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Embolus vs. thrombus

Posted Dec 02 2012 10:16pm

Here are a few very good questions about CNS infarcts. There are two types: red (hemorrhagic) and pale (ischemic). This is an area that is often problematic for students – until someone explains it (then it’s easy!). On to the questions.
Q. Is it true that hemorrhagic infarcts happen because of emboli + reperfusion and ischemic infarcts happen because of thrombi?
A. Yes – that’s right. In a red infarct, there is extravasated blood (blood that has leaked out of the blood vessel and is now in the tissue) – probably from a burst blood vessel due to reperfusion. In this type of infarct, a blood clot gets lodged in a vessel, and then after a bit it either dissolves (from the body’s own fibrinolytic actions) or it moves farther downstream. The unblocked vessel now gets a full blast of blood again, and because it was damaged when the clot was present, it isn’t able to handle a normal blood volume, and it leaks.
In pale infarcts, a clot builds up in the blood vessel over time, slowly cutting off the blood supply until it completely occludes the vessel and no blood can get through at all. You can read more about the difference between hemorrhagic and ischemic infarcts in  this post .
Q. I don’t understand the difference between an embolus and a thrombus.  When I looked up the terms that didn’t really clarify much either-it said that an embolus is also called an thromboembolus.
A thrombus is an abnormal clot that is formed in a vessel. In the vessels in the head, thrombi are usually due to atherosclerosis (and they commonly occur in the middle cerebral artery – though they can occur anywhere). In the rest of the body, thrombi most commonly occur in the deep veins of the legs.
An embolus, on the other hand, is a floating clot that lodges somewhere. Emboli usually break off from thrombi. For example, part of a thrombus in a deep leg vein can break off and float upstream, usually traveling nicely all the way up the inferior vena cava, into the heart, and out through the pulmonary arteries, lodging in whatever vessel is too small to get through. “Embolism” implies movement, and it usually refers to a blood clot (coming from a thrombus elsewhere), though there are other types of emboli too (fat, for example, or air, or even bone marrow, as in the photo above).
This is important in the discussion of red vs. pale infarcts. Pale infarcts are due to thrombi. They form right in the vessel, and over time, occlude the vessel, preventing blood from getting through, causing a pale (bloodless) infarct. Red infarcts occur when emboli (from a thrombus in the carotid artery, maybe, or from a blood clot that formed in the left atrium) travel through vessels in the brain, lodging in a vessel that’s too small to get through. Once lodged, the body sometimes is able to dissolve these emboli; and sometimes they move farther downstream. Then the damaged vessel gets blood going through it again (reperfusion) and blood leaks out through the damaged area.
Sometimes the two terms (thrombus and embolus) will be lumped together (strokes are sometimes called “thromboembolic events”). In some ways, this makes sense, since both thrombi and emboli cause brain infarctions. However, as we just discussed, infarcts due to thrombi are often very different from infarcts due to emboli. It’s important to separate these two types out, because they are treated differently.
Q. Also, in the case of ischemia is the long term result if the person survives a hole in the brain?
Just to clarify: ischemia means that blood flow has been cut off; it is potentially reversible. If ischemia continues long enough, though, an infarct will occur (infarct means that the tissue is dead; it is irreversible).
In the case of a tiny infarct, you won’t see anything much grossly. But in larger infarcts, the dead tissue is eventually cleared away, leaving a hole. Since the brain doesn’t really form scars like the rest of the body does (with granulation tissue and collagen formation), the hole will remain there permanently.

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