Versatile Fill Coils: Initial Experience as Framing Coils for Oblong Aneurysms

Summary

Coil embolization of oblong aneurysms is difficult because the majority of commercially available coils are manufactured with a helical or spherical tertiary structure. While adopting framing strategies for oblong aneurysms (aspect ratio ≥ 2: 1), traditional coils may be undersized in the long axis but oversized in the short axis, resulting in increased aneurysmal wall stress, risk of re-rupture, and difficulty creating a basket that respects the aneurysmal neck. We review three cases in which versatile filling coils (VFCs) were used as the initial coils for embolization of oblong aneurysms and report coil distribution characteristics and clinical outcomes. Packing density after VFC implantation was assessed using the software AngioSuite-Neuro edition and AngioCalc.

Illustrative case: a 58-year-old woman experienced a subarachnoid hemorrhage from a ruptured anterior communicating artery aneurysm (7.5 mm × 3.5 mm). A 3-6 mm × 15 cm VFC was selected as the first coil because the flexibility of its wave−loop structure facilitates framing of an irregularly shaped aneurysm. The loop portions of the structures tend to be pressed to the extremes of the aneurysmal sac by the wave component. The VFC was introduced smoothly into the aneurysmal sac without catheter kickback. We were then able to insert detachable filling coils without any adjunctive technique and achieved complete occlusion. Complete occlusion without severe complications was achieved in all three cases in our study. Average packing density after the first coil was 15.63%.

VFC coils may have a specific role in framing oblong aneurysms given their complex loop-wave design, allowing spacing of the coils at the dome and neck while keeping sac stress to a minimum.

Introduction

Aneurysms can take many three-dimensional forms, including egg-shaped, mushroom-shaped, multilobed, and elliptical forms ^1,2, and shape is a significant risk factor for rupture ^3-5. An oblong aneurysm is defined as a lesion with a width <50% of length (2:1 aspect ratio). They are classified as elliptical aneurysms, which are considered to be more prone to rupture than spherical aneurysms ¹.

Coil embolization is among the most widely used and safe procedures for occlusion of aneurysms. The first coil inserted (the framing coil) is meant to fill the periphery of the aneurysmal sac, stabilizing the structure for subsequent insertion of filling coils. However, most coils are designed to frame spherical aneurysms (Figure 1A,B). Framing strategies for oblong aneurysms may involve coils that are undersized in the long axis but oversized in the short axis. This mismatched geometry may result in increased aneurysmal wall stress and introduce difficulties creating a stable basket across the neck. In cases where complete occlusion is not achieved using the simple single-catheter technique, surgeons may resort to adjunct techniques which may increase procedure complexity, radiation, and complication rates.

Figure 1.

Schematic diagrams of a concept of strategy for proper aneurysm framing. A) A spherical aneurysm: it is easy to make basket to cover the whole aneurysm cavity with a conventional 3D framing coil without stressing the aneurysmal wall. B,C) An elliptical aneurysm: the whole aneurysm cavity can be covered with a conventional coil with a diameter equal to the shorter diameter of the aneurysm. Compared with a spherical aneurysm, the coil distribution is unbalanced (B). D,E) Oblong aneurysm: a conventional bare coil cannot cover whole aneurysm cavity with one conventional coil with a diameter equal to the shorter diameter of the aneurysm (C). Therefore some helical or spherical coils are stacked one upon another to fill both the dome and the neck - the “stacked” or “snow-man” approach (D). E) Otherwise, framing can be done with a framing coil with a diameter equal to the aneurysm dimensions (width × height)/2. A versatile filling coil with loop segments and wave segments provides a stable initial frame that entirely filled the peripheral volume of the aneurysmal sac (C,E).

The initial design of the versatile filling coil (VFC) was not for framing (Figure 2), given that the smaller loop size may have a tendency to exit the aneurysm neck. In typical oblong aneurysms, the neck is slightly smaller than the cross-sectional diameter orthogonal to the long axis of the aneurysm, thus the loops of an appropriately sized VFC would be smaller than the width of the aneurysm, yet larger than the neck, with the waves acting as a spacing element.

Figure 2.

