INTRODUCTION
A peripheral aneurysm in the posterior circulation is a rare pathology, and treatment strategies rely on institutional experience. Microsurgical procedures achieve definitive treatment with or without the reconstruction of blood flow using a bypass combination, such as occipital artery-posterior inferior cerebellar artery (PICA) or PICA-PICA anastomosis [1-3]. These procedures are potentially complex, lengthy, and invasive [4-7]. Difficulties are also associated with the endovascular treatment of these distal lesions, such as the limited number of devices capable of navigating small-caliber peripheral and tortuous vessels and the meticulous manipulation required. Flow-guided microcatheter with high peripheral accessibility is now being employed with the advent of coils that may be used with these microcatheters [4,8,9]. We herein present our patient series of peripheral aneurysms in the posterior fossa that were treated endovascularly, and discuss key technical tips and potential pitfalls.
MATERIALS AND METHODS
Study Description
Between 2017 and 2024, 4 ruptured peripheral posterior fossa aneurysms (3 in the PICA and 1 in the anterior inferior cerebellar artery [AICA]) were treated by endovascular parent artery occlusion (PAO). Patient demographics, aneurysm locations, aneurysm sizes, parent artery diameters, and microcatheters are summarized in Table 1. No patient showed clinical signs of systemic infection. All aneurysms were categorized as either dissecting or hemodynamic related aneurysms. All procedures were performed under general anesthesia. A 6 Fr or 7 Fr guiding catheter was used either via the trans-femoral or brachial approach. After evaluating the collaterals on 3-dimensionally (3D) reconstructed cross-sections of digital subtraction angiography scans or high-resolution cone beam computed tomography, all patients underwent PAO. TACTICS (Technocrat) as a distal access catheter (DAC) was navigated to the parent artery of the ruptured aneurysm. The specific flow-guided microcatheters were the Carry Slim Leon Selective (UTM) or the DeFrictor Bull 1.5 Fr, 165 cm (Medico’s Hirata). The Carry Slim Leon selective series included 2.4 Fr, 145 cm and 1.5 Fr, 165 cm (at the tip) microcatheters, which enabled the coaxial system. An ASAHI CHIKAI X 010 (Asahi Intecc) micro-guidewire was used for all procedures. Coil embolization for PAO was achieved thereafter using i-ED coils Complex Silkysoft (Kaneka) using an electronic detachable coil system. We assessed procedural complications, the presence or absence of new cerebral infarction on postoperative MRI, and modified Rankin Scale (mRS) at 6-months outpatient follow-up.
RESULTS
All 4 patients successfully underwent PAO and progressed without re-rupture or recurrence of the culprit aneurysm, with a mRS score of 0 in 2 patients. One patient recovered to mRS 2 because of cerebellar infarction in the PICA cortical territory. The remaining patient was disabled due to initial brain damage, but did not have procedure-related brain infarction. The Carry Slim Leon selective was used in 1 patient, whereas the DeFrictor Bull was used in 3. The average number of i-ED Silkysoft coils to complete PAO was 6 (range, 5–7).
Illustrative Cases
Case 2
An adult patient with chronic basilar artery occlusion presented with subarachnoid hemorrhage and cerebellar hemorrhage due to the rupture of a peripheral aneurysm in the AICA (Fig. 1A, E). The aneurysm had developed presumably due to hemodynamic forces. Since collateral circulation from the PICA and the superior cerebellar artery and anterior circulation via the posterior communicating artery were sufficient, we performed PAO of the underlying diseased vessel involving the ruptured aneurysm (Fig. 1B–D). Using the Carry Slim Leon Selective 2.4 Fr as a DAC, the Carry Slim Leon 1.5 Fr successfully reached the proximal portion of the AICA aneurysm (Fig. 1F) and PAO was performed using 6 i-ED coils (Fig. 1G). Post-procedural angiography did not show any residual aneurysm, and no new ischemic lesion was observed (Fig. 1H, I). The procedure in this case is shown in Supplementary Video 1. The unruptured distal PICA aneurysm was microsurgically clipped 2 months after treatment because no collateral was expected.
Case 3
An adult patient presented with subarachnoid hemorrhage originating from a ruptured aneurysm in the left PICA telovelotonsillar portion (Fig. 2A). The left AICA was clearly visualized in the periphery, and a collateral pathway to the PICA was expected (Fig. 2B). Due to the acute angle between the left vertebral artery (VA) and PICA, the microcatheter was navigated into the diseased left PICA via the right VA beyond the vertebrobasilar junction (VBJ). A 6 Fr guiding catheter was advanced into the right VA, and a 1.5 Fr microcatheter, DeFrictor Bull, was navigated to the left PICA using TACTICS as a DAC. Since the microcatheter was unable to reach the aneurysm itself, PAO was performed proximally to the aneurysm (Fig. 2C). The i-ED coil, featuring a flexible delivery wire, allowed for stable deployment without kickback, even when navigated through the VBJ or severe vessel tortuosity. Seven i-ED coils were deployed, resulting in successful occlusion (Fig. 2D). Angiography after PAO showed collateral circulation from the left AICA to the PICA perfusion territory (Fig. 2E). No new ischemic lesion was observed (Fig. 2F). The procedure in this case is shown in Supplementary Video 2.
