Q. I’m a little confused about the difference between an aortic dissection and a false aneurysm.

In diagrams of aortic dissection, it looks like all three layers of the vessel have been damaged and blood is leaking out of the vessel BUT still contained by connective tissue etc. Isn’t that what a false aneurysm is? So what’s the difference between the two?

A. I can see what you mean – diagrams of aortic dissections can be misleading!

Diagrams of aortic dissection (like this one above, from Wikipedia) often focus on the three types of dissections. The point of these diagrams is just to show the place in the aorta where the dissection begins (near the heart vs. more distally). They can be kind of misleading, though, if they don’t clearly depict how the blood tunnels through the wall of the aorta. Like you described, it can look more like the aorta has actually ruptured all the way through, and the blood is collecting outside the aorta.

So let’s quickly review the definitions of false aneurysm and dissection.

In a false aneurysm, all three walls of the vessel have been broken through, and blood collects just outside the vessel. It doesn’t tunnel down or up into the wall of the vessel at all. After some time, the blood organizes and becomes firm – kind of like a blood clot – and it prevents further blood from escaping the damaged vessel. 

In a dissection, only the inner portion of the vessel wall is damaged. Blood enters into that damaged area, and tunnels up or down within the wall of the vessel. Unlike a false aneurysm, in which blood bursts through all three layers of the vessel, in a dissection, the outer layers of the vessel are still intact, and blood forms a channel within the vessel wall itself.

So you had the right idea! It was just the diagram that threw you off. In the diagram above, the arrangement of the two little white arrows incorrectly implies that blood is busting all the way through the aorta at a single point – but the rest of the aortic wall looks intact. To really show what’s happening in an aortic dissection, the wall of the aorta should be more clearly depicted, and the second arrow should point up or down within the wall itself, showing the path of blood as forms a tunnel within the vessel wall.