Endovascular aneurysm repair (EVAR) has become the standard procedure used to treat abdominal aortic aneurysm in patients with suitable anatomy, which is mainly attributed to its lower perioperative mortality, morbidity, and shorter length of hospital stay than conventional open surgery [1, 2, 3]. However, postoperative endoleaks, the persistence of blood flow within the aneurysm sac, are the main causes of sac growth and reintervention compromising the long-term durability of EVAR [4, 5].

There is no debate that type I and type III endoleaks, which involve the stent landing zones or junction points between the stents, should be treated promptly. However, the management of type II endoleaks (T2ELs), which account for approximately half of all endoleaks and are caused by retrograde flow from the aortic side branches, remains controversial [6, 7, 8]. Some reports have shown that the T2ELs are a benign entity [9], while others have associated T2ELs with adverse outcomes such as aneurysm sac growth and aneurysm rupture [10, 11]. Current guidelines such as the 2019 European Society for Vascular Surgery (ESVS) guideline and Society for Vascular Surgery implementation of clinical practice guidelines have recommended conservative management and intervention was indicated for significant sac expansion ($\ge$10 mm or 5 mm) [7, 12], whereas some have advocated an immediate treatment [13, 14].

To explore the optimal management strategy for T2ELs, understanding the natural course of T2EL and its influence on long-term outcomes is necessary. Thus, this study aimed to report the natural history of T2EL based on a 10-year study at a single medical center and analyze the long-term effect of T2EL.

Given the conflicting reports regarding whether T2ELs are a benign entity or related to adverse outcomes, debates remain regarding the optimal management strategy for T2ELs. The present study was conducted to report the natural history of T2EL and its influence on long-term outcomes based on a 10-year study at a tertiary medical center. The presented study found that the incidence of an isolated T2EL was 27.5% (79/287), and 36.7% (29/79) of T2ELs showed spontaneous resolution; patients with an isolated T2EL showed a higher aneurysm sac growth rate than patients without endoleak; no significant difference in overall survival was found between patients with or without an isolated T2EL; no significant differences in overall survival and sac growth were identified in patients with an early T2EL and patients with a late T2EL.

The incidence of isolated T2EL in the present study was in line with the incidence in previously published articles (8%–44%) [16, 17]. 36.7% (29/79) of T2ELs showed spontaneous resolution in the present study, which was also consistent with the published reports (30% to 50%) [12]. There was only one (1.3%, 1/79) aortic rupture that occurred in patients with T2ELs, which was similar to the rupture rate in current literature (close to 1%) [6]. Patients in the isolated T2ELs group showed a higher incidence of aneurysm sac growth than patients in the no endoleak group (21.5% vs 4.3%, p 0.001). However, the all-cause mortality between the two groups had no significant differences, and other studies have also shown similar results [18, 19]. In Cox regression analysis, higher age was independently associated with worse survival, which was consistent with the results in another study [18]. All the data mentioned above confirmed that the majority of isolated T2ELs are clinically not of concern. A “watch and wait” strategy for the management of T2ELs is feasible. Based on the results in the present study, just as recommended by the 2019 ESVS [7] and 2018 SVS guidelines [8], a less firm surveillance protocol for patients undergoing EVAR might be safe. And more future studies are needed to confirm the reasonability of the recommended follow-up protocol. Moreover, the present study included a relatively large sample size in the Asian population, thus the experience could be better applied to manage T2ELs in the corresponding population. Our study showed similar results with western studies which have been mentioned above. Thus, the incidence and management strategy of T2ELs might be comparable in the two different populations (Asian and Western).

Among current reports, the treatment modalities for persistent T2ELs management included transarterial embolization, transcaval or translumbar embolization, laparoscopic ligation, sacotomy, or graft explant [20]. However, the treatment of T2ELs is technically challenging and time-consuming, with unsatisfactory clinical success. The recurrence of T2ELs after intervention can be as high as 60% [14, 20]. In the present study, a total of 11 patients with T2ELs received the intervention, but only 2 patients (2/11, 18.2%) showed complete resolution of the endoleak during the follow-up. In Loy’s study, there were 14 patients with T2ELs underwent secondary interventions and only 5 patients (5/14, 35.7%) had complete and sustained resolution of T2EL [21]. Moreover, Sana Mulay et al. [18] recently reported that neither survival nor aneurysm-related outcomes will be improved for patients treated for T2ELs. Based on the above-mentioned results, a more conservative protocol for selecting high-risk T2ELs patients for intervention is rational.

Some studies have reported prophylactic embolization of aortic side branches or aneurysm sac before or during EVAR to reduce the incidence of T2ELs, and results identified this method might be associated with a lower rate of sac growth and incidence of T2ELs [13, 14, 22]. However, most of them are single-center retrospective studies with a small sample size. The exact clinical effect other than the reduction of endoleaks is still needed to be explored. The benefit of selective prophylactic embolization and the economic cost, technical fitness also should be considered. Thus, prospective randomized trials with a large sample size are needed to evaluate the advantages of prophylactic embolization.

In the present study, no differences were found in spontaneous resolution rate, aneurysm sac growth rate, and all-cause mortality between patients in the early T2ELs group and late T2ELs group. While other studies reported that late T2ELs were less likely to have spontaneous resolution than early T2ELs and patients with early T2ELs were less likely to survive than patients with late T2ELs [18]. The different results might be attributed to the following reasons: First, the sample size in our study was small which may cause selection bias. Second, there is no exact definition for the endoleak stage in current literature. For example, the early T2EL was defined as an endoleak visualized on first-year follow-up in Sidloff DA’s study [23]; the early T2EL was defined as an endoleak visualized on 3-month follow-up in Mulay S’s study [18], and the early T2EL was defined as an endoleak visualized on 1-month follow-up by other researchers [14, 17]. Third, the present study may cause information bias. It is possible that some endoleaks were incorrectly categorized or not detected because the CTA imaging mainly detects relatively large endoleaks, more sensitive imaging such as blood pool MRA may be needed to detect small T2ELs [18]. Thus, more studies are needed to explore the exact clinical significance of the stage of T2ELs.

There are some limitations in the present study. First, 36.1% of patients undergoing EVAR were excluded in the present study because most of our patients undergoing EVAR were lost to follow-up. And this could not be improved due to its retrospective nature and low patients’ compliance. Thus, the results of this study should be viewed with an inherent bias due to the retrospective nature and small sample size. Second, this is a single-center study and the follow-up time was variable (range, 2–119.7 months), which may lead to selection bias. Finally, the cause of death in some cases was unknown, it was not possible to evaluate the impact of T2ELs on aneurysm-related mortality.

Type II Endoleak was significantly associated with aneurysm sac growth and no association with survival was observed.

Study conceptualization and design—YLW, FY, YWB; Study supervision—BX; Data collection—WY, CZ, JCL, SGJ, CYW, CTY; Data analysis and interpretation—SJH, CTY, YC, YLW, TQL, JCL; Draft writing—YLW; Draft revising—FY, YWB, CZ, CYW, SGJ, BX; All authors contributed edits, read and approved the final manuscript version.

The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Ethics Committee of Wuhan union hospital and written informed consent was waived because of the retrospective nature (Approval number: 20211101).

We would like to express my gratitude to Huimin Zhou (Wuhan Tongji Hospital) who helped us edit the language. Thanks to all the peer reviewers for their opinions and suggestions.

This research received no external funding.

The authors declare no conflict of interest.