Etiology
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 etiology 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 intraocular 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, intraretinal hemorrhage, and edema in the affected region of the retina. Furthermore, as a result of ischemia, local elevation of growth factors such as vascular endothelial growth factor (VEGF) may cause macular edema and neovascularization.[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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