LAUSR

INTRODUCTION Retinal vein occlusion (RVO) is an obstruction of the retinal venous system by thrombus formation and may involve the central, hemicentral or branch retinal vein. The most common aetiological factor is compression by adjacent atherosclerotic retinal arteries. Other possible causes are external compression or disease of the vein wall e.g. vasculitis. Central retinal vein occlusion (CRVO) results from thrombosis of the central retinal vein when it passes through the lamina cribrosa [1, 2]. It is classically characterised by disc oedema, increased dilatation and tortuosity of all retinal veins, widespread deep and superficial retinal haemorrhages, cotton wool spots, retinal oedema and capillary non-perfusion in all four quadrants of the retina. A previous CRVO may show evidence of optic disc and retinal collaterals, a telangiectatic capillary bed and persistent venous dilation and tortuosity, perivenous sheathing, arteriolar narrowing and macular abnormalities (chronic macular oedema (MO) and retinal pigment epithelial changes). Branch retinal vein occlusion (BRVO) is caused by venous thrombosis at an arteriovenous crossing where an artery and vein share a common vascular sheath [3, 4]. It has similar features to CRVO except that they are confined to that portion of the fundus drained by the affected vein. Hemi-retinal vein occlusion affects either the superior or inferior retinal hemisphere, and the retinal haemorrhages are nearly equal in two altitudinal quadrants (the nasal and temporal aspects) of the involved hemisphere. The two main complications of RVO are MO and retinal ischaemia leading to iris and/or retinal neovascularisation. Thrombosis of the retinal veins causes an increase in retinal capillary pressure resulting in increased capillary permeability and leakage of fluid and blood into the retina. Co-existent retinal ischaemia (see below) may exacerbate this process by the production of vascular endothelial growth factor (VEGF) which in turn promotes retinal capillary permeability and leakage into the extracellular space resulting in further development of MO. MO is the most common cause of visual impairment in RVO, followed by foveal ischaemia. Varying degrees of retinal ischaemia due to non-perfusion of retinal capillaries may occur and principally depends on the degree of retinal vein thrombosis. These changes result in increased production of VEGF and other cytokines, which promote new vessel formation principally but not exclusively involving the iris and angle in CRVO and the retina in BRVO. These complications can lead to neovascular glaucoma, vitreous haemorrhage, and tractional retinal detachment with severe visual impairment. Both CRVO and BRVO can be broadly classified into ischaemic and non-ischaemic types based on the area of capillary nonperfusion, and this distinction is useful for clinical management. It is arguable if these are two separate entities or just ends of a spectrum. The Central Retinal Vein Occlusion study defined ischaemic CRVO as fluorescein angiographic evidence of more than ten disc areas (DA) of capillary non-perfusion on seven-field fundus fluorescein angiography [5, 6]. However, this definition of >10DA is not appropriate with widefield or ultra-widefield imaging given the larger area imaged and unclear clinical significance of ischaemia in the far periphery. Capillary non-perfusion >10DA in the posterior pole of eyes with CRVO irrespective of imaging modality would suggest a high risk of neovascularisation [7]. An ischaemic index (ratio of capillary non-perfusion/total area visible) of >45%, total area of nonperfusion >75DA on ultra-widefield angiography or >10DA of posterior pole nonperfusion has been found to correlate with neovascularisation [7, 8]. It is also important that a clear distinction is made between macular ischaemia and an ischaemic RVO (i.e. global retinal ischaemia). Furthermore, the definition of an ischaemic CRVO may not simply depend on angiography findings but also other parameters such as visual acuity, relative afferent pupillary defect and electrodiagnostic test findings.