Embolism

In medicine, an embolism (plural embolisms; from the Greek ἐμβολισμός "insertion") is the event of lodging of an embolus (a detached intravascular mass capable of clogging arterial capillary beds at a site far from its origin) into a narrow capillary vessel of an arterial bed which causes a blockage (vascular occlusion) in a distant part of the body.

Embolization is a procedure that purposely creates such a lodging and occlusion of specific blood vessels with thrombo-emboli in order to deprive tumors (or other pathologic processes) of their perfusion (blood supply).

Classification
There are different types of embolism, some of which are listed below.

Arterial or venous
Embolism can be classified as whether it enters the circulation in arteries or veins. Arterial embolism are those that follow and, if not dissolved on the way, lodge in a more distal part of the systemic circulation. Sometimes, multiple classifications apply; for instance a pulmonary embolism is classified as an arterial embolism as well, in the sense that the clot follows the pulmonary artery carrying deoxygenated blood away from the heart. However, pulmonary embolism is generally classified as a form of venous embolism, because the embolus forms in veins, e.g. deep vein thrombosis.

Arterial
Arterial embolism can cause vessel occlusion in any part of the body. It is a major cause of infarction (which may also be caused by e.g. arterial compression, rupture or pathological vasoconstriction).

Arterial embolis may be starting in the heart (from a thrombus in the left atrium secondary to atrial fibrillation or septic emboli from endocarditis).

An embolus landing in the brain from either the heart or a carotid artery will likely cause an ischemic stroke.

Emboli of cardiac origin are also frequently encountered in clinical practice. Thrombus formation within the atrium in valvular disease occurs mainly in patients with mitral valve disease, and especially in those with mitral valve stenosis with atrial fibrillation (AF). In the absence of AF, pure mitral regurgitation has low incidence of thromboembolism. Absolute risk of emboli in idiopathic AF depends on other risk factors such as increasing age, hypertension, diabetes, recent heart failure, or previous stroke. Thrombus formation can also take place within the ventricles, and it occurs in approximately 30% of anterior-wall myocardial infarctions, compared with only 5% of inferior ones. Some other risk factors are poor ejection fraction (<35%), size of infarct, and the presence of AF. In the first three months after infarction, left-ventricle aneurysms have a 10% risk of embolization.

Patients with prosthetic valves also carry a significant increase in risk of thromboembolism. Risk varies, based on the valve type (bioprosthetic or mechanical); the position (mitral or aortic); and the presence of other factors such as AF, left-ventricular dysfunction, and previous emboli.

Emboli often have more-serious consequences when they occur in the so-called "end circulation": areas of the body that have no redundant blood supply, such as the brain and heart.

The main complication of arterial embolism is infarction, that is, tissue death (necrosis) caused by blockage of the tissue's blood supply.

Venous
Assuming a normal circulation, a thrombus or other embolus formed in a systemic vein will always impact in the lungs, after passing through the right side of the heart. This forms a pulmonary embolism that can be a complication of deep-vein thrombosis. Note that, contrary to popular belief, the most common site of origin of pulmonary emboli are the femoral veins, not the deep veins of the calf. Deep veins of the calf are the most common site of thrombi, not emboli origin.

The main complication of venous embolism is pulmonary embolism, that is, blockage of the main artery of the lung or one of its branches.

Paradoxical (venous to arterial)
In paradoxical embolism, also known as crossed embolism, an embolus from the veins crosses to the arterial blood system. This is generally found only with heart problems such as septal defects (holes in the cardiac septum) between the atria or ventricles. The most common such abnormality is patent foramen ovale, occurring in about 25 % of the adult population, but here the defect functions as a valve which is normally closed, because pressure is slightly higher in the left side of the heart. In certain circumstances, e.g. if patient is coughing just when an embolus is passing, passage to the arterial system may occur.

Direction
The direction of the embolus can be one of two types:
 * Anterograde
 * Retrograde

In anterograde embolism, the movement of emboli is in the direction of blood flow. In retrograde embolism, however, the emboli move in opposition to the blood flow direction; this is usually significant only in blood vessels with low pressure (veins) or with emboli of high weight.