Branch retinal vein occlusion (BRVO) and central retinal vein occlusion (CRVO) occur when veins in the retina become blocked, usually due to high blood pressure. This causes bleeding and leakage from retinal capillaries, reducing vision through macular edema, hemorrhage, or ischemia. Treatment aims to reduce edema and prevent further vision loss through injections, laser therapy, or vitrectomy surgery.

1. Retinal Vein Occlusion
Branch retinal vein occlusion (BRVO) and central retinal vein
occlusion (CRVO) are both blockages in the veins of the retina. The
blockage in both is thought to be caused by pressure on the vein from
outside. These are NOT ‘like a stroke in the eye’ and are not generally
caused by problems with blood clotting. The most common cause of
these is high blood pressure.

2. Retinal Vein Occlusion
Branch retinal vein occlusion
When a retinal artery lies on top of a retinal vein, any ‘hardening’ of the artery
from high blood pressure can compress the vein. This can suddenly stop the blood
flowing through the vein, leading to a pressure build-up in the smaller veins and
capillaries that drain through it.
The damaged capillaries bleed into the retina at the time of the occlusion
(blockage) and can start to leak, which may continue permanently. If the capillaries
have been so badly damaged that no blood flows through them, this is known as
an ischaemic retinal vein occlusion; if not, then it is known as non-ischaemic.
Both can have a variable immediate effect on vision that can change with time, but
ischaemic vein occlusions can lead to later problems. Only a part of the field of
vision is usually affected, but this may well include the macula and cause central
visual loss.

3. Retinal Vein Occlusion
Central retinal vein occlusion
In central retinal vein occlusion, the site of blockage cannot be seen, as
it does not occur in the retina but within the optic nerve instead.
Bleeding and leakage of retinal capillaries is similar to that in BRVO but
involves the whole retina, causing the entire field of vision to be
affected.
CRVOs can also be non-ischaemic, but it is ischaemic CRVOs that are a
potentially serious threat to the eye and its vision. Without treatment,
ischemic CRVOs can lead to significant sight loss and very painful eyes.

4. Retinal Vein Occlusion
What causes retinal vein occlusion?
Branch retinal vein occlusions usually occur in people over 50 years old and
are more common in people with high blood pressure (hypertension).
If there is not already a history of hypertension, it is definitely worth being
tested thoroughly for this, which may well include a 24 hour blood pressure
monitor. In many patients, no obvious cause is found.
Central retinal vein occlusions also occur in the same age group and are
associated with high blood pressure, smoking, and glaucoma.

5. Retinal Vein Occlusion
How is vision affected?
Patients with both branch and central retinal vein occlusions usually
describe a sudden painless blurring of vision in one eye that may affect
near and distance vision. Sometimes, this is not noticed until the
unaffected eye is covered by chance. This blurring does not usually
change very much over the first few weeks. However, it is very
important to be checked urgently, as these symptoms are very similar
to those of other conditions that may need immediate or urgent
treatment.

6. Retinal Vein Occlusion
How does retinal vein occlusion reduce vision?
There are three ways in which retinal vein occlusions cause reduced vision:
• Macular Oedema
When a vein occlusion occurs and there is an increase in the pressure within
the vein, there can be either temporary (weeks to months) or permanent
damage to the capillaries that makes them leaky. As a result, the macula
becomes waterlogged and swollen. This is called macular oedema and can
be shown both on OCT scanning and fluorescein angiography (FA). OCT
scanning is better for monitoring the amount of macular oedema, as
measurements can be made of the thickness of the macula as well as its
volume and changes in both over time. FA is better at demonstrating which
capillaries are leaking and may need treatment.

7. Retinal Vein Occlusion
How does retinal vein occlusion reduce vision?
• Haemorrhage
When a retinal vein occlusion occurs, the sudden rise in pressure in the capillaries can
cause blood to escape from the capillaries - bleeding. The blood, which is in patches within
the retina, blocks light getting to the photoreceptors (light-sensing cells), resulting in vision
being blocked in patches. If this involves the fovea, it can cause severe visual loss.
This bleeding may well be a one-off event and the blood may clear over weeks to months.
In many, however, persistent damage to the capillaries means that blood may continue to
leech out and the vision can be affected for much longer.
• Ischaemia
If the damage to the retinal capillaries at the time of the vein occlusion is severe, they may
become permanently damaged and blocked. This is called ischaemia, and if it occurs, the
area of retina in which this occurs will never work normally again. When the fovea or
macula becomes ischaemic, central vision can be permanently lost.

8. Retinal Vein Occlusion
How is retinal vein occlusion treated?
There are several types of treatment used in retinal vein occlusions.
These include:
• Intravitreal injection
• Laser photocoagulation
• Vitrectomy surgery
The choice and timing of treatment is dependent on many factors as
well as the results of tests.