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Video transcript

- [Voiceover] There are two main types of stroke, ischemic stroke and hemorrhagic stroke. And in this video we'll cover hemorrhagic strokes. Hemorrhagic strokes account for approximately 25% of all strokes. So let me just orient you to what we're looking at here. Right now we're looking at an underside view. An underside view of a brain. Because this view of the brain allows us to see the vessels that are most commonly involved with hemorrhagic strokes. Vessels of the Circle of Willis here. So what's a hemorrhagic stroke? Well, a hemorrhagic stroke is essentially when one of your blood vessels in your brain springs a leak, and begins to leak blood out of the vessel and in to the skull cavity. That's bad for two reasons. One, you're losing blood from within your cerebral circulation. And I'll show you on our friend here, if you start bleeding in to your cranium, that blood leaking out of your vessels is gonna take up room, right? Your cranium is a closed space, so your blood has nowhere to go. So if you start bleeding in to this closed cranium space, you're gonna increase pressure in there and you're gonna start to actually compress your brain. Another thing to think about, all of that blood leaking out of the vessel is blood obviously not continuing through the vessel to downstream vessels that serve downstream parts of the brain. So you'll start to have brain tissue die off in that distribution. And it gets even worse. You see all this blood leaking out on to this underside surface of the brain? It turns out that blood contacting the outside of blood vessels irritates those blood vessels. So when these vessels feel all this blood, they start to get irritated and start to clamp down a bit. Let me show you what I mean here. Here's your blood vessel as it normally looks. It's at a nice, wide diameter, and it's letting lots of blood cells through. But if blood is sitting around here on the outside of other blood vessels, it eventually starts to break down in to it's components. And we'll call those blood breakdown products. These products start to irritate this normal blood vessel. So what do normal blood vessels do in response? They undergo what's called vasospasm. They sort of close up a little. So you can probably immediately appreciate that not as much blood is gonna get through this artery after it's become irritated and has spasmed and clamped down a bit. So let's say that all of these blood vessels clamped down in response to irritation by blood breakdown products, and so they don't deliver enough blood to the brain tissue in their area. Then our patient would have developed a secondary stroke. So you really have to be careful of vasospasms causing secondary strokes after an initial hemorrhagic stroke. So those are three ways in which hemorrhagic strokes are really bad, but what causes a hemorrhagic stroke? Let's look at two of the most common causes. What might cause a cerebral blood vessel to just rupture? Well usually they have to have an underlying weakness first, such as an aneurysm, an out-pouching of the blood vessel walls. So what happens in an aneurysm is that an area of the Circle of Willis, the vessels in the Circle of Willis, actually balloon out like that. And they happen at a few places more commonly than others. They happen here, where I've drawn it, at the posterior communicating artery. They happen out here, in the middle cerebral artery. They happen in the anterior communicating artery. And they can happen here, sort of near the junction of the internal carotid, and the MCA, and the ACA. They could happen in other places as well, but these are some of the more common places that aneurysms can pop up. So I said that these aneurysms are weakened out-pouchings of the artery wall, and they can be caused by long-standing hypertension putting lots of stress on these artery walls, or they can happen if you have a problem with your arterial walls just for some genetic reason. So these aneurysms can spontaneously rupture, or rupture if you get hit in the head, or if you get tackled, or checked in hockey, or something like that. If you sustain some major trauma, they can also rupture. So if one of them ruptures, then all of a sudden you'll start to leak blood out of your cerebral circulation. So that's one of the major causes of hemorrhagic strokes, a ruptured aneurysm. And by the way, these are colloquially called berry aneurysms because they look like little berries that have sort of developed off of your arteries. The second major cause of hemorrhagic stroke that we'll talk about is a arteriovenous malformation, or an AVM. So I'll just draw this for you down below. In the sort of normal setup of our circulation, we have big arteries that then give off smaller arterials, they sort of divide in to smaller arterials, that then go on and divide in to smaller arterials, until you reach the level of the capillary, which is super small. And also the level where gas exchange between your blood and your body tissues takes place. On the flip side, on the other side, we have the other half of the capillary, which drains blood from the first half of the capillary. And then you have a little venule, which is a small vein. And then you have a vein. A proper vein. And that might empty in to a really, really big vein, let's say the inferior vena cava, or something like that. A really big vein. Okay? So the important thing to keep in mind here is that blood in the arterial system is at really high pressure. And I'll use P for pressure. High pressure. And as blood progresses through the arterial system to get to the capillary, it decreases in pressure. Okay? So when it's in the capillary it's at really low pressure, and essentially the same pressure as the venule side. And the reason for this is because our capillaries are really thin and delicate. And if we hit them with some high pressure blood, they would just rupture and explode and release blood everywhere. And actually, it turns out that that's sort of what happens with an arteriovenous malformation. This whole system here isn't formed the way that I've drawn it in an arteriovenous malformation. What actually happens is at the arterial end is that this whole capillary system, this whole capillary segment is bypassed. And you get arterials that are actually just directly connected to the small veins on the other side. So you just end up getting blood, at still really high pressures, going straight in to the veins. So that's really not good for the veins because they've sort of evolved to deal with lower pressures. And now, without that dampening-down effect of the capillaries, that step down of pressure that's sort of afforded by the capillaries, you get this high pressure blood going straight from arterial system in to venous system. And this high pressure puts this arteriovenous malformation at a high, high risk for rupturing. So you can imagine if that happened in your brain, you'd end up with a hemorhhagic stroke. So, while AVMs can occur really anywhere in your body, they often appear within the brain or within the central nervous system. And why do they happen? Well, they're congenital. It's due to sort of a defect of blood vessel formation in development. So those are a couple of the major causes of hemorrhagic stroke.