Contributor: Gordon K. Klintworth
Embolism is the sudden blockage of an artery or a branch of it by a thrombus or foreign material, which has been brought to its site of lodgement by the current of the blood circulation. It is one of the causes of occlusovascular disease. Many cases of central retinal artery occlusion result from an embolus. Sources of retinal emboli include cholesterol crystals and other pieces of atherosclerotic plaques [atherosclerosis], a thrombus on an atherosclerotoc plaque, thrombus on the endocardium beneath a myocardial infact, endocarditis, a cardiac myxoma, fat [embolism - fat], air [embolism - air] and talc [embolism - talc].
Emboli derived from small particles such as fat, air and talc may be identified within the retinal blood vessels even when they are too small to cause significant ishemia. Younger patients often have a cardiac myxoma. The effect of embolus depends on the nature of the embolus, the size of the occluded blood vessel and the degree of resultant ischemia. Neurons of the retina are extremely susceptible to hypoxia and coagulative necrosis of the inner retinal layers often follows acute central retinal artery occlusion. Intracellular edema causes retinal pallor in the acute stages of the retinal ischemia and this is especially prominent in the macula. Cellular dissolution and nuclear pyknosis of the affected retinal layers are common. Ischemic atrophy of the inner retinal layers (the nerve fiber, ganglion cell, inner plexiform, and inner portion of the inner nuclear layers) supplied by the central retinal artery occurs follows chronic retinal ischemia.
The clinically significant traumatic retinopathy of Purtscher [Purtscher retinopathy] that develops several days after compression or crush injuries of the abdomen or chest, long bone fractures, or head trauma results from different mechanisms, including fat embolism that occludes the retinal and choroidal arterioles producing retinal areas of non-perfusion. Intravenously administered illicit drugs are often contaminated with talc, cornstarch or other foreign particles that occasionally accumulate in choroidal and other ocular blood vessels and they sometimes elict granulomas. Many cases of central retinal artery occlusion result from an embolus.
Air emboli may be identified within the retinal blood vessels even when they are too small to cause significant ishemia.
Emboli causing central retinal artery occlusion often lodge in the optic nerve within the central retinal artery in the lamina cribrosa. The lumen of the central retinal artery may become totally or partially occluded by an atherosclerotic plaque usually at the lamina cribrosa.
Embolic Occlusion. Emboli of the central retinal artery most often originate from fragmentation of atheromatous plaques of the larger arteries, with formation of calcific or platelet-fibrin emboli. Unusual sources of emboli to the central retinal artery include intravascular metastases from remote neoplasms and endocardial myxomas.