The prevalence of thoracic aorta aneurysm is ≤ 6–10 cases per 100,000 individuals [1]. Shang et al. [2] reported that saccular aortic aneurysms account for 1.5% of all aortic aneurysms. The causes of saccular aneurysm are trauma, infection, autoimmune disorders (e.g., Behcet’s disease or other collagen vascular diseases), or previous cardiovascular surgery [3].
However, the mechanism underlying the occurrence of saccular aneurysms has not been elucidated thus far.
It has been reported that blood flow to one side, which is partly due to the weakness of blood vessels, may cause a saccular aneurysm. The geometry of the aortic arch is characterized by curvature and the presence of major branches on the outer curvature. These characteristics play important roles in the development and natural course of saccular aortic aneurysms in this curved region [4]. In fact, cerebral arteries often branch at acute angles, and 90% of cerebral aneurysms are saccular aneurysms [5].
The most common site of saccular aneurysm occurrence is at the bifurcation of the anterior commissural artery. In this bifurcation, the antegrade blood pressure impinges on one side and, depending on the bifurcation angle, the wall stress is stronger. These characteristics increase the incidence of saccular aneurysms [6].
Morphologically, the risk of rupture is mainly evaluated based on diameter of the aneurysm. The relationship between aneurysm diameter and rupture is supported by Laplace’s law. Laplace’s law (Pierre-Simon Laplace, 1806) is a principle of physics stating that the tension on the wall of a sphere. Thus, larger aneurysm diameters are linked to greater wall stress.
However, in recent years, computational biomechanical modeling has been used to evaluate aortic wall stress and possibly predict the behavior of aortic aneurysms to improve risk stratification [9, 10]. The peak wall stress has been identified as a more sensitive and specific predictor of rupture than aneurysm diameter alone [11, 12].
Based on finite element analysis, a sac depth/neck width > 0.8 [7] and aspect ratio (vertical diameter/horizontal diameter) of < 1.0 indicate a high risk of rupture [8]. However, although the aneurysm in the present case had a large diameter and extremely prominent meeting conditions i.e., a sac depth/neck width > 0.8 and aspect ratio < 1.0 in the neck, it did not rupture. The existence of such cases suggests the involvement of factors other than the diameter and type in aneurysm rupture.
Interestingly, in finite analysis, the neck of the aneurysm is the highest peak wall stress and is linked to the rupture point [13, 14].
Surprisingly, in this case, the neck of the aneurysm was completely covered with mural thrombus, and the rupture point was located beneath those thrombus. Consequently, the rupture point was not exposed to wall stress. This was potentially attributed to polycythemia, which is associated with the formation of mural thrombi [15].
Furthermore, in this case, similar to coil embolization of the aneurysm [16,17], the mural thrombus that rapidly formed due to polycythemia may have formed a thrombus within the aneurysm while maintaining the intima.
Additionally, this may prevent rupture because the walls of ruptured saccular aneurysms are characterized by loss of endothelial cells [18]. The combination of endothelial cell loss, and the proteolytic injury by wall stress predisposes the saccular aneurysm wall to rupture. Hence, the loss of endothelial cells is a key event that leads to the degeneration and eventual rupture of the saccular aneurysm wall.
In addition, it has been reported that the disappearance of endothelial cells introduced by blood flow and conversion into a pseudoaneurysm are associated with a high risk of rupture. Moreover, the occurrence of intima hyperplasia reduces the risk of aneurysm rupture [18].
Conversely, the presence of a mural thrombus reduces blood flow from within the blood vessel to the wall, causing ischemia and weakening the blood vessel wall [18]. Indeed, very high endogenous peroxidase activity is observed in peroxidase-stained samples of saccular aneurysm walls with a luminal thrombus [19]. Furthermore, the presence of adherent inflammatory mononuclear cells on the endothelium indicate dysfunction of the endothelium in this area. Dysfunction may be associated with impaired rheology in this vasa vasorum vessel. Violation of the blood circulation to the aortic wall causes media necrosis and contributes to the development of aortic aneurysm dissection [20].
However, in this case, mural thrombus was promoted by polycythemia, and the location in the thoracic, where vasa vasorum was abundant [21], avoided the effects of ischemia.
In addition, it has been reported that patients with a mural thrombus have a lower peak wall stress and a lower risk of rupture than those without a wall thrombus [12].
In this case, a mural thrombus (likely formed due to polycythemia) led to vascular necrosis and formation of a huge aneurysm; however, the mural thrombus covered the neck of aneurysm as a rupture point, effect of preserving intima like coil embolization. Additionally, a decrease in peak wall stress may be possible to reduce the rupture risk even for huge saccular aneurysms. Moreover, the aneurysm was formed in the thoracic aorta, where the vasorum is abandoned reducing the ischemic effect.
It would be desirable to evaluate not only the aneurysm diameter and type but also peak wall stress, presence of an intramural thrombus covering the neck of the aneurysm, tissue changes (i.e., presence or absence of endothelial cells), and development of the vasa vasorum. This approach would allow physicians to make judgments based on more multifaceted findings and, consequently, perform a more accurate assessment of the risk of rupture.
There are some limitations in this study. Firstly, the actual peak wall stress around the aneurysm in this case is unknown. Secondly, it is unknown whether endothelial cells remained in the aneurysm. Thirdly, it is difficult to evaluate endothelial cells and vasa vasorum preoperatively. It would be beneficial in the future, if we could use modalities, such as CT or magnetic resonance imaging for evaluation. Finally, this is a case report; thus, verification of the findings in a study involving a larger population is required in the future.