The light-sensitive tissue at the back of your eye is called the retina. Retinal arteries provide the retina with nutrients, whilst the retinal veins take the blood away from the retina. The smaller 'branch' retinal veins all converge at the optic nerve, to form one large 'central' retinal vein.
Uncommonly, these veins can become blocked (or occluded), resulting in a reduction in blood flow, retinal haemorrhage and oedema of the retina. In the worst cases, abnormal blood vessels can grow, causing significant scarring and severe glaucoma.
A Central Retinal Vein Occlusion (CRVO) occurs when the central vein is blocked, whereas a Branch Retinal Vein Occlusion (BRVO) occurs when a smaller branch vein is occluded.