In the United States, diabetic retinopathy ranks as the leading cause of new blindness among adults. Although the etiology of this condition remains uncertain, hyperglycemia is thought to play a significant role. Retinal damage generally occurs secondarily to ocular-blood barrier alteration with subsequent neovascularization. Since these pathological changes may be delayed or diminished through early glucose control and photocoagulation techniques, the detection of diabetic retinopathy is extremely important.
Diabetic retinopathy is a major cause of morbidity. It has been estimated that 10 million people in the U.S. have diabetes mellitus. Within this group, the risk of developing retinopathy increases with diabetes duration. Among those persons who have had insulin-dependent diabetes for more than 30 years, approximately 12% are blind (Parnes & Singerman, 1994, p. 236).
The normal retina consists of a "delicate nervous membrane (Gray & Goss, 1962, p. 1108)." Its primary function is to receive the images of external objects. Retinal circulation derives from the central artery of the retina; this small vessel is typically the first branch of the ophthalmic artery (Gray & Goss, 1962, p. 636). Retinal capillaries generally contain a lining of endothelial cells. These cells rest on a basement membrane and are interconnected by structures similar to tight junctions. In conjunction with the basement membrane, the intercellular connections form a selective permeability barrier. The ocular-blood barrier restricts the passage of large blood proteins and various smaller molecules out of the capillary lumen. In addition to the endothelial cell layer, retinal capillaries also have a surrounding cell layer composed of pericytes embedded in the basement membrane. The ratio of pericytes to endothelial cells is approximately 1:2 (Mandarino, 1992, p. 1892). Pericytes may be involved in contractility. In addition, various ev...