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Risk factors for stroke
- [Voiceover] What you see here is an anterior view of a guy with his brain showing. And you can see all these blood vessels that are supplying his brain here. And if anything happened to any of these blood vessels, they would deliver less blood to parts of the brain, which would likely result in a loss of brain function, which is what a stroke is. So we don't want him to have a stroke. So let's look at a few things that predispose one to developing a stroke, things called risk factors. And I think the easiest and, sort of, most intuitive way to do this is to pick a vessel, so we'll pick this one here, in the brain, and we'll look at the different things that can happen to that vessel that might predispose you to developing a stroke. So let's just zoom in on that area there. So we've zoomed in quite a bit here. So you can see the artery here. This is a cerebral artery. And this is all just sort of surrounding brain tissue, all this stuff in pink here. And all these wavy lines, these are all the, the grooves and ridges that sort of cover your brain. So what sort of things might affect your blood vessel and predispose you to having a stroke? Well, young blood vessels are usually nice and healthy. So the walls of the arteries are pretty strong and have a lot of tensile strength, whereas, when you get older, these walls sort of almost get a little bit weaker. So that sort of causes two issues. The first one is that, if this blood vessel wall is a little bit weaker, then it's more likely to rupture. And if it ruptures, then blood traveling within your arteries can more easily leak out and cause a hemorrhagic stroke. So that's the first thing. The second things is that, as you get older, your arteries, the walls of them, also get a bit stiffer. And stiffer blood vessels put you at a higher risk for developing high blood pressure. And that in itself is a risk factor for stroke. And we'll talk about that in a few minutes here. So age, age is a risk factor. And actually, your risk is the greatest after about 65 years old, although strokes can happen in younger people, as well. And actually, risk factors can be sort of separated or thought of in two groups: things you can do something about, and things you can't do something about. So we'll call the things you can't do anything about non-modifiable risk factors, and we'll call the things you can do something about modifiable risk factors. So age is obviously one of those things you can't do anything about, unfortunately, for people like me. So let's just keep our running list up there in the corner. So what are some other non-modifiable risk factors? Well, men are more likely than women to have a stroke, and it's not exactly clear why. People of African descent are at a higher risk of having a stroke. And that's thought to be because they have high blood pressure more often. And again, we'll talk about blood pressure in a minute. And the last one I'll mention is that, if you've had a prior stroke before, you'd have a higher risk of having another stroke. So those are the major non-modifiable risk factors, the things that we can't do anything about. But what can we do something about? Well, the biggest modifiable risk factor for stroke is hypertension, or high blood pressure. Let me show you two ways in which hypertension predisposes you to stroke. So let's say you have another little vessel here, a little blood vessel coming off one of the bigger arteries in your brain. So this is our little blood vessel. Well, when you have normal blood pressure, all your little red blood cells, they sort of travel through your cerebral vessels at a normal speed, right? So they're not really going too fast. They're going just at the right speed to not damage your blood vessels, to not, sort of, bump into your blood vessel walls enough to damage them. But what happens when you have really high blood pressure is that these blood cells move a lot faster. And when they move a lot faster, they do two things. The first way that's a problem is because they're moving a lot faster and they tend to bump into the walls of the blood vessel a lot more. And so, while that's not a huge problem in bigger blood vessels, when they get into, sort of, these smaller blood vessels, like this one, that high pressure is a little bit too much for the smaller blood vessel to handle. And so the walls of these smaller blood vessels start to get a bit thicker, to try to compensate for the extra pressure that they're feeling. So this process of the smaller blood vessel undergoing thickening is called hyaline arteriolosclerosis. So that's quite a mouthful. So let's actually break that down so we remember what it means. "Hyaline" just refers to how a cross-section of the vessel would look under a microscope. It looks kind of glassy, and "hyaline" is derived from the Greek word for something that's glassy, or crystal-like. "Arteriolo" just refers to the fact that it's happening in a smaller artery, so an arteriole, and "sclerosis" just refers to hardening. So hyaline arteriolosclerosis. Well, why is this bad? Well, you can imagine that, if the hypertension is long-standing, then this vessel will just continue to get smaller in diameter until not very much blood, and sometimes no blood at all, can get out the other end of the arteriole. So what do you think's gonna happen to the brain tissue that that arteriole serves? Well, it's gonna die off and you're gonna have a stroke in that area. So that's the first way that hypertension is bad for your cerebral blood vessels. What's the other way? Well, as your blood cells rush past this junction here, they're gonna continuously sort of bang into this little edge here. And if they continue to do that over time, they cause a ballooning out, an aneurysm, of the artery wall. So here's our aneurysm. And the problem with aneurysms is that, as they're, sort of, the weakening of the vessel wall, they have this tendency to