This sounds very much like a right central retinal vein occlusion. The most common cause is a blood clot, which prevents the flow of blood along the vein. Other causes include disease of the vein wall and external compression of the vein by a nearby artery. Retinal arteries and arterioles and their corresponding veins share a common space called the adventitial sheath. Thickening of an artery or arteriole compresses the vein, eventually causing occlusion and preventing the flow of blood out of the eye. Blood comes in along the central retinal artery but the blood cannot get out.
Blood backs up in the venous system and stagnates. New blood can’t get into the eye and the lack of fresh oxygen leads to hypoxia results in. The components of blood leak out into the retina so haemorrhages and white exudates are visible. Ischaemic damage to the retina stimulates increased production of vascular endothelial growth factor (VEGF) which, in turn, may lead to the growth of new and fragile blood vessels, which rupture leading to more blood in the retina. Neovascular glaucoma is also a possibility. Macular oedema causes a large reduction in visual acuity and this may lead to a macular hole. Retinal detachment may follow.
Your patient deserves immediate same-day referral to an on call ophthalmologist.