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What is Macular edema and explain in detail?
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.
Blockage of a vein in the retina can cause engorgement of the other retinal veins causing them to leak fluid under or into the retina. The blockage may be caused, among other things, by atherosclerosis, high blood pressure and glaucoma.
A number of drugs can cause changes in the retina that can lead to macular edema. The effect of each drug is variable and some drugs have a lesser role in causation. The principal medication known to affect the retina are:- latanoprost, epinephrine, rosiglitazone, timolol and thiazolidinediones among others.
A few congenital diseases are known to be associated with macular edema for example retinitis pigmentosa and retinoschisis.

Diagnosis
Classification
Cystoid macular edema (CME) involves fluid accumulation in the outer plexiform layer secondary to abnormal perifoveal retinal capillary permeability.  The edema is termed "cystoid" as it appears cystic; however, lacking an epithelial coating, it is not truly cystic