Aetiology
Central retinal vein occlusions (CRVOs) are caused by blockages of venous drainage in the region of the lamina cribrosa.[1] Specifically, thrombus formation in the lumen of the central retinal vein interrupts blood flow.[11]
Branch retinal vein occlusions (BRVOs) most commonly occur at arteriovenous crossings, where an adventitial sheath is shared.[1][4][12][13][14] Arterial disease is therefore a common aetiology for BRVO.
The lumen of the central retinal vein or a branch retinal vein may undergo compression from an adjacent atherosclerotic central retinal artery or a branch retinal artery at an arteriovenous crossing or, less frequently, from vascular inflammatory disease. In the case of a CRVO, the central retinal vein may be compressed by impingement from the posterior bowing of the lamina cribrosa secondary to high intra-ocular pressure. Thus, controlling systemic hypertension and ocular hypertension may prevent a CRVO from occurring in the fellow eye.
Pathophysiology
In both CRVO and BRVO, increased venous pressure can cause venous tortuosity, intra-retinal haemorrhage, and oedema in the affected region of the retina. Furthermore, as a result of ischaemia, local elevation of growth factors such as vascular endothelial growth factor (VEGF) may cause macular oedema and neovascularisation.[15]
Classification
Location of occlusion
CRVO: blockage of central retinal vein in the region of the lamina cribrosa; affects the entire retina (4 quadrants).
BRVO: blockage of a branch of the central retinal vein; affects one quadrant of retinal tissue.
Hemiretinal vein occlusion (HRVO): blockage of a branch of the central retinal vein; affects superior or inferior half of retinal tissue (2 quadrants).
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