What is retinal vein occlusion?
The retina is a thin tissue at the back of the eye, which works a bit like a film in a camera. The blood flow to the retina involves ‘arteries’ which bring blood from the heart and ‘veins’ which drain blood away to the heart. The word ‘macula’ refers to center of the retina. Being central, macula provides detailed sharp vision, especially near vision such as needed for reading, knitting etc.
Retinal vein occlusion occurs when one of these veins is blocked by a blood clot that prevents the blood draining from the retina. This causes blood and fluid to leak into the retinal tissue. The fluid leakage predominantly affects the macula.
Why does retinal vein occlusion occur?
The exact cause for this is not known. It is thought that the pressure from the retinal artery crossing over the vein may cause the clot to form. The common risk factors for developing retinal vein occlusion are high blood pressure, diabetes, high cholesterol, smoking and glaucoma. Less commonly, patients suffering from rare conditions affecting the blood (Myeloma) or inflammation of the blood vessels at the back of the eye (Behcet’s disease, Sarcoidosis etc) can develop RVO.
What are the types?
Depending upon where the blockage is, retinal vein occlusion is divided into two types.
1. Central Retinal Vein Occlusion (CRVO) - the blockage occurs in the main retinal vein causing damage to the entire retina (Figure: 1)
2. Branch Retinal Vein Occlusion (BRVO) – the blockage occurs in one of the branches of the main vein causing damage to the part of the retina in the area drained by that vein (Figure: 2)
Why is sight affected?
There are two reasons for this
1. Swelling of the macula (Macular Oedema): This is the most common cause. It means waterlogging of the macula caused by leakage of fluid. Since the macula is responsible for detailed sharp vision, macular oedema causes blurred vision.
2. New blood vessels growth (Neovascularisation): Occasionally fragile and leaky blood vessels can grow as a consequence of the vein occlusion. These abnormal blood vessels may bleed inside the eye, causing what is called a vitreous haemorrhage, or put the pressure up in the eye causing a condition called rubeotic glaucoma.
What tests are required?
Specific blood tests and blood pressure checks are carried out. In addition to this, specialized photographs of the retina are taken to help decide what treatment, if any, is needed.
1. Fundus fluorescein angiography – in this test a dye is injected in the arm and photographs are taken of the dye as it passes through the blood vessels at the back of your eye. The figure 3 shows delayed appearance of the dye in bottom half due to BRVO.
2. OCT retinal scan – It is a simple test which uses a light beam to build up a detailed picture of the retina. In retinal vein occlusion there is macular oedema as seen in figure 4.
What are the treatment options for macular oedema?
There is no treatment to unblock the vein. But treatment is aimed at resolving macular oedema to improve vision. The treatment options include
1. Observation: In a small proportion of patients, macular oedema can resolve spontaneously.
2. Laser treatment to macula: This involves applying laser burns to the macular region. Laser treatment only works in some cases of BRVO but not in CRVO. It is carried out as an outpatient procedure.
3. Injection of anti-vascular endothelial growth factor (anti-VEGF) drugs: The anti-VEGF drugs work on leaky blood vessels to help reduce fluid in the macula. Lucentis, Eylea and Avastin are the drugs available in this category. Injections are given in an outpatient setting under local anaesthesia. One injection is given every month for the first three months. Thereafter monthly assessments continue during which further injections may be needed.
4. Injection of Ozurdex: Ozurdex is a very small steroid implant, which is injected in the eye under local anaesthesia. The effect of treatment lasts 3-4 months and it can be repeated thereafter.
Which treatment is most effective?
In research trials, anti-VEGF injections have shown to be the most effective in improving sight, followed by Ozurdex implant and then laser. A combination of treatments may be more effective and Mr Patwardhan will discuss this with you at the time of initial consultation if applicable.
In BRVO the success rate of improving sight is approximately 60% while in CRVO the success rate is approximately 45%.
What is the treatment for new blood vessels growth (Neovascularisation)?
If new vessels (neovascularization) have developed, the treatment is aimed at limiting damage but does not improve vision. This is achieved by extensive laser treatment to the peripheral retina to prevent or treat vitreous haemorrhage or rubeotic glaucoma.
What are the risks of treatment?
Generally speaking benefits far outweigh the risks. The injection carries a small risk (0.3%) of sight threatening infection in the eye (endophthalmitis). With Ozurdex treatment the main risks are development of glaucoma (increased pressure in the eye) and cataract (cloudy lens) in approximately 25% cases. With anti-VEGF injections there is a small risk (1-2%) of cardiovascular events such as heart attack and stroke.
How long is the treatment required?
The treatment is carried on for up to 2 years. After that further treatment will only continue if it is deemed beneficial.
Why does the treatment not work in everyone?
The most common reason is structural damage to the retina caused by the vein occlusion itself. Sometimes the treatment does not improve the macular swelling and sometimes the vision gets worse because of a worsening of the vein occlusion.