Diabetic macular edema: Swelling of the retina in diabetes mellitus due to leaking of fluid from blood vessels within the macula. The macula is the central portion of the retina, a small area rich in cones, the specialized nerve endings that detect color and upon which daytime vision depends.

As macular edema develops, blurring occurs in the middle or just to the side of the central visual field. Visual loss from diabetic macular edema can progress over a period of months and make it impossible to focus clearly.

Macular edema in common in diabetes. The lifetime risk for diabetics to develop macular edema is about 10%. The condition is closely associated with the degree of diabetic retinopathy (retinal disease). Hypertension (high blood pressure) and fluid retention also increase the hydrostatic pressure within capillaries which drives fluid from within the vessels into the retina. A common cause of fluid retention in diabetes is kidney disease with loss of protein in the urine (proteinuria).

Diabetic macular edema is classified into focal and diffuse types. This is an important difference because the two types differ in treatment. Focal macular edema is caused by foci of vascular abnormalities, primarily microaneurysms, which tend to leakage fluid whereas diffuse macular edema is caused by dilated retinal capillaries in the retina.

Two types of laser treatment for diabetic macular edema are focal and grid. Focal laser treatment is used to treat focal diabetic macular edema; the aim is to close leaking microaneurysms. Grid laser treatment is used to treat diffuse diabetic macular edema and is applied to areas of retinal thickening in which there is diffuse leakage; the aim is to produce a retinal burn of mild to moderate intensity.

The patient is rechecked several months after treatment and, if the diabetic macular edema is not responding to treatment, the laser treatment is repeated. The goal of treatment is to maintain current visual acuity and reduce the chances of progressive visual loss. Even with successful treatment, visual acuity often does not improve.

Macular edema occurs when fluid and protein deposits collect on or under the macula of the eye, a yellow central area of the retina, causing it to thicken and swell. The swelling may distort a person's central vision, as the macula is near the center of the retina at the back of the eyeball. This area holds tightly packed cones that provide sharp, clear central vision to enable a person to see form, color, and detail that is directly in the line of sight.

Cystoid macular edema is a type of macular edema that includes cyst formation.