**3. Natural course and associated symptoms**

ERMs tend to remain stable or to present limited progression over time, with most patients experiencing mild or no symptoms following the initial diagnosis, indicating that membrane contraction possibly occurs at an early phase after its original formation and generally stabilizes thereafter. According to the findings of the population based "Blue Mountains Eye Study", epiretinal membrane progression was encountered in 28.6% of cases, stability was noticed in 38.8% of eyes, while 25.7% of cases regressed during a five-year follow-up period [35]. Accordingly, former studies have supported the non-progressive character of the disease [36-37], since only 10 to 25% of eyes seemed to show a significant decrease in visual acuity over time, with variable rates of progression [2, 9]. In fact, previous reports suggest that less than 5% of ERM cases present visual acuity of 20/200 or worse [38-39]. Moreover, rare cases of spontaneous ERM separation from the retina with associated visual improvement have also been documented [9, 40].

ERMs are symptomatic only if the macular or peri-macular area is involved. In its mildest forms, such as in cases of cellophane maculopathy, the disease is usually asymptomatic [38-39]. Symptomatic patients usually complain of decreased visual acuity (VA), metamorphopsia or vague visual disturbances. Other less common symptoms include micropsia and monocular diplopia [38-39]. In general terms, the extent of the visual effect of the disease is determined mainly by the degree of the induced retinal distortion, the position of the membrane in association to the macula, as well as its thickness and transparency. The decrease in VA can generally be attributed to the filtering effect of the ERM that prevents light from reaching the photoreceptors, the distortion of the retinal surface, as well as the macular oedema and the associated vitreoretinal traction due to incomplete PVD, if present. The distortion of the retinal surface due to ERM contraction, which in some cases can involve the entire retinal thickness, is the primary cause for metamorphopsia, which is usually the leading and most disturbing symptom of the disease.
