What is a stroke?

- [Voiceover] So before we sort of jump into what strokes are, let me just sort of orient you to this, to what we're looking at here. So this is a side view of a person. And we can see the contents of their skull. So let's do some labeling here. We've got the skull here, and obviously, the skull protects the brain. We have the cerebrum, the most obvious part of the brain, in pink. We have the cerebellum down here, and we have the brain stem. Now, in terms of vessels, in terms of blood vessels, all of the blue that you see here, these are all veins. These are veins that drain the brain. And now let's label some arteries. So here in front, we have the internal carotid artery. And remember, we're only looking at the left side here. So we can only see the left one. And here, sort of running up inside the vertebrae, we have the vertebral artery. Now it gets a little bit tricky here. This is the basilar artery. This here is the middle cerebral artery. I'll call it the MCA. Here we have the posterior cerebral artery. We'll call that the PCA. And in front here, we have the ACA, or the anterior cerebral artery. And you can sort of see the Circle of Willis is formed right there. All right. So let's jump into it. So all those vessels we just talked about, that's your blood supply to your brain. And if you lose some or all of that blood supply to your brain, then you lose some or all brain function. So the loss of some blood supply causing the loss of brain function, that's a stroke. So there's two main ways to disturb this blood supply. The most common type of disturbance is where you get stoppage of blood flow to a part of your brain. So let me draw this out for you. Let's say that this bit of the anterior cerebral artery is blocked off. This means that blood won't be able to go from this previous part of the ACA over to this next part of the ACA. And if blood can't get through, then the brain tissue downstream, that depends on that blood for its oxygen needs, that brain tissue would start to get injured and start to die off. And the second kind of disturbance, once that's not as common, but still really, really serious, is where one of the blood vessels of your cerebral circulation, where one of the blood vessels ruptures. Let's say that this bit of the ACA, right at the junction between the ACA and the anterior communicating artery, started to balloon out. Right? It started to form an aneurysm. Well, eventually, that aneurysm could rupture. And if it ruptured, you'd start leaking blood out of your cerebral circulation and into the space around your brain. So if that happens, there's two major consequences. First, because you're leaking blood out of your cerebral circulation, these downstream parts of, say, your anterior cerebral artery here, obviously won't be getting any blood. So again, this downstream brain tissue is gonna have a lack of oxygen around. So it'll start to get damaged. Also, the blood will start to collect and sort of push on your brain tissue and cause damage to your brain that way. So those are really the two types of stroke. You can get stoppage of blood flow to an area, which is also called an ischemic stroke, "ischemic" referring to an environment, the cellular environment where there's not enough oxygen, or you can get this rupture of a vessel that we saw earlier. And that's also called a hemorrhagic stroke. Hemorrhagic. And "hemorrhagic" refers to, sort of, a sudden torrential bleeding outburst. So, ischemic stroke and hemorrhagic stroke. But regardless of whether you have an ischemic stroke or a hemorrhagic stroke, either way your brain cells are gonna start to malfunction after about three minutes or so. And that's because they have now a lack of oxygen, right? Because they're not getting proper blood flow, not getting proper blood supply, and they're not getting enough glucose. And they need glucose in your bloodstream to function properly, to carry out their complicated functions. So what do you think might happen if a part of your brain just sort of stopped working, started to malfunction? Well, intuitively, you might start to think, "Well, hey, if a part of my brain starts malfunctioning, "I might lose those abilities "that that part of the brain performs," right? So let's look at a few examples here. So let's say we get a clot. This is a clot here. Let's say we get one in the middle cerebral artery. Well, if we get a clot here, then that means that blood isn't gonna be able to get through the MCA, right? Here's blood in the MCA. All of a sudden, we're gonna lose the abilities that the downstream parts of the brain perform, at least temporarily. Now, the middle cerebral artery's pretty important. It actually supplies blood to two really important areas of the brain, one called Broca's area, on the outside of the brain, and one called Wernicke's area, or Wernicke's area, depending on if you wanna use the true German pronunciation. And these two areas are really, really important in determining your speech, determining how you speak and whether or not you can understand what people are saying to you. So if you get a big clot in your middle cerebral artery, and you happen to stop blood supply to Broca's area and/or Wernicke's area, you're gonna end up with problems with your speech. Now, let's do another example. Let's say you get a clot here, in this little artery coming off the