DME, the third form of diabetic retinopathy, is characterized by swelling of the central part of the retina that mediates high-resolution vision. DME frequently coexists with and is superimposed upon NPDR or PDR. When the area of swelling is located more than 1 disc diameter (approximately 1500 mm) away from the center of the fovea, the swelling constitutes a low threat to visual acuity and is regarded as non-clinically significant macular edema (Early Treatment Diabetic Retinopathy Study [ETDRS] Research Group 1985). Vision deteriorates when DME involves the foveal center. DME that either directly involves the fovea or is at high risk of doing so is referred to as Clinically Significant Macular Edema (CSME) (ETDRS Research Group 1985).