CRVO has two types:
• Nonischemic (70%): which is characterized by vision that is better than 20/200, 16% progress to nonperfused; 50% resolve completely without treatment; defined as <10 disk diameter (DD) of capillary nonperfusion.
• Ischemic (30%): which is defined as more than 10 DD of nonperfusion; patients are usually older and have worse vision; 60% develop iris NV; up to 33% develop neovascular glaucoma; 10% are combined with branch retinal arterial occlusion (usually cilioretinal artery due to low perfusion pressure of choroidal system) [7].
Central retinal vein occlusion is a disease of the old population (age >50 years old). Major risk factors are hypertension, diabetes, and atherosclerosis. Other risk factors are glaucoma, syphilis, sarcoidosis, vasculitis, increased intraorbital or intraocular pressure, hyphema, hyperviscosity syndromes (multiple myeloma, Waldenstrom's macroglobulinemia, and leukemia), high homocysteine levels, sickle cell, and HIV [8].
Paul O'Mahoney et al. studied the relationship between traditional atherosclerosis risk factors and retinal vein occlusion (RVO). They systematically retrieved all studies between 1985 and 2007 that compared cases with any RVO with controls. They concluded that hypertension and hyperlipidemia are common risk factors for RVO in adults, and diabetes mellitus is less so. It remains to be determined whether lowering blood pressure and/or serum lipids levels can improve visual acuity or the complications of RVO [9].
Open-angle glaucoma is the most common local factor predisposing to RVO as increased intraocular pressure compromises retinal vein outflow and produces stasis [10, 11].
Every eye with CRVO is at risk for developing neovascular glaucoma. Lowering intraocular pressure helps to improve retinal circulation in an eye with CRVO [12], and there is a 10% risk for development of BRVO or CRVO in the fellow eye [13].
Risk factors for developing neovascular iris in patients with CRVO are the amount of nonperfused retina, extent of retinal hemorrhages, male sex, and central vein occlusion of less than one month duration [14]. Visual acuity in patients with CRVO at baseline is a strong predictor for the development of INV/ANV, as is the amount of nonperfusion seen by fluorescein angiogram [1].
The central vein occlusion study (CVOS) data did not support the recommendation for prophylactic panretinal photocoagulation (PRP). The CVOS found that early PRP decreased the rate of iris neovascularization (INV); however, the reduction was not statistically significant. Moreover, the study showed that early PRP reduced, but did not eliminate, the possibility of anterior-segment neovascularization. The CVOS recommended close follow-up of eyes with CRVO during the first 6 months (including gonioscopy and undilated slit lamp examination of the iris) and prompt PRP of eyes in which iris neovascularization (INV)/Angle neovascularization (ANV) develops [14].
Although PRP was better than selective PRP or photodynamic therapy (PDT) at determining INV and anterior segment neovascularization regression, selective PRP or PDT can also be safely used to manage anterior segment neovascularization secondary to ischemic CRVO [15].