Often known as a “silent killer,” ascending thoracic aortic aneurysms (ATAAs) may grow asymptomatically until they rupture, at which point, mortality is over 90%.
But and by extension, for those who have one, a significantly reduced risk for coronary artery disease and myocardial infarction (MI).
“We noticed in the operating room that many patients we worked on who had an ATAA had pristine arteries, like a teenager’s,” said John Elefteriades, MD, William W.L. Glenn Professor of Cardiothoracic Surgery and former chief of cardiothoracic surgery at Yale University and Yale New Haven Hospital, New Haven, Connecticut. “The same was true of the femoral artery, which we use to hook up to the heart-lung machine.”
Elefteriades and colleagues have been investigating the implications of this association for more than two decades. Many of their studies are highlighted in a recent review of the evidence supporting the protective relationship between ATAAs and the development of atherosclerosis and the possible mechanisms driving the relationship.
“We see four different layers of protection,” said Sandip Mukherjee, MD, medical director of the Aortic Institute at Yale New Haven Hospital and a senior editor of the journal AORTA. Mukherjee collaborated with Elefteriades on many of the studies.
The first layer of protection is lower intima-media thickness, specifically, 0.131 mm lower than in individuals without an ATAA. “It may not seem like very much, but one point can actually translate into a 13%-15% decline in the rate of myocardial infarction or stroke,” Dr. Mukherjee said.
The second layer is lower levels of low-density lipoprotein (LDL) cholesterol. Lower LDL cholesterol levels (75 mg/dL) were associated with increased odds of ATAAs (odds ratio [OR], 1.21), whereas elevated levels (150 mg/dL and 200 mg/dL) were associated with decreased odds of ATAAs (OR, 0.62 and 0.29, respectively).
Lower calcification scores for the coronary arteries are the third layer of protection (6.73 vs 9.36 in one study).
The fourth protective layer is a significantly reduced prevalence of coronary artery disease. A study of individuals with ATAA compared to controls found 61 of those with ATAA had coronary artery disease vs 140 of controls, and 11 vs 83 had experienced an MI. Of note, patients with ATAAs were protected despite having higher body mass indices than controls.
Other MI risk factors such as age increased the risk even among those with an ATAA but, again, much less so than among controls; a multivariable binary logistic regression of data in the team’s review showed that patients with ATAAs were 298, 250, and 232 times less likely to have an MI than if they had a family history of MI, dyslipidemia, or hypertension, respectively.
Why the Protection?
The ligamentum arteriosum separates the ascending from the descending (thoracoabdominal) aorta. ATAAs, located above the ligamentum, tend to be pro-aneurysmal but anti-atherosclerotic. In the descending aorta, below the ligamentum, atherosclerotic aneurysms develop.
The differences between the two sections of the aorta originate in the germ layer in the embryo, Dr. Elefteriades said. “The fundamental difference in tissue of origin translates into marked differences in the character of aneurysms in the different aortic segments.”
What specifically underlies the reduced cardiovascular risk? “We don’t really know, but we think that there may be two possible etiologies,” Dr. Mukherjee said. One hypothesis involves transforming growth factor–beta (TGF-beta), which is overexpressed in patients with ATAA and seems to increase their vulnerability to aneurysms while also conferring protection from coronary disease risk.
Some studies have shown differences in cellular responses to TGF-beta between the thoracic and abdominal aorta, including collagen production and contractility. Others have shown that some patients who have had an MI have polymorphisms that decrease their levels of TGF-beta.
Furthermore, TGF-beta plays a key role in the development of the intimal layer, which could underpin the lack of intimal thickening in patients with ATAA.
But overall, studies have been mixed and challenging to interpret, Dr. Elefteriades and Dr. Mukherjee agreed. TGF-beta has multiple remodeling roles in the body, and it is difficult at this point to isolate its exact role in aortic disease.
Another hypothesis involves matrix metalloproteinases (MMPs), which are dysregulated in patients with ATAA and may confer some protection, Mukherjee said. Several studies have shown higher plasma levels of certain MMPs in patients with ATAAs. MMPs also were found to be elevated in the thoracic aortic walls of patients with ATAA who had an aortic dissection, as well as in the aortic smooth muscle cells in the intima and media.
In addition, some studies have shown increased levels of MMP-2 in the aortas of patients with ATAAs compared with patients with coronary artery disease.
Adding to the mix of possibilities, “We recently found a gene that’s dysregulated in our aneurysm patients that is very intimately related to atherosclerosis,” Dr. Elefteriades said. “But the work is too preliminary to say anything more at this point.”
“It would be fabulous to prove what it is causing this protection,” Dr. Mukherjee added. “But the truth is we don’t know. These are hypotheses.”
“The most important message from our work is that most clinicians need to dissociate an ATAA from the concept of atherosclerosis,” Dr. Elefteriades said. “The ascending aorta is not an atherosclerotic phenomenon.”