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What is an aortic dissection?

Created by Vishal Punwani.

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Video transcript

An aortic dissection is a life threatening condition where there's a separation of the layers in the wall of the aorta. Now, in case that doesn't quite make sense to you, let's go over some background information. We know that every cell in our body needs oxygen to survive. It's really important that our cells have constant access to oxygen, and in fact, many of our cells can start to die if they're deprived of oxygen for even a few minutes, so to get oxygen to all of our cells, we actually have special delivery cells dedicated to carrying oxygen to every other cell. These delivery cells are red blood cells within our blood, and they get around our body by way of our circulatory system, which is made up of our arteries, our veins, and our heart. So now that we know the basic components of our circulatory system and why they're so important, again, to circulate blood around our bodies so we can stay alive, I just want to focus for now on the main artery in our body called the aorta. So now let's look at where exactly the aorta is. So this here is a side view of the guy, so we're looking at him from the side, and this type of view is called a sagittal view, and I purposefully chose this view so I could easily show you the different regions of the aorta. So the aorta stems off the top of the heart, and it arches up to about the upper chest area, and then back downward again, and it heads down behind the heart, sitting right in front of the spine, the vertebrae, all the way down to just below the bellybutton, where it splits into two other arteries. The ileac arteries. We actually divide up the aorta into different regions, so this region up here, we call this the aortic arch, and then this region here in the chest area is called the thoracic aorta, thoracic referring to the thorax, or the trunk, and then we have this big section down here that starts underneath the diaphragm, called the abdominal area, so now we've looked at the aorta from a side view, but to get an even better appreciation of what the aorta looks like, let's look at it from a front view, or an anterior view, so that's this view here, and hopefully you can imagine that the aorta's sitting about midline in our bodies, and it runs down behind the heart, and sort of follows the spinal column, so back here would be our vertebral bodies, our spine, and the aorta sort of tracks down along our vertebral bodies. So the aorta is really the main artery in our bodies because it's sort of the big highway that blood takes to get to every other part of the body, so blood gets pumped out of the chamber of the heart here, the left ventricle, and into the aorta, which has lots of little arteries branching off of it, to take blood to different parts of the body, so going back to the highway analogy, these two smaller arteries here would be exits one and two, the left and right coronary arteries, and this up here would be exit three, the brachiocephalic artery that supplies blood to your brain and to your right arm, and down here we have the left and right renal arteries that make sure your kidneys get enough blood, and then down here we have the left and right common ileac arteries that supply blood to your pelvis and your lower limbs, and there are a lot of other arteries that come off the aorta, but I've just shown you a few. So by now, it's pretty clear that the aorta's really important, right? You definitely want your aorta to stay nice and healthy so it can continue to be the main pipeline for your blood, but sometimes things go wrong, so this brings us back to the condition I mentioned at the start of the video. One of the things that can go wrong is something called an aortic dissection, and an aortic dissection is a life threatening condition where there's a separation of the layers in the wall of the aorta, and to really understand what an aortic dissection is, let's look at the anatomy of its walls to see how separation might happen. So let's take out this piece of the aortic wall here, and use that to look at the anatomy of the aorta. So the walls of the aorta have three main layers, and we call each of these layers a tunic, so we say that the aorta has three tunics, or in latin, tunica, so that's the outermost layer called the tunica adventitia, and then you've got this really thick middle layer called the tunica media, or just the media, and the media's this really, really thick, muscular layer of the aorta that's really necessary to allow the aorta to withstand all the pressure of the blood, the blood pressure that it has to deal with, and finally, on the very inside, so facing the inside of the aorta, we've got the innermost layer called the intima, the tunica intima, and the intima's just this really thin layer that lines the inside, so three layers. The outer layer that sort of keeps the aorta in place. The media, which is the really thick, muscular layer, and the intima, which is the really thin, inner layer, and this here is our lumen, where the blood flows through the aorta, and if you think about it, the heart is here, pumping blood out of the heart at pretty high pressure every second, and so that means the aortic wall is sort of exposed to really high pressures and sheer stress, which makes it pretty susceptible to injury, purely from a mechanical stress point of view, so an aortic dissection is when you get a tear in the intima here, and blood enters the layer between the intima and the media, and that blood will just continue to sheer away the connection between the intima and the media, so with each heartbeat, blood will sort of jet through this tear and cause even more tearing, sort of like when you're peeling a hard-boiled egg and you get through that little membrane under the shell, right? After that membrane gets broken, you can easily peel off the rest of the eggshell. It's a similar concept, and as more and more blood enters the plane between the intima and the media, this dissection sort of keeps just getting bigger and wider, so from here, usually one of two things happen. You either get continuation of the tear, just continuing to be propagated, or you can get an exit tear that has blood able to re-enter the lumen of the aorta. So if the tear continues to propagate and more and more blood just leaves the circulation to enter the dissection, that just means more and more blood is not getting around the the rest of the body to do its job, and conceivably, you could lose liters of blood into