Retinal vein occlusion. Ophthalmoscope image of the retina of a patient suffering from occlusion of a branch vein in the retina. The occlusion occurs as the circulation of blood is obstructed, usually by an adjacent blood vessel. This leads to haemorrhages (red streaks) and a lack of blood flow to part of the retina. In a branch vein occlusion, one of the smaller veins supplying the retina is affected, and the symptoms include sudden blurring of part of the visual field, or the development of a large blind spot. This may resolve over time as the haemorrhage dissipates. Risk factors include hypertension, diabetes and cardiovascular disease.