DISCUSSION
An optimal treatment strategy for peripheral aneurysms in the posterior fossa has not yet been established. A combination of microsurgical clipping and bypass procedures is the treatment of choice. Due to well-developed leptomeningeal anastomoses in cerebellar arteries, many cases may tolerate PAO if an alternative blood supply is present [4,7,10,11]. Given the vital brainstem perforators originating from its anterior medullary to tonsillomedullary segments, occlusion of the PICA in these region carries a high risk of fatal ischemic injury [4,10]. The coil should be placed as close as possible to the aneurysm neck, and the length of the occlusion should be minimized to secure adjacent perforating arteries or cortical branches. Conventional microcatheters for coiling are often large in diameter and lack flexibility for peripheral navigation. Therefore, achieving distal access beyond peripheral tortuosity is substantial. Super-small bore and flexible microcatheters, such as the Marathon 1.5 Fr, 165 cm (Medtronic) and the 2 which were presented in this technical presentation, designed for liquid embolization, offer superior distal navigability compared to standard coiling catheters, making them more suitable for treating peripheral posterior fossa aneurysms [4]. Furthermore, the use of a small-bore DAC, such as TACTICS, improves the stability of microcatheters [12]. The emergence of super-small and flexible microcatheters, like the DeFrictor Bull and Carry Slim Leon, which are compatible with small-bore DACs like TACTICS, has further facilitated access to more distal lesions. The Carry Slim Leon with its 2.4 Fr outer diameter and 1.5 Fr inner lumen, allows for a sophisticated quadplex coaxial system when used with a 3.4 Fr DAC, providing improved maneuverability and accessibility to challenging distal targets.
Sacrificing the parent artery for ruptured peripheral intracranial aneurysms is an established concept in neurovascular intervention [5-7,11,13]. Currently, n-Butyl Cyanoacrylate (NBCA) is commonly used as an embolic agent in these microcatheters. However, its use carries significant risks, including procedural complications such as unexpected ischemic complications in the territory of perforating arteries or distal critical collaterals. In contrast, coil embolization offers the potential for more controlled, length-specific embolization, theoretically preserving collateral circulation. A significant current limitation, however, is the lack of commercially available detachable coils compatible with such small-bore flow-directed microcatheters. Although it remains off-label use, the i-ED coils have unique flexibility and detachment system, providing compatibility with flow-guided microcatheters such as the Marathon [4,8,9]. Moreover, the low profile of i-ED Complex Silkysoft coils allows for insertion into flow-guided microcatheters with lumens as small as 0.012–0.014 inches. In vitro demonstration is presented in Supplementary Video 3. As presented in our series, the i-ED coils were successfully inserted without issues even in tortuous anatomical configurations. Additionally, the i-ED coil features a unique electronic detachment mechanism that provides auditory and visual cues at the detachment point, enabling its use with single-marker microcatheters, as demonstrated in Supplementary Video 3. This procedure is not feasible with conventional coils due to issues with the diameter and detachment system, which potentially requires 2 markers at the tip of microcatheters. Nevertheless, there were some technical issues. Excessive tortuosity of peripheral vessels in vivo could impede smooth coil insertion, potentially leading to microcatheter kickback and procedural difficulties. Therefore, we recommend selecting small and short coils (e.g., 1 cm or 2 cm in diameter). The development of this i-ED coil compatible with super-micro catheters would significantly broaden the scope of endovascular treatment techniques for highflow shunt diseases as arteriovenous malformations or fistula lesions, offering easier modulation of high blood flow before liquid embolization with agents like ONYX (Medtronic) or NBCA.
Due to the limitations associated with quantitatively assessing collateral circulation in cases with subarachnoid hemorrhage, the risk of peripheral cerebellar infarction following PAO needs to be considered. While the clinical symptoms of partial cerebellar infarction often improve with rehabilitation, occlusion proximal to perforating branches of the brainstem carries a higher risk of brainstem infarction and its associated permanent neurological deficits [2,4-6,10,11,13]. Therefore, a careful assessment using a detailed analysis of collaterals with angiography and the occlusion, including the culprit aneurysm with the preservation of perforating arteries or cortical branches, is crucial for selecting the optimal treatment of choice for each individual patient.
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