DIABETES CAN AFFECT THE EYES IN SEVERAL WAYS:

Variations in vision are caused by changes in the sugar content of the lens of the eye or the blood sugar level changes. All increase in blood sugar thus tends to make a person relatively more short sighted and a decrease relatively more long sighted. 
Episodes of uncontrolled blood sugar can cause cataracts in young patients. Diabetes also causes the usual old age problems to occur at a younger age and more frequently. 
Diabetes most importantly causes retinopathy. Normally the blood vessels in the retina do not leak. But with diabetes, the retinal blood vessels can develop tiny leaks which causes fluid of blood to seep into the retina. The retina then becomes wet and swollen and cannot work properly. The effect on vision is especially grave when this swelling involves the macula. Diabetes also causes the retinal blood vessels to close resulting in malfunction and death of those parts of the retina supplied by these blood vessels. The areas of the retina in which blood vessels have closed then foster the growth of abnormal new blood vessels called neovascularization. Neovascularization causes bleeding and scar tissue that can result in vitreous haemorrhage, retinal detachment and blindness. 
TREATMENT OF DIABETIC RETINOPTATHY

LASER SURGERY

Laser surgery either seals the leaking blood vessels of the macula or reduce their leakage causing the macula to dry, or are used to destroy the diseased portions of the retina (without blood supply) in order to stop the growth of abnormal new blood vessels. The major purpose of laser surgery is to prevent further visual loss. Laser surgery do not necessarily improve vision.

VITRECTOMY SURGERY

Haemorrhage into the vitreous usually presents as floaters, spiderwebs or spots in front of the eyes, or as blurred vision. Vitreous haemorrhages usually disappears with time, but can be removed by an operation called a vitrectomy (during which the gel in the back of the eye is removed). In advanced retinopathy new blood vessels and scar tissue grow on the surface of the retina and are attached firmly to the back surface of the vitreous gel. The vitreous gel pull on these blood vessels and scar tissue, and causes a retinal detachment. The patient will then notice a shadow or very large dark area in the vision. The retina can be re-attached by doing a vitrectomy and by removing the scar tissue and new blood vessels.

LOW VISUAL AIDS

If the retinal blood vessels in the macula close, the macula stops working. This causes loss of central or detailed vision. Although there is no treatment for this form of the disease, eyesight may be helped somewhat with the use of low visual aids.

PREVENTING DIABETIC RETINOPATHY :

Controlling blood sugar is the single most important thing a patient can do to prevent diabetic retinopathy. 
Smoking aggravates the closure of retinal blood vessels and is very bad for patients with diabetes. 
High blood pressure is very bad for people with diabetes and should be strictly controlled. 
Regular eye examinations by an Ophthalmologist (at least yearly) are important in order to diagnose retinopathy early. Early laser treatment is much more effective. The severity of retinopathy is often related to the length of time the patient has had diabetes. Patients who has had diabetes for many years should be followed up more regularly. 
Pregnancy often aggravates retinopathy and an eye examination is essential when a pregnancy is planned