In PDR, retinal blood vessels close off, and large areas of retina lose their source of nutrition. When this happens, peripheral or side vision is usually reduced, and the patient's ability to see at night and to adjust from light to dark is often diminished.

As a result of this loss of nourishing blood flow, the retina responds by developing new blood vessels that are abnormal and are called neovascularization. The development of neovascularization is the retina's method of coping with the closing of its own blood vessels and the loss of nourishment. Many people with diabetes have some closing of retinal blood vessels without ever developing neovascularization. But the problem is that when neovascularization develops, it is never always a problem. It is, in fact, dangerous to the eye. Neovascularization does not nourish the retina properly, and it may cause other problems. One problem is bleeding into the vitreous cavity (called vitreous hemorrhage). A second problem that occurs when neovascularization develops is the growth of scar tissue on the retina; the scar tissue can pull the retina off the back wall of the eye (called a traction retinal detachment). Either of these serious problems, vitreous hemorrhage or traction retinal detachment, can lead to severe loss of vision or even total blindness.

A third problem that can occur is when neovascularization grows on the iris, the colored part of the eye, rather than just on the retina. When neovascularization grows on the iris (called rubeosis), it may close off normal flow of fluid out of the eye and cause the pressure in the eye to rise to dangerously high levels. The high pressure (called neovascular glaucoma) can cause permanent changes, resulting in visual loss, pain, even loss of the eye.

It is very important to understand that the closing of retinal blood vessels and the development and growth of neovascularization may occur without any noticeable change of vision. So, it may be impossible for the person with diabetes and early PDR to know that such changes are occurring. For this reason, it is essential that every person with diabetes be examined regularly by a specialist who is familiar with diagnosing diabetic retinopathy. Such examinations should occur regularly--probably every six to twelve months, or more or less frequently--for the lifetime of the person with diabetes. The earlier neovascularization is discovered, the better the chance that laser surgery will save vision. The later neovascularization is discovered, the greater the chance for blindness.