This study investigated the results of embolization methods applied to T2EL after EVAR. The results highlighted the significant impact on sac diameter when successful direct TAE of both the nidus and its associated branches was achieved. Furthermore, apart from previously reported risk factors for sac enlargement, there was a notable correlation between direct embolization techniques, using coils and incorporating NBCA glue as needed, and sac enlargement. Among patients who underwent successful direct TAE, the rates of sac non-expansion were 100%, 95%, and 95% at 1, 3, and 5 years, respectively.
Previous reports suggested that sac diameter before embolization influenced the embolic effect of TAE (Horinouchi et al. 2020). Contrary to the findings in those reports, this study revealed that successful direct TAE of the nidus and branches, with the adjunctive use of coils and as-needed NBCA glue, was the critical factor influencing sac diameter reduction. A history of dyslipidemia and the use of antiplatelet medications are associated with multiple embolization procedures, and smoking and the presence of a moyamoya nidus, characterized by unclear boundaries, were known factors that weaken the effect of TAE on the inhibition of sac diameter increase (Charitable et al. 2021; Iwakoshi et al. 2023; Sarac et al. 2012). The larger the thrombus volume in the sac, the more effectively it inhibited the increase in sac diameter (Fujii et al. 2020). The results of the current study suggest that moyamoya endoleak is associated with the use of antiplatelet agents, suggesting that inadequate thrombosis of the aneurysm may lead to moyamoya endoleak. Although achieving effective embolization in the presence of moyamoya endoleak is challenging (Iwakoshi et al. 2023), it is speculated that the antiplatelets effects, in addition to the small size of the nidus making it difficult to embolize, may contribute to the reduced efficacy of the embolization process.
And then, based on the results of this study, two important technical factors in endoleak embolization were identified, the first being direct nidus embolization. This was consistent with many reports indicating the usefulness of embolization via direct puncture of the nidus (Guo et al. 2020; Mewissen et al. 2017; Nana et al. 2022). Furthermore, in some reports, embolization of the branch was not always necessary if the nidus was properly embolized (Yu et al. 2017). In a meta-analysis, it was observed that direct puncture embolization of the nidus achieved a higher clinical success rate than TAE. This difference in clinical success could be attributed to variations in the rate of embolization directly to the nidus (Guo et al. 2020). In contrast, embolization of the branch alone increased sac diameter even when the endoleak was properly embolized (Sarac et al. 2012). This finding was consistent with the conclusion that proper nidus embolization was necessary. The study also found that embolization of only the branches was less effective than direct TAE of the nidus and branches in preventing sac enlargement, even if the embolization was performed at a level directly connected to the nidus. These results supported that direct TAE of nidus and branches was an important technical component in endoleak embolization.
Another important consideration was using coils for embolization to prevent loss of embolic material in the nidus. TAE with liquid embolic material was reported as a useful tool for the embolization of T2EL. However, it is essential to note that these reports had a limited follow-up period of only 2 years (Abularrage et al. 2012). Also, TAE involving the nidus and branches using coils has been documented as beneficial. However, the particular report had a relatively short follow-up period of approximately 1 year (Kasirajan et al. 2003). This study revealed that embolizing branches exclusively with NBCA glue resulted in a statistically significant increase in sac diameter while using coils in the embolization of the nidus and its associated branches might contribute to inhibiting an increase in sac diameter. Based on these findings, it can be concluded that using non-dissipating embolic materials for nidus embolization might be an effective strategy to prevent sac enlargement.
This study had some limitations. First, owing to its retrospective and limited-scale design. The relatively small size of the patient subgroups poses a challenge for robust statistical interpretation. Second, accurately determining T2EL associated branches may be challenging because of the inability to perform contrast-enhanced CT scans in certain patient categories, including those with renal dysfunction, contrast medium intolerance, or absence of sac enlargement. And, angiographic visualization via a 1.9 Fr catheter typically fails to adequately opacify the nidus and the entirety of inflow/outflow branches, potentially obscuring the true anatomical and pathophysiological details of the endoleak. Should two or more branch vessels remain patent, the specific embolic agent used within the nidus and/or branch vessels may be inconsequential, as persistent flow can facilitate aneurysmal growth and allow the embolic material to displace from its initial placement. Therefore, this study does not assert the efficacy of using NBCA alone to embolize all potentially nidus-related inflow/outflow branches. Consequently, this study may underrepresent the occlusive efficacy of NBCA when used as the sole embolic agent.Third, it is important to note that this study followed the Japanese guidelines, focusing on EVAR treatment for smaller AAAs. This could potentially introduce a bias into this results, as the European Society for Vascular Surgery guidelines recommend considering larger AAA sizes. So, given that aneurysmal sac diameter is known to influence natural growth rates, this discrepancy could potentially affect T2EL efficacy outcomes in larger AAAs. Therefore, future studies focusing on larger AAA are required to verify these preliminary findings.