Blockage of a retinal artery by a thrombus (blood clot) typically affects only one eye, causing sudden blindness or loss of part of the visual field.
It requires urgent treatment, which may include eye massage, draining fluid from the eye, or medication; even with treatment, loss of vision is usually permanent.
Blockage of a retinal vein also usually affects only one eye and tends to cause deterioration of vision over several days, although sometimes visual loss may occur suddenly.
Treatment may involve medication injected into the eye or laser surgery.
Even with treatment, there is likely to be some permanent loss of vision.