Most blood vessels are fenestrated, meaning that they have tiny openings that allow fluid to pass through the vessel wall. Retinal blood vessels, on the other hand, have tight junctions between the cells of the blood vessel wall, so all fluid and molecules exiting the vessel have to pass through the cells. In diabetic retinopathy, the vessels become more permeable. Water, blood cells, proteins, fats, and other large molecules may leak out into the surrounding retinal tissue. Macular edema is the most common cause of decreased vision in patients with background diabetic retinopathy.
Exudates are the result of fats and proteins leaking out of the permeable vessels along with water.
The water can be quickly reabsorbed into the vessels or into the tissue under the retina, but the fatty material is absorbed only very slowly. Swelling in the retina is fairly common in background diabetic retinopathy, but it is not always significant swelling. In other words, retinal edema does not always affect vision and does not always need to be treated. If possible, it is best to find macular edema when it is clinically significant, but before it affects the vision, since treatment is most effective at this stage. The treatment of macular edema is primarily by laser treatment.