Diabetic macular edema (DME or DMO) is characterized by retinal thickening or hard exudates, and is the most common cause of visual impairment in diabetic patients. Damaged retinal capillaries and the microaneurysms leak fluid, resulting in retinal edema. Hard exudates are indicative of current or previous retinal thickening, and result from the accumulation of lipoprotein and lipid-filled macrophages within the retina. Involvement of the macula has the potential to cause severe impairment of central vision.
DME is usually asymptomatic until edema or lipid deposits affect the fovea; then central vision becomes impaired. Patients may also notice distorted shapes of objects (metamorphopsia).
Capillary leakage results in areas of dull, edematous retina. The macular edema often renders the retina semi-transparent (translucent) causing a blurring of the appearance of the retinal pigment epithelium and choroid. The edema is excess fluid in the extracellular spaces between retinal cells causing excess scattering of light. Stereoscopic techniques (most commonly slit-lamp examination with an accessory lens) enable detection of retinal thickening. Occasionally, pockets of fluid are visible at the macula (cystoid macular edema, CME).
Eventually, well-circumscribed hard (lipid) exudates form, often at the junction between normal and thickened retina. Hard exudates in a ring pattern are known as a circinate retinopathy.
The DME Disease Severity Scale allows grading with a direct ophthalmoscope based on the presence of hard exudates, which are readily visible evidence of previous or current retinal edema.
- DME absent: no retinal thickening or hard exudates present in the posterior pole.
- DME present: some retinal thickening or hard exudates present in the posterior pole.
- Mild: distant from the centre of the macula.
- Moderate: approaching the centre of the macula.
- Severe: involving the centre of the macula.
Usually other signs of diabetic retinopathy are also present (see Diabetes – Retinopathy Introduction).
DME is the most common cause of loss of vision in diabetes.
See Diabetes – Macular Edema Management
Laser treatment is used in the treatment of PDR and macular edema. Surgical removal of the vitreous (vitrectomy) is also performed in patients with PDR in certain settings.
Mild non-proliferative diabetic retinopathy with hard (lipid) exudates, dot and blot hemorrhages and macular edema. The macular edema reduces the transparency of the retina, blurring the retinal pigment epithelium and choroid appearance.
Fluorescein angiogram of the eye shown in figure 1. The macular edema is evidenced by the diffuse fluorescence and the microaneurysms show as multiple dots of hyperfluoresence. The central zone of hypofluorescence indicates the foveal avascular zone.