Understanding the natural history of thoracic aortic aneurysms (TAAs) is essential to patient care and surgical decision making. In this evidence summary we discuss some of the most clinically relevant features of the disease. The true incidence of TAAs is likely to be higher than currently reported because of the inherently silent nature of TAAs. However, TAAs can become rapidly lethal once dissection or rupture occurs, highlighting the need for more robust screening. The impressive discovery of familial patterns and novel genetic loci for TAAs challenges the idea that most TAAs are simply sporadic. Although the aorta grows in an indolent manner, its rate of growth and its current diameter both have important clinical implications. Biomechanical studies have supported clinical findings of 6.0 cm as a dangerous threshold. Surgical extirpation of TAAs is currently the mainstay of effective treatment. Although endovascular TAA repair is becoming increasingly common, long-term safety remains unproven. We still need more data to support the concept that any medical therapy is effective.

Background: Current guidelines recommend a diameter of 5-5.5 cm as the threshold for surgery on the ascending aorta. However, a study from the International Registry of Acute Aortic Dissection showed that nearly 60% occurred at <5.5 cm (the ‘aortic size paradox') - leading to a debate whether the size threshold should be lowered. However, the study showing dissection at small size had no knowledge of the population at risk. Herein, we aim to calculate the relative risk of aortic dissection at sizes <5.5 cm by analyzing both the number of occurring dissections (numerator) and the population at risk at each aortic size (denominator). Methods: Using a publicly available database of 3,573 multiethnic subjects (46% male, mean age 60.7 years) from the general population, we plotted a distribution curve of ascending aortic size (by magnetic resonance imaging). The relative risk of aortic dissection was calculated by dividing the proportion of dissections occurring at each size (numerator) by the proportion of aortas of that same size in the general population (denominator). Results: The mean ascending aortic diameter of the reference population was 3.2 cm (±0.4 cm). The largest diameter was 4.9 cm in women and 5.0 cm in men. The proportion of subjects with an aorta <3.5 cm was 79.2%, that of subjects with 3.5-3.9 cm was 18.0%, that of subjects with 4.0-4.4 cm was 2.6%, and that of subjects with ≥4.5 cm was 0.22%. The relative risk of dissection in those categories was found to be 0.055, 2.5, 4.9, and 346.8, respectively. Patients with an aorta ≥4.5 cm were 6,305 times more likely to suffer aortic dissection than those with an aorta <3.5 cm. Conclusions: The normal aorta is deceptively small, most commonly <3.5 cm. The aortic size paradox is a byproduct of the very large number of patients in small size ranges. This study fully supports current recommendations for surgical intervention at 5-5.5 cm.

Recent studies have confirmed a close association between various medical conditions (intracranial aneurysm, abdominal aortic aneurysm, temporal arteritis, autoimmune disorder, renal cysts), certain aortic anatomic variants (bovine aortic arch, direct origin of left vertebral artery from aortic arch, bicuspid aortic valve), and family history of aneurysm disease with thoracic aortic aneurysm and dissection. This paper reviews these associations. We propose to capitalise on these associations as powerful and expanding opportunities to diagnose the virulent but silent disease of thoracic aortic aneurysm. This can be accomplished by recognition of this ‘guilt by association’ with the other conditions. Thus, patients with associated diseases and anatomic variants should be investigated for silent aortic aneurysms. Such a paradigm holds substantial potential for reducing death from the silent killer represented by thoracic aortic aneurysm disease.

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