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.
