- There's two main types of stroke. Ischemic stroke which is
where brain tissue dies because it's not getting enough blood and therefore not getting
enough oxygen to stay alive. This is often caused by some type of obstruction of a blood vessel. And hemorrhagic strokes which is where a blood vessel in the brain bursts. And neither of them are particularly good because they both cause
brain tissue to die off. So in this video, we'll
look at ischemic strokes which actually make up
about 75% of all strokes. And let me orient you to
what this over here is. This is an underside view of the brain just so you can get a really good look at the blood vessels
that originate underneath the brain and supply it with blood. So, there's three major
causes of ischemic strokes and we'll sort of go
through each one in turns. So, the first one we'll
talk about is an embolism or an embolic cause of stroke. And let me just bring in my guy here to help us out with visualizing. So we're looking at the underside. An embolism is some type of traveling mass in your blood stream and it's most often a little blood clot. So, in an embolic stroke, this little blood clot will travel up to one of your cerebral
arteries and get stuck and cut off your blood
circulation to that area. So let's say it get stuck
somewhere about here. It's traveled up through
the internal carotid artery and is lodged here in the
middle cerebral artery. Now, parts of the brain
served by that MCA, that middle cerebral artery, are all gonna start to die off. So, where would a clot come from? Well, they often come from the heart. And I know that sounds like a line from a Boyz II Men song. But you can literally
get blot clots forming within your heart. So, either on the valves
or within the walls of usually your left atrium
or your left ventricle that then break off away
from the walls of the heart to travel up to your brain and then they get stuck just like you see here. Where else do emboli come from? Well, you might have
some cholesterol built up in one of your carotid arteries or maybe in your basilar artery as part of a process
called atherosclerosis. And from these collections
of cholesterol that build up, you can have little pieces of cholesterol that sort of flick off. And just as we saw before, they sort of travel up into your brain and they get stuck. So, this little piece of cholesterol has gotten stuck in one of our anterior cerebral arteries here. And so, it will choke off blood supply to the distribution of this
anterior cerebral artery. So we'll have death of
brain tissue on this side. And remember the anterior cerebral artery supplies blood to some of the more midline parts of the brain. So that's the first major
type of ischemic stroke, ones caused by emboli. Now, another major type of ischemic stroke are ones called thrombotic strokes. So, how did those happen? So, it's similar to
embolic strokes in that a blood vessel that supplies a part of the brain
gets completely blocked. But the difference is that emboli arise away from the brain. Whereas in a thrombosis, a problem actually arises
in that blood vessel and it gets blocked off at that very spot. So, let me show you some
examples to clarify this. So, we talked about how you can get some carotid atherosclerosis happening here. The build up of a fatty
cholesterol deposit inside your arterial wall. Actually, let's show it
happening in the basilar artery because it's a bit easier to
see on the underside picture. So, you get this build up of cholesterol and this actually the most common reason for a thrombotic stroke. And eventually, that cholesterol deposit is gonna sort of crack open. It's gonna break open
which actually causes a blood clot to form on top of it. And that blood clot can
get really, really big, and so, as you can see in this picture it actually got so big that blood can't even get through this
basilar artery anymore. And so obviously, no
blood is being allowed to get through into the
cerebral circulation from at least the basilar artery which will pretty quickly cause a stroke. So, that's an example
of a thrombotic stroke. A disruption of blood caused by a problem with that blood vessel locally. And actually, this
atherosclerosis commonly happens in this middle cerebral artery here. And of course, possibly
on the other side's middle cerebral artery. Inside these internal carotid arteries ride as they sort of turn into the middle cerebral arteries and in this basilar artery here. So these are three key locations that atherosclerosis happens. So that's atherosclerosis and that generally happens in larger arteries in the brain. So, the ones I sort of
pointed out to you already. But there's another sort of
subtype of thrombotic strokes and this one happens in primarily smaller vessels of the brain and these are called lacunar strokes. So let's actually look
at these vessel here. I'm gonna just draw in a few
tiny little vessels, okay. And now, we'll blow one up. So, that's our small vessel. If you have hypertension which is longstanding high blood pressure, then the walls of these
small blood vessels can't really handle the
pressure and the force with which red blood cells
travel through them with. Because this really high blood pressure puts a lot of strain and stress on these blood vessel walls. So, what these blood
vessels do in response is they sort of thicken up. They get a lot thicker. And so immediately, you can kinda see that not very much blood is
gonna get through them as they start to get
thicker and thicker, right? And if the high blood
pressure isn't dealt with, they're just gonna get thicker. And so, you can imagine that now not very much blood is gonna be able to get out of these blood vessels because not really that much blood can get through these blood vessels. And so, what do you think might happen to the piece of brain that
this blood vessel serves? Well, it's not really gonna be very happy. It's not gonna get much oxygen and so, it's gonna die off. So that's a lacunar stroke, a small vessel stroke. You can also get tiny
bits of atherosclerosis happening in the small vessels which would block them off to some extent. But really, hypertension
and this wall-thickening is the major cause of
these lacunar strokes. So, the last type of ischemic stroke I want to talk to you about is something called the watershed stroke. Now what does that mean? Well, let's just go
over some basics first. The blood flow within your brain goes from inside to out and from outside to in. So, you have some redundancy there to make sure that all of
this brain area is covered. It's all getting enough blood. But let's say that something happened to reduce total blood flow to your brain. So for example, if you
got two large atheromas or atherosclerotic deposits in your internal carotid arteries. Okay? So not that much blood. Actually, really little blood is getting through up to your brain, all right? Well, the total amount
of blood that's getting to your brain is gonna go down, right? You'll be in a state of
what we call hypoperfusion. Hypo meaning lowered and perfusion just refers to how much blood your brain is being filled with. So as a result of all this, you're still gonna have a little bit of cerebal blood flow. But you're not gonna
have that much, right? So, we'll make these arrows a bit smaller. You get some blood flow on this side from outside to in but you're not gonna have as much because overall you're getting less blood up to your brain, right? Because of these two blockages. And so, what do you notice here? I'm sure you noticed that
there's this huge area in the middle that is not getting that blood supply from
either circulation now. >From either the inside-to-out or the outside-to-in circulation. So this area will start to die off. And actually in watershed infarcs there's a classic pattern almost exactly like what I'm drawing here. And so, as a result of that this area is colloquially
called the watershed area. Areas where say this part, this half, is normally perfused by this blood supply and this half is normally perfused by this blood supply. But when you sort of scale both blood supplies back a bit, this watershed area is
prone to being damaged. And in this case, this
person has developed a watershed stroke. So, these are three main
causes of ischemic stroke: Embolism, thromboses,
and watershed strokes caused by hypoperfusion.
