Unilateral diabetic macular oedema secondary to central retinal vein congestion

. Purpose:  To describe two cases of strictly unilateral diabetic retinopathy with macular edema where the precipitating factor appears to have been retinal venous congestion.Methods:  Retrospective interventional case study.Results:  Examination of fundus photographic records demonstrated generalized venous dilation in the affected eyes years in advance of the development of unilateral diabetic macular edema. Relentless progression of edema and visual loss followed despite retinal photocoagulation treatment and ultimately the affected eyes in both patients developed central retinal vein occlusion. The fellow eyes in both patients remained without retinopathy throughout the period of observation.Conclusion:  The sequential and strictly unilateral appearance of retinal venous dilation, diabetic macular edema, and central retinal vein occlusion in diabetic patients without fellow‐eye retinopathy suggests that venous congestion induced the progression from hyperglycemic insult to vascular injury in the affected eyes. Presumably, diabetes may cause retinal vascular dysfunction that results in little or no vascular damage unless venous congestion is present. This supports that venous congestion and increased sensitivity to congestion are important components of the pathogenesis of diabetic macular edema.