Retinal artery occlusion is characterized by sudden, painless, monocular loss of vision, most often due to an embolus occluding the central retinal artery or its branches. The diagnosis of the condition depends on clinical presentation and the classic funduscopic appearance of the retina.
Retinal artery occlusion (RAO) was first described in 1859 by von Graefe . It mainly occurs due to the occlusion of the central retinal artery (CRA) or its branches, usually by an embolus and is reported to affect approximately 2 out of every 100,000 individuals in the United States . The patients present classically with a sudden onset of unilateral, painless, visual loss which may rapidly deteriorate to the level of mere finger counting ability or even complete loss of vision  . Up to 30% of individuals have a cilioretinal artery , a branch of the short posterior ciliary artery, which may lead to significant sparing of the vision as the cilioretinal artery supplies part or the entire fovea and is unaffected in the central retinal artery occlusion.
Clinically RAO has four different entities:
As already mentioned, the commonest cause of RAO is a thrombus or an embolus occluding the central retinal artery as it pierces the dural sheath of the optic nerve . Retinal recovery after RAO depends on the dislodgement of the embolus or thrombus and on the tolerance time of the retina  .
The diagnosis of RAO is suspected based on the clinical presentation and can be confirmed with clinical evaluation, fundoscopy, and fluorescein angiography. An ophthalmic examination may reveal a relative afferent pupillary defect while the funduscopic appearance of a pale, opaque fundus with a red fovea (cherry-red spot) is confirmatory. Occasionally, a cholesterol embolus (known as Hollenhorst plaque) may also be seen. In cases where the central retinal artery is spared and only a major branch is occluded, visual loss may be restricted to the corresponding sector of the retina. Fluorescein angiography detects the lack of perfusion in the occluded artery.
If non-arteritic permanent RAO due to thrombus or embolus is suspected then the physician should inquire about risk factors for atherosclerosis, a past history of vascular disease, cardiac disease, smoking, transient ischemic events, angina, and renal disease  . In young patients, if there is no suspicion of atherosclerotic events, other etiologies like vasculitis, hemoglobinopathies, myeloproliferative disorders, hypercoagulable conditions, and the use of oral contraceptives should be excluded .
After confirming the diagnosis of RAO, carotid Doppler ultrasonography and echocardiography are performed to locate the source of the embolus and to prevent further embolization. An electrocardiogram (ECG) is ordered to detect atrial fibrillation and a 24-hour Holter monitoring may be indicated in patients with suspected arrhythmia. Laboratory tests are useful to diagnose the etiology and may include complete blood count, erythrocyte sedimentation rate (ESR), fibrinogen levels, antiphospholipid antibodies, activated partial thromboplastin time (aPTT), prothrombin time (PT), lipid panel, blood culture, and work up to exclude sickle cell disease.
Imaging studies such as echocardiography or carotid Doppler ultrasonography are performed urgently while magnetic resonance angiography (MRA) and computed tomography (CT) angiography can be performed later to detect atherosclerotic disease.