Macular edema: definition and basic concepts.

Macular edema is the result of an accumulation of fluid in the retinal layers around the fovea. It contributes to vision loss by altering the functional cell relationship in the retina and promoting an inflammatory reparative response. Macular edema may be intracellular or extracellular. Intracellular accumulation of fluid, also called cytotoxic edema, is an alteration of the cellular ionic distribution. Extracellular accumulation of fluid, which is more frequent and clinically more relevant, is directly associated with an alteration of the blood-retinal barrier (BRB). The following parameters are relevant for clinical evaluation of macular edema: extent of the macular edema (i.e., the area that shows increased retinal thickness); distribution of the edema in the macular area (i.e., focal versus diffuse macular edema); central foveal involvement (central area 500 microm); fluorescein leakage (evidence of alteration of the BRB or 'open barrier') and intraretinal cysts; signs of ischemia (broken perifoveolar capillary arcade and/or areas of capillary closure); presence or absence of vitreous traction; increase in retinal thickness and cysts in the retina (inner or outer), and chronicity of the edema (i.e., time elapsed since initial diagnosis and response to therapy). It is essential to establish associations and correlations of all the different images obtained, regardless of whether the same or different modalities are used.