The main pitfall in the direct surgical clipping of a basilar tip aneurysm concerns the thalamoperforating arteries, as the visualization and dissection of these perforators are very difficult in some cases of basilar tip aneurysm. The thalamoperforators originate from the proximal PCA (namely, the P1 segment); thus, in cases of coil embolization, the PCAs should also be preserved. Balloon-assisted coil embolization, firstly introduced by Moret et al., shows high rates of delayed coil compaction and aneurysm recurrence.
6) Thus, various stenting techniques have been introduced to provide a discrete buttress for coil embolization and scaffolding that diverts blood flow away from aneurysm. Representative stent remodeling techniques include Y stenting and waffle-cone stenting. The Y stenting technique is relatively easy compared to horizontal stenting; however, the probability of thromboembolic complications is higher compared to that with single stents.
1) The waffle-cone technique, firstly introduced by Horowitz et al., can warrant the origin of both parent arteries, but the neck itself cannot be blocked.
4) Horizontal stenting can meet these two demands with less thrombogenicity and higher neck protection. With basilar tip aneurysms, the anatomic routes (namely, both P-coms) allow an additional horizontal stenting technique, which was firstly introduced by Cross et al.
3) This horizontal technique completely depends on P-com anatomy and course, as the horizontal stent is mounted through the P-com. Yashar et al. subsequently described “P-com independent” horizontal stenting, known as antegrade horizontal stenting.
8) The access route in this antegrade horizontal stenting technique is actually from the basilar artery to the unilateral PCA, and final stent deployment occurs in a narrow working area between the unilateral PCA and the end of the aneurysm neck or the just-proximal point of the contralateral PCA. Thus, the neck coverage is subordinate to retrograde horizontal stenting, and stent herniation into the aneurysm can occur. In the retrograde technique, a stent is mounted in the bilateral PCAs through the ICA and P-com. This retrograde approach is completely dependent on the thickness of the P-com, and the long route may contain many anatomic and technical pitfalls. However, compared to that in the antegrade technique, the neck coverage is more complete and stable. Thus, to prevent stent miss-lodging and to maximize coil insertion, the retrograde technique is the first choice in the horizontal stenting of basilar tip aneurysms. In the cases reported in the present study, the retrograde approach was used to mount horizontal stents.
2) According to a review of reported case studies using an retrograde approach, stenting is attempted if the thickness of the P-com is over 1 mm, and the success rate is high in those cases. As shown in
Fig. 2A and 2B, the contrast was clear in the lt. P-com in the present cases. Because the thickness of the lt. P-com was 1.9 mm and 2 mm, respectively, it was easy to move catheters and stents through the P-com. In addition to P-com caliber, equally important point is “angulation” of P-com and P1. Before the application of a horizontal stent, the stable launch of a microwire into the contralateral PCA (through the ipsilateral PCA) is mandatory. In both of the present cases, the left P1 had a downward sagging appearance. However, the passage of the microwire was much easier in Case 1 than in Case 2 for the following reasons: 1) the larger aneurysm permitted the inevitable upward jumping of the microwire, and 2) the upward sagging of the right P1, which has extension line of downward sagging of the left P1. In other words, 1) V-kink of the left P-com of the ICA, 2) short aneurysmal height, 3) much sharper downward sagging of the left P1 caused difficulties in the microwire handling in Case 2. To overcome these obstacles, we introduced a microwire into the proximal basilar artery, advanced the microcatheter, introduced the microwire to the right PCA, and finally launched the microcatheter to the target position.
The present cases highlight that a thorough understanding of the angioarchitecture and flexibility in the procedural strategy are mandatory for the successful deployment of a horizontal stent in basilar tip aneurysms. In summary, to succeed retrograde horizontal stenting for coiling of wide neck basilar tip aneurysm, the main check points and technical summaries are same as follows ; 1) P-com caliber (wider is better), 2) P-com angulation (less acute is better), 3) Both P1 angulations (less acute is better), 4) Aneurysmal size (larger is better)