The retina is the thin, light detecting layer at the back of the eye. Blood flows into the eye from the central retinal artery and out of the eye through the central retinal vein. A blockage to the retinal vein prevents blood from leaving the eye causing it to leak onto the retina. A blockage to a vein can occur because of a hardening of the arteries which causes compression on a vein. Several factors are related to retinal vein occlusion. These include: the presence of raised blood pressure and high cholesterol; Glaucoma and Diabetes, and certain rare blood disorders and smoking. If a blockage occurs at one of the branches of the retinal vein, it results in a bleed in that area of the retina. However, if it occurs in the main vein, it is called a central retinal vein occlusion which generally results in more bleeding over the retina. Many people see a dark patch in their vision that moves with the eye. The severity of vision loss depends on the extent of the bleeding. A retinal vein occlusion may not require any treatment as the blood absorbs. If the bleed is located at or near the centre of vision, then it may cause a swelling in this area and permanent changes to vision. Intravitreal injections can help improve central vision and stabilise any swelling. Another complication is the growth of abnormal new blood vessels. These occur in about 20% of patients with retinal vein occlusion. The growth of these vessels can be stabilized with retinal laser treatment.