Retinal Vein Occlusion
Retinal vein occlusions manifest in four forms: central retinal vein occlusion (CRVO), hemi–central retinal vein occlusion (HRVO), branch retinal vein occlusion (BRVO) and papillophlebitis.
CRVO and HRVO develop in similar ways: The central retinal vein compresses while passing through the lamina cribrosa, blocking drainage from the retina. The cause of this blockage is thought to be thrombosis due to compression from a sclerotic central retinal artery, altered blood flow or a combination of these factors. BRVO develops when a sclerotic retinal arteriole compresses a thin–walled venule, blocking drainage. 12 Papillophlebitis is essentially a CRVO in a young, healthy adult, the difference is that papillophlebitis is thought to develop from an inflammatory process. 13, 14.
CRVO appears as dilated, tortuous veins in all four retinal quadrants, deep retinal dot and blot hemorrhages, superficial NFL hemorrhages, and macular and disc edema. There may be neovascularization of the disc, retina or iris. Retinal exudates and collateral vessels on the optic disc are also common. 15, 16 Papillophlebitis and HRVO share this appearance, though the latter involves only half the retina. 13–17 BRVO affects only one quadrant, 15, 16 and the hemorrhaging tends to be triangular with its apex at an A–V crossing.
In these vein occlusions the major vessels themselves do not leak. Rather, the leakage comes from the thin–walled retinal capillaries draining into the major vessels. A profound occlusion can destroy the integrity of the retinal capillaries. The subsequent ischemia puts the patient at risk for neovascularization. In ischemic CRVO, patients are likely to develop iris neovascularization and neovascular glaucoma. 18 In ischemic HRVO and BRVO, patients tend to manifest neovascularization on the optic disc and retina, this raises the risk of vitreal hemorrhage and tractional retinal detachment. 19, 20 Papillophlebitis rarely develops any noticeable ischemia. In rare case where there is marked ischemia, the condition behaves much like ischemic CRVO. 13, 14.
Macular edema is the most common cause of vision reduction in retinal vein occlusions. In BRVO or HRVO, macular edema is amenable to argon laser photocoagulation, but in many cases it resolves without treatment. Clinicians usually wait at least three months before taking this step. If the edema persists beyond 18 months, however, it will permanently disrupt the RPE with irreversible vision loss, and photocoagulation will bring no benefit. 21, 22.
As for macular edema in CRVO, doctors used to treat it with argon laser photocoagulation. Research shows this treatment doesn’t improve vision in these patients. 23