The retina is a transparent layer at the back of the eye that receives visual information and converts it to electrical impulses which is transmitted to the brain via the optic nerve. The retina is analogous to the film at the back of a traditional film-based camera. The blood supply of the retina is supplied predominantly by the central retinal artery while the blood flows out via the central retinal vein. Both these blood vessels travel along the optic nerve which is located at the back of the eye.

Retinal vein occlusion (RVO) is a common cause of painless visual loss that occurs suddenly and can lead to severe loss of vision. It occurs in about 0.5% of the population, so in Malaysia, it is estimated that more than 100000 people have this problem. It is more common in patients above the age of 50 years. There are two main types of RVO – central retinal vein occlusion (CRVO) and branch retinal vein occlusion (BRVO). BRVO occurs twice as commonly compared with CRVO.  Visual loss occurs due to swelling in the macula (macula oedema) which happens in both these types of RVO. The macula is the most important part of the retina as it detects very fine detail and colour and any damage to it will cause reduction in our vision.

Figure 1: Normal colour picture of the back of the left eye. The optic nerve transmits visual information from the retina to the brain. The macula is the most sensitive part of the retina and detects colour and fine details. The retinal veins drain blood out from the retina via the optic nerve.

Figure 2: Central retinal vein occlusion. There is extensive bleeding throughout the back of the eye due to blockage of the central retinal vein at the optic nerve. The patient’s vision is severely affected due to macula oedema.
Figure 3: Branch retinal vein occlusion. There is a blockage of one of the retinal veins in the lower half of the retina leading to bleeding in the lower part of the retina. There is macula oedema present but the patients vision is not as badly affected compared to that in CRVO.

What causes retinal vein occlusion?

RVO occurs due to blockage of the vein. This usually happens when the retinal veins are compressed by the retinal artery next to it. Like all arteries in our body, the retinal artery naturally thickens with age especially if we have diseases like hypertension and high cholesterol levels in our blood. This thickening of the retinal artery causes narrowing of the retinal vein which is usually located next to the artery. There is reduction of blood flow in the retinal vein leading to blood clot formation and finally total blockage of the vein.

When the retinal vein is blocked, the pressure within the vein increases causing leakage of blood and fluid out into the retina. The retina also becomes starved of oxygen (ischaemic) and there is an increase in the levels of  a  growth factor called vascular endothelial growth factor (VEGF). VEGF causes further leakage of fluid from the blood vessels and macula oedema. Sometimes, there is growth of abnormal new blood vessels on the surface of the retina. These blood vessels are fragile and easily bleed into the eye. These blood vessels can also grown in the front of the eye on the pupil and can bleed and cause raised eye pressure. This can be very painful and difficult to treat

What are the risk factors for retinal vein occlusion?

There are several major risk factors for developing RVO. These are: high blood pressure, high levels of serum cholesterol, diabetes, and glaucoma. For patients younger than 50 years old,  blood clotting diseases like high serum homocysteine and antiphospholipid syndrome, increase the risk of RVO.

What is the prognosis for retinal vein occlusion?

Patients who develop CRVO tend to do badly without any treatment. Only 20% of patients have an improvement of vision without treatment. Most patients (80%) lose significant amounts of their vision with this condition and a large percentage of untreated eyes can also develop glaucoma and bleeding in the eye.

BRVO has a better prognosis than CRVO as the patient’s vision is usually not as badly affected. If there is macula oedema present, the visual acuity does improve with treatment. Less than 1% of patients will develop bleeding in the eye in BRVO.

Why is retinal vein occlusion important?

In a few large population studies like the Beijing Eye Study, RVO was associated with a significantly increased risk of death from cardiovascular disease. Men who developed RVO had a 2 times increased risk of developing stroke. As such, when a patient develops RVO, it is often a sign of significant cardiovascular disease like high blood pressure and they are at higher risk of developing heart attacks or stroke in the future.

In part 2 of this article, the author will discuss the latest preventive and treatment options for retinal vein occlusion.