Most cases of vein occlusion are due to high blood pressure and atherosclerosis. Other less common causes include inflammatory and infectious diseases such as lupus, sarcoidosis, syphilis; blood thickening diseases such as sickle cell disease, polycythema, lymphoma, etc. Rarely, this has also been linked to oral contraceptives and diuretic drugs.
In the eye, the arteries and veins cross over each other. If the artery has high pressure and is hardened it can compress the veins which are generally less rigid. Like stepping on a garden hose, this creates back pressure and then leakage of blood and fluid into the retina from the veins.
Observation is usually the treatment. If there is no improvement in vision after 3-6 months then laser treatment is often used to try to help dry up the fluid. Controlling the cause, such as high blood pressure, is also essential.
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Branch retinal vein occlusion 
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