Diabetic macular edema (DME) is caused by a complication of diabetes called diabetic retinopathy. Diabetic retinopathy is the most common diabetic eye disease and the leading cause of irreversible blindness in working-age Americans. Diabetic retinopathy usually affects both eyes.

Diabetic retinopathy is caused by sustained damage to the small blood vessels of the retina. The leakage of fluid into the retina can cause swelling of the surrounding tissue, including the macula.

DME is the most common cause of vision loss in people with diabetic retinopathy. Poor blood sugar and additional medical conditions, such as high blood pressure, increase the risk of blindness in people with DME. DME can occur at any stage of diabetic retinopathy, although it is more likely to occur later as the disease progresses.

Experts estimate that about 7.7 million Americans have diabetic retinopathy, and of these, about 750,000 also have DME. A recent study suggests that non-Hispanic African Americans are three times more likely to develop DME than non-Hispanic Caucasians, most likely due to the higher incidence of diabetes in the African American population.

Macular edema can develop after any type of surgery performed in the eye, including surgeries for cataracts, glaucoma, or retinal disease. A small number of people who have undergone cataract surgery (experts estimate only 1-3 percent) can do it within a few weeksthe surgery develop macular edema. If one eye is affected, there is a 50 percent chance that the other eye will also be affected. Macular edema after eye surgery is usually mild, short-lived, and responds well to eye drops that treat inflammation.

Age-related macular degeneration (AMD) is a disease characterized by deterioration or breakdown of the macula, which is responsible for sharp central vision. In neovascular AMD, also called "wet" AMD, blood vessels begin to grow from the choroid (the bed of blood vessels below the retina) to the retina. These new and abnormal blood vessels leak fluid into the macula and cause macular edema.

When retinal veins are blocked (retinal vein occlusion), blood does not drain properly and leaks into the retina. If it leaks into the macula, macular edema develops. Leakage is made worse by the severity of the blockage, the number of veins, and the pressure in them. Retinal venous occlusion is most commonly associated with age-related atherosclerosis, diabetes, high blood pressure, and eye conditions such as glaucoma or inflammation.

Uveitis describes a group of inflammatory diseases that cause swelling in the eye and destroy eye tissue. The term "uveitis" is used because the diseases most often affect a part of the eye called uvea. However, uveitis is not limited to the uvea. Uveitis can affect the cornea, iris, lens, vitreous humor, retina, optic nerve, and whites of the eyes (sclera).

Inflammatory and immune system disorders can also affect the eye and cause swelling and breakdown of tissue in the macula. These conditions include cytomegalovirus infection, retinal necrosis, sarcoidosis, Behçet's syndrome, toxoplasmosis, Eales disease, and Vogt-Koyanagi-Harada syndrome.