What is a retinal vein occlusion?
The retina is the thin neural membrane that lines the inner surface of the back of eye. Developmentally, the retina is an outpouching of the brain and its function can be considered analogous to that of the film in a camera. It has a blood supply consisting of branches of what is called the central retinal artery which enters via the optic nerve at the back of the eye and is drained by a network of branch retinal veins which join together to form a central retinal vein which leaves the eye also via the optic nerve.
Occlusions of either the branches of the retinal vein or the central retinal vein itself are relatively common causes of visual problems.
In the case of a retinal vein occlusion, a blockage forms in the vein, usually due to a blood clot, and obstructs the blood flow. The backpressure causes bleeding into the retina, swelling of the retina from leakage of other fluids into the retina, and also a lack of oxygen delivery which can permanently devitalise the neural tissue. The condition is uncommon under the age of 60 but becomes more frequent in later life.
There are two main types of retinal vein occlusion;
- Branch retinal vein occlusions – which are due to blockage of one of the four main branches of the retinal vein, each of which drains about a quarter of the retina. Smaller branches of the four veins can also be obstructed which are termed tributary branch vein occlusions.
- Central retinal vein occlusions occur when the main retinal vein becomes obstructed at the point it leaves the eye. This type of obstruction affects the drainage of blood from the whole retina.
In general, visual loss is more severe when the central retinal vein is blocked.
What causes retinal vein occlusion?
The exact cause of retinal vein occlusions is unknown, but there are several risk factors which make the condition more likely to occur. These include;
- High blood pressure
- High cholesterol
- Certain rare blood disorders.It is important to identify and treat underlying risk factors to prevent further vein occlusions in the affected eye and also to reduce the risk to the other eye. In a small number of cases, no risk factors will be identified, and the cause will remain unknown.
Treatment of retinal vein occlusions
Not everybody who develops a retinal vein occlusion needs treatment, but in approximately 2/3 of cases treatment will be recommended to prevent vision loss. There are two treatable reasons why patients with retinal vein occlusions lose vision;
- The development of swelling and haemorrhage in the central retina or macula.
- The development of abnormal vessels at the back of the eye on the surface of the retina and optic nerve, or at the front of the eye involving the iris and the drainage mechanism which controls the pressure in the eye. The process of new vessel development is termed ‘neovascularisation’. Patients with retinal vein occlusions are checked frequently at the onset of the condition to check for the development of these two complications. Vision loss resulting from the loss of oxygen delivery – so called ‘ischaemic damage’ – cannot be reversed.
Swelling at the centre of the retina (the macula) is the most common cause of vision loss in retinal vein occlusion. The swelling is caused by damage to the blood vessels and increased pressure in the veins and capillary vessels which then leak fluid.
Different medicines such as anti- vascular endothelial growth factor (anti-VEGF) medicines or steroids may be helpful in reducing this leakage. These medicines are given by intravitreal injection into the eye and the injections often need to be repeated when the effect of the drugs wear off. Laser treatment is sometimes helpful in restoring central vision in branch retinal vein occlusions.
About 20% of patients with a retinal vein occlusion develop abnormal blood vessels which can grow on the iris at the front of the eye or at the back of the eye on the surface of the retina or the optic nerve.
These abnormal blood vessels can bleed or cause a pressure rise in the eye which can cause pain and lead to further loss of vision. New vessel formation can normally be prevented by a combination of intravitreal injections or laser treatment to the retina.
What can I expect from treatment of a retinal vein occlusion?
Most patients with retinal vein occlusions do well, with over 75% of patients maintaining good vision following appropriate treatment. However about 50% of patients require regular intravitreal injections to maintain this improvement and a proportion of affected people will unfortunately experience a permanent reduction in their eyesight despite proper management.