The central retinal artery, a branch of the ophthalmic artery, enters the eye through the optic disc and divides into multiple branches to perfuse the inner layers of the retina. A branch retinal artery occlusion (BRAO) occurs when one of these branches of the arterial supply to the retina becomes occluded. Examples are shown in the images below.
Most commonly, a branch retinal artery occlusion occurs secondary to an embolus. Emboli typically originate within vessels upstream where they dislodge and travel within the circulatory system to ultimately become lodged downstream in a vessel with a smaller lumen. The most common include cholesterol emboli from aorto-carotid atheromatous plaques, platelet-fibrin emboli from thrombotic disease, and calcific emboli from cardiac valvular disease. Various other endogenous emboli as well as exogenous emboli and nonembolic causes have been reported. [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]
Ischemia of the inner layers of the retina leads to intracellular edema as a result of cellular injury and necrosis. This intracellular edema has the ophthalmoscopic appearance of grayish whitening of the superficial retina. Primate studies have shown that complete occlusion of arterial supply to the retina results in reversible ischemic injury in up to 97 minutes. This may help explain why patients may give a history of transient loss of vision prior to an episode of branch retinal artery occlusion. Possibly, these episodes are secondary to emboli transiently becoming lodged, causing temporary occlusions and then reperfusing the retina as the emboli are released.
Branch retinal artery occlusion is most likely to occur at the bifurcation of an artery because bifurcation sites are associated with a narrowed lumen. In 90% of cases, branch retinal artery occlusions involve the temporal retinal vessels. Whether the temporal retinal vessels are affected more often or whether the nasal retinal vessel occlusions are more often undetected is unclear.
Patients with branch retinal artery occlusion have a higher risk for morbidity and mortality secondary to cardiovascular and cerebrovascular disease. A thorough medical workup is indicated for all patients with branch retinal artery occlusion, and an etiology can be identified in as many as 90% of patients.
Central retinal artery occlusions (CRAOs) account for 58% of acute retinal artery obstructions, branch retinal artery occlusions account for 38%, and cilioretinal artery occlusions account for 5%.
Multiple studies have shown increased mortality in patients with retinal arterial emboli. Increased mortality secondary to fatal stroke has been shown in studies, but the most common cause of death in this population is cardiovascular disease.
One study reported a 10-fold increase in the annual rate of stroke in patients with retinal emboli compared to controls after a follow-up period of 3.4 years.  Another study found a 3-fold higher risk of 8-year mortality from stroke in patients with documented retinal emboli at baseline compared with patients without emboli. A case series reported that 15% of patients with retinal emboli died within 1 year, and a mortality rate of 54% was shown within 7 years.
The incidence of neovascularization in all retinal artery obstructions is less than 5%. In branch retinal artery occlusion, the incidence is even more rare. Neovascularization, when it does occur, is more likely in persons with diabetes. Clinical cases have been reported in which neovascular glaucoma developed after branch retinal artery occlusion. 
One study compared retinal artery occlusions in black and white patients and found that both groups have the same risk factors for retinal arterial occlusive disease.  This study also suggested that whites were more likely to have identifiable carotid disease than blacks.
Among elderly patients, men are 2.5 times more likely than women to have retinal emboli. This correlates with the higher rate of stroke found in men.
Typically, branch retinal artery occlusion presents in the seventh decade of life. Branch retinal artery occlusion due to embolic causes is rare in patients younger than 30 years. Less than 1 per 50,000 outpatient visits to the ophthalmologist are estimated to involve a person younger than 30 years with retinal arterial obstruction. These cases are more likely to be nonembolic causes of retinal arterial occlusions.
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