In medicine, an embolism occurs when an object (the embolus, plural emboli) migrates from one part of the body (through circulation) and causes a blockage (occlusion) of a blood vessel in another part of the body. The term was coined in 1848 by Rudolph Carl Virchow. This is in contrast with a thrombus, or clot, which forms at the blockage point within a blood vessel and is not carried from somewhere else.

Classification

There are different types of emboli.

Material

They can be classified based on the embolic material:

• Thromboembolism – embolism of thrombus or blood clot.
• Fat embolism – embolism of fat droplets.
• Air embolism (also known as a gas embolism) – embolism of air bubbles.
• Septic embolism – embolism of pus-containing bacteria.
• Tissue embolism – embolism of small fragments of tissue.
• Foreign body embolism – embolism of foreign materials such as talc and other small objects.
• Amniotic fluid embolism – embolism of amniotic fluid, foetal cells, hair, or other debris that enters the mother's bloodstream via the placental bed of the uterus and triggers an allergic reaction.

Pathway

The pathway of the embolus can be one of three types:

• Anterograde
• Retrograde
• Paradoxical

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. 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 between the atria or ventricles.

Pathophysiology

In thromboembolism, the thrombus (blood clot) from a blood vessel is completely or partially detached from the site of thrombosis (clot). The blood flow will then carry the embolus (via blood vessels) to various parts of the body where it can block the lumen (vessel cavity) and cause vessel obstruction or occlusion. Note that the free-moving thrombus is called an embolus. A thrombus is always attached to the vessel wall and is never freely moving in the blood circulation. This is also the key difference for pathologists to determine the cause of a blood clot, either by thrombosis or by post-mortem blood clot. Vessel obstruction will then lead to different pathological issues such as blood stasis and ischemia.

However, not only thromboembolism will cause the obstruction of blood flow in vessels, but also any kind of embolism is capable of causing the same problem.

Fat embolism usually occurs when endogenous (from sources within the organism) fat tissue escapes into the blood circulation. The usual cause of fat embolism is therefore the fracture of tubular bones (such as the femur), which will lead to the leakage of fat tissue within the bone marrow into ruptured vessels. There are also exogenous (from sources of external origin) causes such as intravenous injection of emulsions.