rupture and spill blood out of the circulation. So hypertension can cause aneurysms, which can then cause hemorrhagic strokes. Now, these next three risk factors sort of work together. So I'll talk about them all together. I'm referring to diabetes, smoking, and high blood cholesterol. So what happens in diabetes is that you get lots and lots of blood sugar floating around in your blood vessels. And this glucose, what it does at high concentrations is, it damages blood vessel walls, it makes them hard and stiff. And we all know that smoking, right ... So here are the toxins from smoking, tobacco smoking. These toxins go ahead and damage your blood vessels, as well. Now, our third player, cholesterol, these little particles called LDL cholesterol, what they do when they see these damaged blood vessel walls is they sort of hop into the damaged blood vessel walls because they think it's a really cool, comfortable place to hang out, and they sort of build up. So I'll draw it over here, but let's say they built up to a really, really significant extent. And I'm not sure if if looks like it, but just to be clear: this cholesterol is actually building up within the blood vessel wall. And by the way, this cholesterol building up within blood vessel walls is called atherosclerosis. >From here, two things can happen. Either this big cholesterol formation can sort of crack open, cause a clot to form on top of it, and block off the entire blood vessel that way, or a piece of this cholesterol collection can sort of break off and float downstream to block off a smaller vessel. But either way, you deprive downstream brain tissue of blood, and a stroke would develop that way. So diabetes, smoking and cholesterol can all work together to increase your risk of having a stroke. And I just wanna clarify that they all independently increase your risk of having a stroke. But here I've just shown you them all working together, just for simplicity's sake. So now let's zoom out a little bit to look at some other risk factors. So that atherosclerosis we talked about, that can actually affect these internal carotid arteries. And in fact, they often are affected by atherosclerosis. So these are the internal carotid arteries, and you can get that fatty build-up of atherosclerosis within them. So if that happened, you're effectively reducing blood supply to large parts, almost all of your brain. And you could end up having a massive stroke because, although there are, sort of, redundancy mechanisms in place to provide collateral blood flow to the brain, like the circle of Willis here, if you get blockages, say, in your internal carotids, still, the internal carotids provide the majority of blood to the brain. So if you block one of these off, you're really, really gonna increase your risk for having a stroke and having a lot of your brain tissue die off. So the next set of risk factors I wanna talk about are ones caused by heart conditions. So here's your heart. Just quickly, here's your left atrium, your left ventricle, your right ventricle, and your right atrium. Now, the heart condition that most strongly predisposes you to having a stroke is called atrial fibrillation. So what happens in atrial fibrillation? Well, let's actually just look at the left atrium here, because that's the one that's relevant for stroke. So in atrial fibrillation, your atria don't contract properly, and they actually, sort of, just quiver instead of properly contracting. And so, because they're sort of quivering, blood is allowed to stick around in them for a little bit longer than it normally is, and that blood that sticks around can start to clot, because when blood isn't moving, it starts to clot, just spontaneously. And these clots, they can break off, travel into the left ventricle, and then out of the left ventricle, where they can head up to the brain. And they can get lodged in blood vessels in the brain, and therefore, they cut off blood supply to parts of the brain and cause an ischemic stroke. Atrial fibrillation. Another heart condition that predisposes you to stroke is a myocardial infarct, or a heart attack. So let's say, for example, you had a heart attack that affected this part of your left ventricle here. Normally this left ventricle pumps blood and contracts really, really nicely and strongly. But after you've had a heart attack, it doesn't really contract properly. So again, you can get this blood sort of sitting around in the left ventricle with nothing to do but clot. And again, this blood clot can sort of break away from the wall and follow the same route as the clot that developed earlier. One more risk factor that has to do with the heart is heart failure. So in heart failure, your heart muscle actually fails to pump properly. It fails to contract properly. And if you had really, really bad heart failure, you wouldn't get enough blood ejected out of your heart to supply the brain with enough blood that it needs. So in this case, you would get global ischemia of your brain tissue. In other words, your entire brain wouldn't be getting enough oxygen. So you'd have a pretty big ischemic stroke. So the last risk factor I wanna touch on is something called a TIA, a transient ischemic attack, or a mini-stroke. So I mentioned that some heart conditions can cause clots to travel up to the brain and get stuck in cerebral blood vessels. And I also mentioned that you can get some carotid artery atherosclerosis here that might embolize, right? So a piece might break off and travel up into the brain and get stuck in an artery. Now, if either of these types of clots goes away on its own within 24 hours, so if they dissolve or if the body's own mechanisms break them down and they go away within 24 hours, then you've had something called a TIA or mini-stroke. Simply having had one or more of these mini-strokes greatly increases your risk of having a proper stroke. So that was an overview of some of the non-modifiable and some of the modifiable risk factors for strokes.