How can an embolism cause a hemorrhagic infarct?

Q. I am confused as to how an embolism can cause a haemorrhagic infarct. To me, emboli are little chunks of clot that float around and get stuck in vessels. Shouldn’t this cause an ischemic infarct? Also, a hemorrhagic infarct is named for its color (not for its underlying mechanism), right? But then how do you get a hemorrhagic infarct?

A. Nice questions. This is always a topic that seems confusing. There are two kinds of brain infarcts: pale (ischemic) infarcts, and red (hemorrhagic) infarcts. It’s really important to figure out which one you’re dealing with, so you can treat the patient properly. Ischemic infarcts (if caught soon enough) can be treated with a clot-busting drug; hemorrhagic infarcts should not (the patient is already having excess bleeding, so you don’t want to give anything that could make that worse).

Ischemic infarcts are so named because they are due to straight-up ischemia (you plug a vessel, usually with a thrombus, which is a clot that stays put where it develops), in which blood no longer is supplied to a portion of the brain. Pretty easy to understand.

Hemorrhagic infarcts are a little more difficult to grasp. You’re right: emboli are little moving chunks of stuff. Usually, they are chunks of blood clot (though you can also have chunks of other things, like fat, that can do the same type of damage). Emboli can lead to ischemic strokes, if they plug up a vessel and stay lodged there. However, they frequently dissolve a bit and move downstream, leaving behind a damaged and vulnerable vessel.

When blood flows through that damaged and vulnerable vessel, the wall can break, spilling blood into the tissue and causing an infarct. This is called “reperfusion” injury and the mechanism is not well understood. Presumably, the ruptured vessel is no longer able to supply blood to the region of brain that it previously supported (and also, the tissue is disrupted by the blood pouring out of the vessel), and an infarct occurs.