Coil embolization of intracranial aneurysms has become a common technique with recent developments in devices and adjunctive techniques. Although the overall outcome of coil embolization is deemed favorable, recanalization following treatment has become a greater concern compared with surgical clipping [2, 4, 15-17].
Recanalization rate after coil embolization has been reported to occur in 11.3–49% [1, 2] or 11.3–38.0% [1, 3] of UIAs. Recanalization rates vary widely across reports. This may be because of differences in the definition of recanalization, follow-up duration, image modality used for follow-up, and advances in imaging modalities. Meyers et al. highlighted the importance of consensus recommendations for reporting standards, terminology, and written definitions when reporting radiological evaluations and endovascular treatments of intracranial cerebral aneurysms  and recommended the use of a 6-point CGS for the evaluation of recanalization. These definitions are thus helpful for making direct comparisons between studies. Digital subtraction angiography remains the gold standard modality for follow-up; however, the superiority of the less invasive TOF MRA for follow-up evaluations for coiled aneurysms has been advocated in several reports [9, 18, 19]. Therefore, in our study, recanalization was defined as a worsening of CGS score by more than 1 point, and these patients were evaluated using 3D TOF MRA.
Several factors such as aneurysm size3)-8), neck width7)8), VER 3)6)8), and patient age4) have been reported as risk factors for recanalization. In our study, we found that AR and VER were significant predictors of recanalization. To date, AR has not been reported as a factor influencing recanalization.
AR is a pre-interventional morphological feature of aneurysms that is calculated by dividing aneurysm depth by aneurysm neck width. Generally, aneurysms with a high AR are embolized with ease compared with those with a low AR. Brinjikji et al. have reported that aneurysms with an AR and dome-to-neck ratio <1.2 usually require adjunctive techniques. Furthermore, AR was an independent predictor of the need for adjunctive techniques20). In this study, the ROC curve analysis showed that an AR of 1.5 was the cut-off value for recanalization after coiling (sensitivity, 56%; specificity, 65%). Therefore, all aneurysms were divided into two groups: group L (AR < 1.5) and group H (AR ≥ 1.5). Mean VERs of group H and L were 25.27% ± 6.06% and 25.74% ± 5.08%, respectively. No significant difference was observed between the two groups (p = 0.713). Our data demonstrated that a high AR is a risk factor of recanalization, whereas VER was equivalent across groups.
Bavinzski et al. investigated gross and microscopic histopathological findings of aneurysms treated with GDC obtained at autopsy. They discovered tiny open spaces between the coils and aneurysm neck on post-interventional angiographies, even when complete obliteration was achieved 21). Mitsos et al. have also performed hemodynamic simulation of aneurysm coiling using an anatomically accurate computational fluid dynamics (CFD) model22). On completion of coiling, flow at the coil mesh/parent vessel interface was detected (i.e., at the aneurysm neck area), even if packing densities over 24–25% were achieved. Moreover, gradual coil introduction resulted in a stepwise relief of wall pressure at the aneurysm dome, and a redistribution of wall pressure was identified at the aneurysm inflow zone. These factors may have contributed to recanalization after coil embolization and may correlate with parent vessel geometry.
Stent-assisted coiling (SAC) of intracranial aneurysms has been proposed for the treatment of fusiform or wide-neck aneurysms when other conventional endovascular techniques are not feasible. Several studies have reported that SAC decreases recanalization after coiling23)24). The coverage of the aneurysm neck prevents coil migration to the parent artery, which then contributes to the increase in packing density. Moreover, the flow-diversion effect, which causes progressive occlusion25) and angulation change of the parent artery due to the straightening effect of the stent 26), was also shown to decrease recanalization after coiling. Conversely, these hemodynamic and morphological changes of the aneurysm and parent artery caused by stent deployment may affect original anatomical features. In our study, the SAC cases were excluded to enable verification of the relationship between aneurysm morphology, geometry, and coil embolization.
AR has been reported as a predictor of aneurysm rupture27-31). There have been several reports on the correlation between parent vessel geometry and risk of aneurysm rupture and growth. Hassan et al. reported a correlation between aneurysm depth and both neck width and caliber of draining arteries32). Furthermore, the incidence of rupture of aneurysms with an AR exceeding 1.6 was 100% in the categories of side wall and side wall with branching vessels. Hoi et al. studied the hemodynamics of 3D saccular aneurysms arising from the lateral wall of arteries with varying arterial curves (starting with a straight vessel model) and neck width using CFD analysis33). They reported that as the degree of arterial curvature increased, flow impingement on the distal side of the neck has also intensified, which led to the enlargement of the impact zone at the distal side of the aneurysm neck, and the large impact zone at the distal side of the aneurysm neck correlated with aneurysm growth and regrowth of treated lesions.
Based on these reports, aneurysms with a high AR may occur in regions of higher hemodynamic stress, and this may lead to recanalization after coil embolization. Direct correlations between AR and recanalization after coil embolization of UIAs have not yet been reported. Further CFD studies investigating the correlation between AR and recanalization are required.
There are several limitations of this study. This was a single-center retrospective study. There was also a degree of selection bias. Therefore, the patients in this study are not representative of all patients with UIAs. Furthermore, the general indications and choice of treatment method is different across institutions.