Retinal vein occlusions happen
when a blockage occurs either in the main vein of the
retina or a smaller branch point in the retinal venous
circulation. Since the main function of retinal veins is
to carry blood away from the eye and back to the heart
(retinal arteries bring blood into the eye from the
heart), blood backs up in the retinal circulation,
causing congestion forcing blood out of the vessels and
into the retina. This causes bleeding and swelling in
the retina, resulting in blurry vision and retinal
damage. Vein occlusions tend to be associated with
underlying high blood pressure, diabetes and glaucoma.
They can also occur, however, in the absence of any
underlying systemic disease.