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Ischemic stroke

Visit us (http://www.khanacademy.org/science/healthcare-and-medicine) for health and medicine content or (http://www.khanacademy.org/test-prep/mcat) for MCAT related content. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Vishal Punwani.

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Video transcript

- 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.