cataract surgery may become necessary when the visual limitation inhibits the individual’s ability to function.
Overall it is very important for diabetics to have yearly dilated eye exams, even if they feel their vision is fine and their blood sugar is well controlled.  It is also important to maintain good blood sugar control to decrease the risk that diabetic retinopathy will start and/or progress.