Introduction
Pathophysiology
- The main causative factor is arteriosclerosis
- This is when the arteries thicken.
- The arteries cross over veins, with adventitial sheath shared
- If the arteries harden and thicken, such as due to hypertension, the underlying vein is compressed
- Is known as BRVO if only a branch of the vein is occluded
- Is known as CRVO if occlusion occurs before the lamina cribrosa
- Typically presents with variable blurred vision to sudden vision loss that is usually painless
Second causative factor:
- Intraluminal thrombus
- This is caused by venous stasis (impaired blood flow), injury to the vessel endothelium, or blood hypercoagulability/stickiness
- If this is seen in younger patients, this can indicate systemic disease
Dead Giveaways
Acute:
Branch RVO
Due to arteriosclerosis and thrombosis
Blocked vessels increase intraluminal pressure, as blood trying to leave retina is blocked
Blood transudates into retina, increasing interstitial pressure, which exacerbates this condition.
Perpetuation of oedema leads to impediment of capillary perfusion
Note that it is primarily flame haemorrhages, due to the blood transudating taking on the shape of the RNFL. Blot/dot haemorrhages also present Cotton wool spots can also be present, following disruption of axoplasmic flow, and serves as a major sign of retinal ischaemia
OCT:
Cotton wool appearance
Central RVO
Characterised by a "blood and thunder" fundus, composed of venous tortuosity, dilation and retinal haemorrhaging
Cotton wool spots, disc and macular oedema may be present
The whole fundus is affected due to the central vein being occluded. Additionally, note how the optic disc has very indistinct margins The disc has increased hyperaemia and redness, especially nasally, making it indistinguishable from the retina
On the OCT:
Optic disc elevation shows oedema 
Macular oedema with cystic spaces. If the typical signs are not present, observe dilations and increased tortuosity of retinal veins by comparison with the fellow eye
The scattered haemorrhages, and tortuosity indicate impending CRVO and BRVO
Chronic VRO
Causes the pressure inside the capillaries to rise, and by Stalin's law and perfusion, will lead to leakage of protein and blood into retina.
Higher osmotic pressure exaggerates the oedema present
Presence of hard exudates imply present or past retinal oedema, due to increased vascular permeability and breakdown of BRB
Hard exudates can be see around the macular Another defining feature is the collateral vessels. Venous occlusions typically form collateral vessels to re-establish circulation. This in turn leads to sclerosis of vessels, and the production of ghost vessels.
Collateral vessels form between vessels, and leave the sclerotic vessel. No collateral vessels can mean poor visual prognosis 
The white vessels are sclerotic, and lack blood flow, and thus are referred to as ghost vessels 
Optociliary shunt vessel. A collateral vessel formed with the choroidal system
diagnostic features
Chronic RVO:
Neovascularisation may occur
The most common place for neovascularisation is the pupil margin, ciliary body, scleral spur and the trabecular meshwork
Rubeosis Iridis:
Reported to be as high as 50% within 2-4 months of ischaemic CRVO, but only 2% of BRVO
Also occurs in DR Can lead to 100 day glaucoma, which needs close monitoring
5% of CRVO and 25% of BRVO develops NVE
Can lead to tractional detachment, or rhegmatogenous detachment due to the formation of the fibrous component
10% of BRVO develops NVD
Prognosis is the least optimistic when chance of neovascularisation is very high. Difficult to distinguish between ischaemic and non-ischaemic
Typically, BRVO has better prognosis than CRVO
Ischaemic vs Non-ischaemic:
Visual acuity is typically better than 20/200, whilst ischaemic is worse
VF defects are rare, but are fairly common in ischaemic
Fundus appears less severe in non-ischaemic, with less areas of non-perfusion
Normal ERG, but in Ischaemic, there is a reduced b-wave amplitude for both photopic and scotopic ERG
Prognosis is good, and low chance of NV in anterior seg. Opposite for ischaemic