We present a single-center series as well as the systematic review and meta-analysis focusing on procedural complications, morbidity and mortality in very small IAs patients who undergoing endovascular or microsurgical treatment. With a 1.7% morbidity and a 0% mortality, our results found that very small UIAs can be effectively treated with very low neurological complication rate. However, the risk of poor outcome in patients with very small RIAs remains high, reaching 17.6%, even after effective endovascular or microsurgical treatment.
The International Subarachnoid Aneurysm Test (ISAT) [23] is a randomized controlled trial designed to compare the safety and efficacy of endovascular coiling and surgical clipping in SAH patients. However, these results can not be applied to very small IAs patients simply, because there are some differences in natural history, aneurysm characteristics and so on. Although no randomized clinical trial (RCT) comparing these two treatment methods, some retrospective and prospective studies [24,25,26,27,28] reported the outcomes of clipping or coiling in very small UIAs. Moroi et al. [29] reported the total morbidity and mortality of 368 cases of UIAs treated with clipping were 0.3% and 2.2% respectively, while the morbidity and mortality of small UIAs in different location of aneurysm were lower than those of large UIAs. Raoul et al. [24] reported the 67 very small UIAs treated by coiling, and the good outcome rate was 97.0%, and the intraprocedural perforation and neurological morbidity rate were 1.7% and 2.8%, respectively. Their study showed that the rate of procedural perforation was lower than previously reported results with technical evolution of endovascular devices. In another retrospective study of 287 patients with small UIAs [30], most of the patients had a favorable outcome after coiling, with the intraoperative rupture rate of 1.2% and the incidence of perioperative thrombotic events of 4.2%, morbidity and mortality of 0.9% and 2.4%, respectively. Because of incomplete occlusion of aneurysm, the recanalization rate of small UIAs within 3 mm is clearly higher than that of 3–5 mm small UIAs. Pierot et al. [25] reported that the failure rate of coiling in very small IAs was higher than that in large IA (13.7% vs. 3.3%), and suggested that for this very small UIAs with low risk of rupture, preventive treatment should be carried out after morphological changes in imaging follow-up. However, Hwang et al. believed coiling of very small aneurysms may be technically feasible with favorable clinical outcomes and relatively low recanalization rate. that recently, another multicenter study [27] reported that clipping is safe and effective in the treatment of very small UIAs. After long-term follow-up, the total morbidity decreased to 2.7% and no operation-related death occurred, and the posterior circulation is a significant risk factor for early neurological deficit. Rahmanian et al. [28] reported 15 cases of treatment outcome for very small UIAs, and believed that surgical clipping is a safe and effective modality of treatment associated with low mortality and morbidity. In our series, 31 patients with very small UIA have good outcomes after either coiling or clipping, with very low morbidity (1.7%) and no death. A comparative effectiveness analysis [31] suggested that endovascular coiling should be performed directly if annual risk of rupture of very small UIA is more than 1.7%. Therefore, the high-risk patients with very small UIAs should be identified and preventively treated.
In a meta-analysis of 1105 patients with very small IAs in 22 studies [32], the morbidity and mortality associated with coiling were higher in very small RIA than that in very small UIA (4.0% vs. 2.0%; 3.0% vs. 2.0%), and the incidence of neurological complications was higher in very small RIAs than that in very small UIAs (6.5% vs. 5.0%). In a recent study [28] of 52 patients treated with surgical clipping, the mortality rate was 0% in very small UIAs and 3.8% in very small RIAs. Due to the complications associated with aSAH, the morbidity and mortality of the very small RIA patients were higher than that of the very small UIA patients. For very small RIAs, either endovascular or microsurgical treatment is more difficult and challenging. Hong et al. [33] reported 49 patients with very small RIAs treated with endovascular coiling. Among them, 44 (89.8%) cases recovered well, 4 (8.2%) had moderate and severe disability, and 1 (2.0%) died. Chung et al. [34] considered that endovascular treatment of very small RIAs is technically feasible, but it is associated with an increased rate of intra-procedural complications. Hwang, et al [35] reported that of 23 patients receiving endovascular coiling, 17.4% had residual neurological dysfunction and 8.7% mortality. Twenty-eight patients (77%) were followed up for more than 6 months by angiography and/or MRA. The minor recanalization rate was 6%, and the major recanalization rate was 3%. Zhang et al. [36] suggests that stent assisted coiling can significantly reduce the recurrence of aneurysms without increasing the risk of additional operation. In a cohort study of 91 coiled patients versus 60 clipped patients, Chalouhi et al. [37] found that surgical clipping of very small RIA was associated with a higher incidence of perioperative complications, but the overall disability outcomes were similar. In our series, the good outcome rate of 48 patients receiving endovascular therapy was 91.7%, and that of 78 patients receiving microsurgical treatment was 76.9%. The difference in outcomes was mainly due to more patients with poor Hunt-Hess grade received clipping in our center. Jian et al. [38] reported that there was no significant difference in the outcomes of 162 patients with very small RIAs after the endovascular coiling or surgical clipping, and the Hunt-Hess poor grade are significant risk factors for poor outcomes.
This study has some limitations. First of all, most of the studies on coiling or clipping are single center retrospective analysis, and there are no RCT studies on coiling versus clipping for very small IAs. Secondly, there is a possible publishing bias, because series with more positive results may be more easily reported and published. Nevertheless, our results have certain guiding significance for individualized treatment decisions for very small IAs due to the treatment risk of UIAs or RIAs is acceptable. Based on our findings, we conclude that very small UIAs can be treated effectively and safely with good long-term functional and angiographic outcomes. However, very small RIAs patients are at high risk of poor outcome and the incidence of neurological complication should not be ignored. The morbidity and mortality of clipped patients with very small RIAs were 2% and 3%, respectively, and the morbidity and mortality of coiled patients with very small RIAs were 4% and 6.5%, respectively. The pooled results indicated that there was no significant difference in poor outcome between the endovascular and microsurgical treatment of very small RIAs. A large-scale randomized controlled trial is needed to verify the difference between clipping and coiling of very small UIAs in the future.