a dissection because as the blood continues to get into the dissection, it will just continue to tear along that intimal, medial plane, and continue to tear down the length of the aorta, collecting blood, and so you can imagine that would pretty quickly lead to really severe conditions, like hypotension or shock, so in an aortic dissection, you've created a true lumen, which is lined by intima on both sides, and a false lumen, which is between the intima and the media, and some people develop other pathologies within the false lumen later on, so now you know what a dissection is, but what causes it? Let's make some space here. The most common cause of a dissection is chronic hypertension, which is pathologically high blood pressure. In fact, hypertension is present in at least two thirds of all cases of dissection, and is due to the stress and degenerative changes that hypertension causes within the aorta. Another cause of dissection is a connective tissue disease, such as marfan syndrome, or ehlers-danlos syndrome. These conditions weaken the walls of the aorta, and other connective tissues in your body. You might have a pre-existing aneurysm, which is a weakening and bulging out of the wall of the aorta, and that can predispose you to a dissection. Trauma to the chest can cause a dissection. For example, if you're in a motor vehicle accident. Men are actually 2-3 times more likely than women to develop an aortic dissection, and increasing age is another risk factor for development of dissection. For example, the peak incidence that we see dissection is between 50 and 65 years old, unless you had one of those underlying connective tissue disorders. Then, the peak incidence would be between about 20 to 40 years old. So, there are two major ways to classify aortic dissections. The Stanford's System, and the DeBakey System, but we'll just look at the Stanford System here, and it's important to find out what classification a dissection falls into, because they have implications for treatment, as we'll see, so let's just make some space here. So there's two major types of dissections under the Stanford's System. There's Type A, and there's Type B. So Type A dissections involve any part of the aorta before the origin of the left subclavean artery, so the left subclavean artery starts here, and that means that a Type A dissection can start anywhere along this part of the aorta, and a Type B dissection involves the descending thoracic aorta after the left subclavean artery origin, so a Type B dissection can involve any other part of the aorta from here down, and again, these are important to find out because they have implications for treatment, so Type A dissections usually require surgery, and Type B dissections are usually treated medically, so without surgery, and so with this A and B system in mind, there's actually a couple sites of tears that are pretty common in people who get aortic dissections, so the most common site of tears is about two centimeters above the aortic root, so right about here, and so that would be a Type A dissection. Another really common site is just after the left subclavean artery, so about here, so that would be a Type B dissection, but overall, about 90 percent occur within ten centimeters of the aortic valve, and the aortic valve is here in the heart, at the top of the left ventricle. Then why are people getting dissections in these areas in the first place? Why in these two most common areas, and why within ten centimeters of the aortic valve? Well, if you can imagine the left ventricle pumping blood out to this proximal area of the aorta over and over and over again, these areas get the most immediate, sheer stress with each pump of the heart, and so over time, they begin to weaken, and get predisposed to tearing. Also, blood is more turbulent in the beginning part of the aorta because it has to sort of navigate this turn of the aorta, and because of that, a lot of blood will sort of bump into the sides of the aorta, and over time, that weakens the walls, as well. Now, someone who's having an aortic dissection, what symptoms might they feel? Well, the most commonly described symptom is a sudden onset of severe and central chest pain that often radiates to the back. Patients often describe a tearing toward the back type of pain, and the pain can actually radiate down the arms, similar to the symptoms of a myocardial infarct, which is what we know as a heart attack, but other symptoms include sweating, nausea, shortness of breath, weakness, or syncopy. That's fainting, and if you have an aortic dissection in your abdominal aorta, you might get abdominal pain that could radiate to your sides or toward your back. In terms of diagnosis, diagnosis can be sometimes a bit difficult because aortic dissection can present similarly to a handful of other conditions, so doctors need to take a good history and examine a patient for certain signs in order to diagnose aortic dissection, but there also needs to be a few different types of imaging done on a patient to directly observe the dissection and to find out exactly where it is, if there is one, because knowing where it is is pretty key to treating it, as we sort of alluded to earlier when we were classifying. A transesophogeal echocardiogram, or TEE, is a test that uses sound waves to produce images of a patient's heart that we can look at. This is useful for dissection in that it's quick to do and cheap, but it can only see the thoracic aorta and the aortic valve. It can't see the abdominal aorta, which is a potential problem since it's possible to get dissections down there, as well. A CT, or computerized tomography scan with contrast, is really the gold standard of diagnosing dissection. For one, you can use it to image the entire length of the aorta to make sure you check all of it for dissections, and two, you can inject a liquid into the patient's circulatory system that contains iodine, and that will make the heart, the aorta, and other blood vessels more visible on the images that you get from the CT scan. Last, a magnetic resonance angiogram, or an MRA, is actually 100 percent sensitive and specific for picking up aortic dissections. That means that if there is a tear, the MRA will pick it up and allow you to accurately diagnose it as an aortic dissection. This scan interacts with your body's magnetic field to create images of the structures in your body. In this case, your aorta, and I'll just emphasize that even though an MRA is 100 percent sensitive and specific, a CT is still the gold standard because doing a CT comes with less risks to the patient than does an MRA.