Diabetic Retinopathy

Diabetic retinopathy affects more than 5.3 million persons age 18 years and older in the United States. Steven A. Smith, M.D., of the Division of Endocrinology, Diabetes, Metabolism, and Nutrition at Mayo Clinic in Minnesota, says, "Epidemiologic studies looking at the major risk factors for diabetic retinopathy have shown a consistent association between retinopathy and duration of diabetes mellitus, degree of hyperglycemia, and presence of hypertension, hyperlipidemia, pregnancy, and renal disease. The less consistent risk factors for diabetic retinopathy are obesity, cigarette smoking, moderate alcohol consumption (more than one alcohol-containing beverage per day), and lack of physical activity."

Proliferative diabetic retinopathy

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John M. Pach, M.D., of the Department of Ophthalmology at Mayo Clinic in Minnesota explains, "Most patients with diabetes mellitus have some degree of diabetic retinopathy after having diabetes for 20 years."

Two Types of Diabetic Retinopathy

Diabetic retinopathy is divided into two types — nonproliferative and proliferative. Proliferative retinopathy refers to retinal neovascularization from ischemia. Nonproliferative changes occur before the onset of neovascularization due to increased capillary permeability and ischemia. The typical fundus findings are microaneurysms and hemorrhages. The retinal hemorrhages, depending on their location, take a dot-blot or flame-shaped appearance. As vascular permeability increases, retinal edema may ensue. Lipoproteins may precipitate out, giving the appearance of hard exudates. Infarctions of the nerve fiber layer may also occur, producing the appearance of cotton-wool spots. As the background retinopathy increases in severity, more numerous retinal hemorrhages are seen, along with irregularity of the veins or venous beading."

Nonproliferative diabetic retinopathy

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In patients with background diabetic retinopathy, the leading cause of decreased visual acuity is macular edema. Dr. Pach notes that diabetic macular edema can be categorized as focal or diffuse:

• Focal diabetic macular edemarefers to leakage primarily from microaneurysms
• Diffuse diabetic macular edema is due to increased widespread leakage from the capillary bed

As retinal ischemia increases, vascular endothelial growth factor is upregulated, which promotes retinal neovascularization. Although neovascularization may occur in the iris or the trabecular meshwork and thus result in neovascular glaucoma, the primary location of the neovascularization is on the retinal surface.

"The natural history of retinal neovascularization is for continued growth and fibrosis," says Dr. Pach. "As the vitreous contracts, it may cause the sites of neovascularization to bleed. Typically, the hemorrhage occurs in the vitreous, causing such symptoms as floaters and the appearance of cobwebs. As the neovascularization progresses, fibrosis develops. The increasing retinal neovascularization and fibrosis may exert tractional forces on the retina. When this traction overcomes the adhesive force of the retina to the retinal pigment epithelium, a traction retinal detachment occurs. Once the macula is detached, visual acuity is poor."

Eye-Directed Treatment

The mainstay of treatment for diabetic macular edema and proliferative diabetic retinopathy is laser photocoagulation. Dr. Pach highlights that prompt treatment is recommended when eyes have high-risk characteristics of proliferative disease. Panretinal photocoagulation in eyes with high-risk characteristics decreases by approximately 60 percent the risk of severe visual loss.

For an eye that does not have the high-risk characteristics, panretinal photocoagulation may still be recommended, depending on the condition of the other eye, as well as the documented progression of the retinopathy.

Focal laser photocoagulation for clinically significant diabetic macular edema decreases the risk of moderate visual loss by 50 percent. Such photocoagulation is most effective in patients with focal or multifocal leakage primarily from microaneurysms."

Dr. Pach continues, "Diffuse macular edema, due to leaking primarily from parafoveal capillaries, may suggest an underlying systemic condition. Often, patients with this type of edema have hypertension, congestive heart failure, or renal disease as a contributing factor. Normalization of blood pressure or diuresis may improve diabetic macular edema."

Control of Systemic Factors

Dr. Smith notes that there is strong evidence in the medical literature showing that the lowering of hemoglobin A1_c levels to approximately 7.0 percent is advantageous in reducing the onset and progression of diabetic retinopathy. Similarly, evidence shows that a decrease in systolic or diastolic blood pressure, or both, is advantageous in reducing the development and progression of diabetic retinopathy. Aggressive lowering of total cholesterol and triglyceride levels may cause regression of the exudates.