Macular edema occurs when fluid and protein deposits collect on or under the macula of the eye (a yellow central area of the retina) and causes it to thicken and swell (edema). The swelling may distort a persons central vision, because the macula holds tightly packed cones that provide sharp, clear, central vision to enable a person to see detail, form, and color that is directly in the centre of the field of view. Cause
The causes of macular edema are numerous and different causes may be inter-related. It is commonly associated with diabetes. Chronic or uncontrolled diabetes type 2 can affect peripheral blood vessels including those of the retina which may leak fluid, blood and occasionally fats into the retina causing it to swell. Age-related macular degeneration may cause macular edema. As individuals age there may be a natural deterioration in the macula which can lead to the depositing of drusen  under the retina sometimes with the formation of abnormal blood vessels. Replacement of the lens as treatment for cataract can cause pseudophakic macular edema. (‘pseudophakia’ means ‘replacement lens’)  also known as Irvine-Gass syndrome The surgery involved sometimes irritates the retina (and other parts of the eye) causing the capillaries in the retina to dilate and leak fluid into the retina. Less common today with modern lens replacement techniques. Chronic uveitis  and intermediate uveitis can be a cause.
These abnormalities include the presence of enlarged oval shaped red blood cells (macroovalocytes) and hypersegmented neutrophils (defined as a neutrophil with six or more lobes). Hypersegmented neutrophils may be seen in the absence of macroovalocytes as hypersegmentation of neutrophils is an early sign of megaloblastic anemia and may precede the appearance of macroovalocytes; they may also be seen in other anemias (e.g., iron deficiency anemia) and thus are suggestive of megaloblastic anemia but not specific for it. An increased red cell distribution width (anisocytosis) also suggests megaloblastosis and is commonly seen in Vitamin B12 deficiency and folic acid deficiency. This type of anemia is caused by impaired DNA synthesis and repair, often from deficient thymidine production. Thiamine responsive megaloblastic anemia syndrome also causes megaloblastic anemia.The red blood cells grow larger because they cannot produce DNA quickly enough to divide at the right time as they grow and thus grow too large before division. Additional causes of megaloblastic anemia include medications that interfere with DNA synthesis or with the absorption or metabolism of Vitamin B12 or folate such as methotrexate, sulfasalazine, metformin, anticonvulsant medications (e.g., valproic acid or phenytoin), certain antibiotics (e.g., trimethoprim/sulfamethoxazole), antiretroviral medications, cholestyramine, triamterene,  and nitrous oxide.