The retina is the light-sensitive tissue lining the back of the eyeball. Because it’s constantly working, it requires a rich network of blood vessels to bring nutrients and oxygen to its tissues as well as carry away metabolic waste material. A disruption to this blood flow can have consequences ranging from being entirely asymptomatic to a potentially serious threat involving permanent damage and vision loss, depending on the affected blood vessels.
Blood Vessels of the Retina
Freshly oxygenated blood is carried into the eye via the central retinal artery, which splits into smaller branch arteries and then even smaller retinal capillaries.
Once the blood has delivered its oxygen to the retina, the deoxygenated blood is removed from the eye via the network of retinal veins.
From branch retinal veins, blood flows into the main retinal vein, known as the central retinal vein, and eventually back to the heart and lungs.
A blocked vein can occur in either a branch retinal vein (branch retinal vein occlusion, BRVO) or the central retinal vein (central retinal vein occlusion, CRVO).
Depending on where the retinal vein occlusion occurs, you may notice a sudden and profound loss of your central vision or, alternatively, no discernable change to your vision at all.
Central Retinal Vein Occlusions
In most cases, a central retinal vein occlusion occurs when blood flow is blocked in the central retinal vein due to a blood clot. A central retinal vein occlusion can occur around the same point where the optic nerve enters the eye or even at a point outside of the eyeball after the central retinal vein has exited the eye. The presence of blood clots prevents blood from flowing out through the retinal vein and backs up blood circulation through the entire network of retinal blood vessels. This results in bleeding into the retina and deprives areas of the retina of fresh oxygen.
Branch Retinal Vein Occlusions
Branch retinal vein occlusions are up to seven times more frequent compared to central retinal vein occlusions. Like a central retinal vein occlusion, a branch retinal vein occlusion is due to a blood clot resulting in poor blood flow through that particular retinal vein, resulting in haemorrhaging and poor oxygen supply. Most instances of a branch retinal vein occlusion occur at the point where the retinal vein crosses with a retinal artery.
How Is Vision Affected During a Retinal Vein Occlusion?
The main threat to the vision from a retinal vein occlusion, whether central or branch vein, is swelling of the macula, known as macular oedema, or the formation of abnormal blood vessels due to low oxygen in the retina. Vision loss from a retinal vascular occlusion is not associated with eye pain or discomfort. Instead, you’re more likely to experience a sudden painless blurring of your central vision. A retinal vascular occlusion does not result in complete blindness even in the event of macular oedema, as there will be other parts of your vision that remain intact.
Macular oedema is the most common cause of vision loss following retinal vessel occlusion. The macula is the part of the retina responsible for central vision, which is why damage and swelling of this area are significant. Macular oedema can lead to permanent central vision loss, even despite immediate or urgent treatment. Instances of macular oedema can develop even months after the original retinal vein occlusion event.
Retinal ischaemia refers to a lack of oxygen. The retina responds by creating new blood vessels, which are fragile and leaky. This, in turn, can contribute to macular oedema and result in poor vision. If these abnormal blood vessels form around the iris and fluid drainage channels of the eye, elevated eye pressure and a type of vision-threatening eye disease called neovascular glaucoma can become a risk.
The majority of retinal vein occlusions occur in just one eye. However, the risk of developing a retinal vein occlusion in the other eye will be elevated over subsequent years.
Risk Factors for Retinal Vein Occlusion
The reasons why some people develop blood clots in a retinal vein are not fully understood, though several risk factors for retinal vein occlusions have been identified. Having any of the risk factors doesn’t mean you’re guaranteed to have a retinal vascular occlusion, while not having any risk factors doesn’t mean you’re immune from it.
Risk factors for a retinal vein occlusion can include:
- High blood pressure
- Systemic conditions that affect blood flow, such as hardening of the arteries (atherosclerosis), blood clots elsewhere in the body, and heart disease
- High cholesterol
- Older age
Diagnosis of a Retinal Vein Occlusion
Both optometrists and ophthalmologists are able to diagnose a retinal vein occlusion, though only ophthalmologists (eye doctors) are qualified to treat it.
Your eye care professional will be able to diagnose a retinal vein occlusion by viewing the retina. This may require a dilated eye exam with the instillation of eye drops that widen the pupil for a better view. Retinal imaging with a specialised camera can also be useful for visualising the retina and any areas of retinal vascular occlusion and haemorrhaging.
Other imaging techniques, such as optical coherence tomography (OCT), are often used, especially for macular edema. This gives a better view of the macula and can be used to monitor the degree of swelling.
Your optometrist or ophthalmologist will also monitor your visual acuity, which is typically measured by reading black letters of decreasing size against a white chart. Other relevant tests can include checking for abnormal blood vessels in the drainage structure of the eye and measuring eye pressure.
How is Retinal Vein Occlusion Treated?
Treatment for macular oedema is with eye injections of a drug known as anti-VEGF therapy. This treatment will usually require monthly injections for at least a few months until the swelling has resolved. This medication can also be used for treating abnormal new blood vessels, reducing your risk of neovascular glaucoma.
In some cases, laser therapy can also be useful for sealing off the leakage from these new blood vessels.
Note: Any surgical or invasive procedure carries risks. Before proceeding, you should seek a second opinion from an appropriately qualified health practitioner.
Retinal Vein Occlusion.
Retinal vein occlusion.