Ischemia/infarction: if an arterial thrombus cannot be lysed by the body and it does not embolise, and if the thrombus is large enough to impair or occlude blood flow in the involved artery, then local ischemia or infarction will result. A venous thrombus may or may not be ischemic, since veins distribute deoxygenated blood that is less vital for cellular metabolism. Nevertheless, non-ischemic venous thrombosis may still be problematic, due to the swelling caused by blockage to venous drainage. In deep vein thrombosis this manifests as pain, redness, and swelling; in retinal vein occlusion this may result in macular oedema and visual acuity impairment, which if severe enough can lead to blindness.