Normally, when you look at an object, light from the object passes through the cornea of your eye, then the lens, and then it hits the retina at the back of your eye. The cornea and the lens both help to focus the light on to your retina.
Nerve messages pass from the cells in your retina, down nerve fibres in your optic nerve to your brain. The messages are interpreted by your brain, which enables you to see. If the retina is damaged, a clear picture cannot be produced.
The macula is a small area of the retina at the back of your eye and it is the part of the retina that is the most densely packed with seeing cells. The macula is responsible for your central (focused) vision. The rest of the retina is responsible for your edge (peripheral) vision.
The retina is a thin, light-sensitive membrane that lines the back of your eye. It is delicate, is made of several layers, and needs a good supply of oxygen. Its rich blood supply is drained away by four retinal veins, which feed into one central retinal vein which drains out of the eye.
What happens in retinal vein occlusion?
Retinal vein occlusion occurs when one of the four retinal veins, or the central vein, becomes blocked by a blood clot. This means that blood cannot drain away from the retina as easily. The blood 'backs up' behind the blockage, and fluid and blood leak from the blood vessels into the delicate tissue of the retina, forcing its layers apart and affecting its ability to respond to light.
If this bruising and swelling occur at the centre of the retina (the macula) then central vision can be affected. This is more likely to occur in central retinal vein occlusion, as drainage of blood from the macula is usually shared by all four retinal branch veins. If only one of the four branch veins is blocked, the other three will still drain blood away from the macula.
There are two main types of retinal vein occlusion:
- Branch retinal vein occlusion - the blockage occurs somewhere along the course of one of the four retinal veins. (One retinal vein drains each quarter of the eye.)
- Central retinal vein occlusion - the blockage occurs in the main vein formed by the four retinal veins coming together.
Branch retinal vein occlusion is about three times more common than central retinal vein occlusion.
What causes retinal vein occlusion?
Retinal vein occlusion usually occurs when one of the following occurs:
- A retinal vein is 'pinched off' through the pressure of an artery lying on top of the vein.
- A retinal vein is blocked with a blood clot or fragment of fatty deposit (atherosclerotic plaque) in the wall of the artery.
- More rarely, the vein can be blocked by some inflammatory conditions.
Some things increase your risk of developing retinal vein occlusion. They include:
Risk factors for cardiovascular disease
Cardiovascular diseases are diseases of the heart or blood vessels caused by atheroma. Patches of atheroma are like small fatty lumps that develop within the inside lining of arteries. Atheroma is also known as 'hardening' of the arteries (atherosclerosis). A patch of atheroma makes an artery narrower.
Your risk of developing retinal vein occlusion is increased if you have risk factors for cardiovascular disease. These include:
- High blood pressure: about 7 in every 10 people with retinal vein occlusion have high blood pressure. If high blood pressure is not well controlled, it can also increase your risk of having another episode of retinal vein occlusion (either in the same eye or the other eye). High blood pressure tends to harden the retinal arteries, making them more likely to push against the veins.
- Atherosclerosis: the retinal arteries and veins run very closely together. If the retinal arteries are narrowed due to atherosclerosis, they can become stiff and rigid. It is thought they then press on nearby veins and disturb the blood flow in them. This means that a clot is more likely to form in the vein, leading to retinal vein occlusion.
- High cholesterol/high lipid levels: these increase the fatty deposits inside retinal arteries - these can break off and lodge in the veins.
- Diabetes (this affects the blood vessels in the eye in a variety of ways - see separate leaflet called Diabetic Retinopathy).
- Smoking: this increases the blood's clotting tendency, in addition to worsening the 'hardening' and narrowing of arteries.
Other risk factors
- Having a blood clotting problem such as antiphospholipid antibody syndrome: this can increase your risk of developing retinal vein occlusion.
- Conditions which 'thicken' the blood: these increase the tendency of the blood in the vein to 'sludge' and slow (particularly where there are narrowings).
- Glaucoma: this increases pressure in the eye, which has the effect of slowing the blood flow through the veins.
The combination of high blood pressure, high lipid levels and diabetes is called metabolic syndrome and is a particularly strong risk factor for retinal vein occlusion.
If you are a smoker, giving up smoking is one of the most beneficial things that you can do to reduce your risk.
Further reading and references
Retinal Vein Occlusion (RVO) Guidelines; Royal College of Ophthalmologists (July 2015)
Aflibercept for treating visual impairment caused by macular oedema after branch retinal vein occlusion; NICE Technology Appraisal Guidance, September 2016
Dexamethasone intravitreal implant for the treatment of macular oedema caused by retinal vein occlusion; NICE Technology Appraisal Guidance, July 2011
Anti-VEGF for Retinal Vein Occlusion: Patient Information; Moorfields Eye Hospital
Patel A, Nguyen C, Lu S; Central Retinal Vein Occlusion: A Review of Current Evidence-based Treatment Options. Middle East Afr J Ophthalmol. 2016 Jan-Mar23(1):44-8. doi: 10.4103/0974-9233.173132.
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