DIABETES & THE  EYE   modified from the Canadian Ophthalmic Society website
DIABETES MELLITUS occurs when the body does not properly process sugar (glucose) for its energy needs, usually because of a failure to produce or appropriately respond to insulin (a chemical messenger from the pancreas). The disease affects as many as 1 million Canadians, 50% of whom do not know they have it.  Diabetes can cause blindness.
How Does Diabetes Affect the Eyes?  Poorly regulated and high levels of sugar in the blood can cause the lens of the eye to swell and shrink, resulting in BLURRED VISION and eyeglass prescriptions that keep on changing. The condition may also interfere with focusing of the eye.  Control of the blood sugar level usually corrects these problems.  Diabetes can also cause CATARACTS, a clouding of the lens inside the eye that blurs vision.  Diabetes can cause DOUBLE VISION when it affects the nerves that control the alignment and movement of the eyes. It can also cause the optic nerve to be more easily damaged by glaucoma.
     The most important cause of visual impairment in people with diabetes is DIABETIC RETINOPATHY, a condition in which changes occur in the tiny blood vessels that nourish the retina (the light-sensitive tissue that lines the back of the eye and changes the light into nerve messages to be transmitted to the brain). In the early stages of diabetic retinopathy, called NONPROLIFERATIVE or background retinopathy, small blood vessels weaken and leak fluid or tiny amounts of blood, distorting the retina. At this stage the person may have normal vision or may note blurred or changing vision. Although 25 % of diabetics have some degree of nonproliferative retinopathy, it does not progress to more severe problems in most.
     In more advanced diabetic retinopathy, blood vessels in the retina are blocked or closed completely, and areas of the retina die. PROLIFERATIVE diabetic retinopathy affects about 5% of people with diabetes, and occurs when new, abnormal blood vessels grow to replace the old ones. These new vessels are fragile and often rupture and bleed into the eye, blocking vision. Scar tissue forms, shrinks and tears the retina, causing bleeding or detaching it from the back of the eye. This can result in severe visual loss or blindness. Fortunately, this occurs only in a small minority of people with diabetes.       
     The chances of having some form of diabetic retinopathy increase the longer a person has had diabetes. Retinopathy is present in 90% of those who have had the disease for more than 20 years.
Detection and Treatment  Research has shown that severe visual loss from diabetic retinopathy can be prevented or delayed by laser treatment, but only if the retinopathy is diagnosed early enough. This is why it is important for most people with diabetes, particularly those who have had the disease for five years or more, to have a yearly eye examination performed by a medical doctor trained to recognize the subtle early signs of diabetic retinopathy.     Severe diabetic retinopathy may be recorded with special photographs of the retina called fluorescein angiograms (injection test with orange coloured fluid).
Treatment with laser photocoagulation is aimed at sealing leaky vessels and preventing the growth of new, abnormal vessels. Laser treatment has risks and side effects, which must be weighed against the benefits for each individual patient. In more advanced retinopathy, the benefits usually outweigh the risks.  Many laser sessions may be required.
Despite treatment, or for lack of it, some people with diabetes bleed may bleed massively into the eye and require a delicate, microscopic operation called a VITRECTOMY to remove blood and scar tissue from the eye. Others also need surgery for retinal detachment.