basilar artery. Right? So that runs up your brain stem. Well, there's a lot of special nerves called cranial nerves that originate on your brain stem and sort of go their separate ways from there. And some of them are involved in controlling your facial muscles. And if these neurons lose oxygen, it means that you might start to have the edges of your mouth droop, it means that your eyelids might start to droop, basically, you might not be able to use some of the muscles of your face. So what exactly determines how bad a stroke is? Well, two things: where in the brain it happens, and how much brain tissue is actually damaged. And what determines that? Well, that's determined by which blood vessels are involved. For example, if you get a big clot right about here, then you're not gonna allow blood to your brain stem. So your brain stem would start to die off, and that would be really, really bad because your brain stem is really responsible for keeping your alive. Your brain stem has all of the regulatory centers that control your breathing and your heart and a lot of your other vital functions. In contrast, if you got a clot, let's say, in this tiny little vessel here, or in this tiny little vessel right about there, on in this one here, then how much brain injury would you end up with? Well, you'd end up with a little in each small area. And while, ideally, you don't have any brain injury, the sort of functional disability that you'd suffer if you had these small strokes, compared to this really, really big and significant stroke, there'd be a pretty big difference there. So just to reiterate, the severity of the stroke depends on where you get your stroke, right ... So again, you don't wanna have it in your brain stem ... And how much brain area is involved. And is this brain injury irreversible? Well, most of the time, yes. Neurons will start to die after about four minutes without oxygen. So what would cause a stroke to happen? Well, there's some common heart conditions, like atrial fibrillation, where the uppermost chambers of your heart don't contract properly. That often creates blood clots. And then those blood clots, when they, when they get pumped out of your heart, they can travel up to your brain and get stuck in your brain blood vessels, like we saw earlier. Another common heart-related cause is a myocardial infarct, or a heart attack. So when you have a heart attack, a part of your heart wall might start to not contract properly. So when that happens, little blood clots can grow on the heart wall and then be pumped out, up into the brain. Another really common cause, and actually one of the most common causes of stroke, is when you get something called atherosclerosis, or build-up of, sort of, this fatty cholesterol plaque in the walls of your arteries. So let's say that you got some cholesterol build-up in the wall of the artery here, in the internal carotid. Well, as you can imagine, blood is gonna have a really, really hard time getting past this, sort of, massive cholesterol that's stuck in the wall of the artery, right, this atherosclerosis. And these can happen in multiple places in the brain, too. And if you actually have one, you're likely to have another. So you might have one there, and you might have one, say, here, you might have one there. So they're really, really dangerous, atherosclerotic plaques. Now, let me switch gears for a second and talk about something called a TIA, or a transient ischemic attack, also known as a mini-stroke. A TIA is essentially a temporary interruption of blood flow to a part of the brain. So the symptoms, the symptoms of the TIA and the stroke, they're really similar. But the difference is that a TIA doesn't actually destroy brain cells and it doesn't cause permanent disability. So one of the key differences is that a TIA will resolve, it'll kind of go away within 24 hours, whereas, if you have a stroke, if you've had a proper stroke, you may not gain normal function again for weeks or months, or maybe even for the rest of your life. So let me just quickly show you what this would look like. So in a proper stroke, let's say this vessel gets blocked. Without treatment, this tissue will die off. But in a TIA, you'd get a little clot, this area would start to become injured, and then the clot would sort of spontaneously break up and go away. And then, all of a sudden, this brain tissue would start to go back to normal again. And so, in the vast majority of situations, you won't be left with any permanent brain cell death. So to finish off, let's just clear up a few misconceptions about stroke. So there's a common thought that strokes are primarily heart-related. And, as we saw earlier, the heart's certainly involved in some causes of stroke. But strictly speaking, strokes are conditions related to a mismatch between cerebral blood flow supply and demand. So it's a cerebral, it's a brain problem. There's another common misconception that strokes only occur in the elderly. And while it's true that about 2/3s of strokes happen in the elderly, and people older than 65, a pretty significant 1/3 happen in people under 65. So strokes can happen in people of varying ages. Last, and probably the most important, strokes are not unpreventable. There's a lot of things you can do to reduce your risk of having a stroke, such as controlling any high blood pressure or stopping smoking.