PROS1 and hyperhomocysteinemia: In addition, compression of the vein at the arteriovenous (A/V) crossing, degenerative changes of the vessel wall and plasma abnormalities, including resistance to activated protein C, deficiencies of protein C, protein S, and antithrombin III, prothrombin gene mutation, anti-phospholipid antibodies and hyperhomocysteinemia may be the other risk factors related to RVO [23].