1/ Ever wonder: Why are abdominal aortic aneurysms (AAAs) more common than thoracic aortic aneurysms (TAAs)? I have! Follow this thread to see how the answer to this question makes use of long-forgotten medical classes. And there’ll be an appearance by The Great Mimicker.

2/ First, let’s confirm that AAAs are more common than TAAs. Here’s the breakdown of discharges/year: AAA: 63% TAA: 26% Thoracoabdominal: 8% Unspecified: 3% circ.ahajournals.org/content/121/13…

3/ What explains this difference? Does it have to do with differences in blood pressure or blood flow experienced at each segment of the aorta? Or, are there differences in anatomy?

4/ It’s doesn't seem to be blood pressure or blood flow. As this diagram shows, the mean arterial pressure doesn’t change much along the aorta and blood flows are, if anything, highest in the thoracic aorta. Are there anatomic differences? Yes!

5/ The differences in anatomy result from distinct embryologic origins. The smooth muscle cells of the TA derive from neural crest cells whereas the AA derives from mesodermal cells. Amazingly, the thoracic and abdominal aorta are distinct organs! linkinghub.elsevier.com/retrieve/pii/S…

6/ As a result of the difference in origin, the TA and AA smooth muscle cells respond differently to cytokines and growth factors. This, and other factors, lead to very different final structures. I’ll highlight two.

7/ First, the thoracic aorta has more elastin and collagen. More elastin results in increased compliance and lessens the propensity of the TA to aneurysmal degeneration. More collagen adds to the "safety net" this strong protein provides. linkinghub.elsevier.com/retrieve/pii/S…

keyboard_arrow_left Previous keyboard_arrow_right Next

8/ Second, the TA is vascular (it has a vasa vasorum, the “vessels of the vessel”) whereas the AA is avascular. Put another way, the TA has a richer supply of nutrients. circres.ahajournals.org/content/20/4/4…

9/ Do these differences play out in other pathologies of the aorta? Of course they do. One disease that becomes easier to explain once you understand the above is syphilitic aortitis. What part of the aorta does this typically affect?

10/ Syphilitic aortitis is more common in the thoracic aorta. But that's odd; the TA has so much "protection"... Recall that treponema likely migrate hematogenously. So, they will affect the body parts that have a rich blood supply. The TA has one of those (the vasa vasorum!).