A photograph of a VFC showing the unique shape, consisting of a loop section (arrow) and a wave section (double-headed arrow). The loop section provides stability and the wave section promotes distribution throughout the aneurysmal space, even for atypically shaped spaces such as oblong aneurysms.

We describe three cases in which VFCs were used successfully as the framing coil for complete occlusion of oblong aneurysms. Cleveland Clinic material use logs were reviewed and cases chosen in which the aneurysm was oblong (aspect ratio of 2:1 or greater) and a VFC (Microvention, Tustin, CA, USA) was used as the initial framing coil. Demographic information and imaging data were also collected and analyzed. Coil packing density was assessed using the software AngioSuite-Neuro edition (AngioSuite Inc.) or AngioCalc.

Case Report

A patient presented with mild nuchal rigidity and headache. Emergent computed tomography (CT) showed Fisher grade 2 subarachnoid hemorrhage (SAH). We diagnosed the patient with Hunt and Hess grade 2 SAH. Emergency angiography of the left internal carotid artery revealed an aneurysm of the anterior communicating artery (ACA) measuring 7.5 mm × 3.5 mm (Figure 3A,B). After the risks, benefits, and alternatives were discussed with the patient, she elected coil embolization under general anesthesia. The patient was fully heparinized. A microcatheter (ExcelsiorTM SL10; Stryker Neurovascular, Fremont, CA, USA) was manipulated into the aneurysm over a 0.014-inch microguidewire (Synchro2TM; Stryker Neurovascular, Fremont, CA, USA) under high-magnification fluoroscopy. The choice of framing coil for such oblong aneurysms is often difficult because the available helical or spherical 7 mm coils are larger than the short axis diameter, and may increase the risk of intraprocedural rupture, while smaller coils will not readily distribute in the space. That is why we selected a VFC of size 3-6 mm × 15 cm (Microvention, Tustin, CA, USA). This coil was introduced smoothly without catheter kickback with a normal embolization technique. The coil deployed entirely within the aneurysmal sac and did not descend from the sac into the parent artery. Angiography revealed no evidence of re-rupture or thrombosis of the parent artery (Figure 3C,D). The packing density after inserting the framing coil was 20%.

Figure 3.

Left internal cervical angiography before coil embolization. A) The working angle of a left internal cervical arteriogram showing a cerebral aneurysm located in the anterior communicating artery (arrow). B) Magnified view showing the dimensions of the oblong aneurysm of size 7.4 mm (long thick double-headed arrow) × 3.5 mm (short thin double-headed arrow). Left internal cervical angiography after first (VFC) coil placement. C) Working oblique view showing that the framing coil fills the entire outer aspect of the aneurysmal sac without an adjunct technique such as balloon assistance. The microcatheter is stationary during coil insertion because VFC coils spread along the aneurysmal wall with minimum recoiling. D) Another working angle showing preservation of flow through the parent artery after framing coil insertion. Left internal cervical angiography after placement of both the framing and filling coils. E) Working angle showing that the aneurysm is occluded completely by five coils. F) Another working angle. Arrow demarcates the coil mass forming a square shape at the aneurysmal neck without use of the balloon-assisted technique. The VFC is shown to fill the entire aneurysm and takes on the peculiar shape of the aneurysm.

Four subsequent coils, one VFC (3-6 mm × 10 cm), and three hydrogel coils (Hydrocoils: 1.5 mm × 2 cm, 1.5 mm × 2 cm, and 2 mm × 2 cm; all from Microvention, Tustin, CA, USA) were then inserted into the aneurysmal sac using the same microcatheter. During this procedure, kickback of the microcatheter was minimal, and we did not require adjunct techniques such as balloon assistance. The coil mass conformed completely to the three-dimensional structure of the aneurysm for complete occlusion (Figure 3E,F). The final packing density attained was 44.5%. After coil embolization, mild mechanical stimulation-induced vasospasm was observed but was rapidly improved by intra-arterial verapamil injection. The patient quickly recovered from anesthesia and exhibited no signs of neurological deficit. She was discharged and sent home. Since the case described above, we have used VFCs for successful framing of oblong aneurysms in two other patients for the same reason mentioned above. All patients had ruptured oblong ACA aneurysms that were completely occluded by coil embolization without severe intraoperative complications. The average packing density after insertion of the first coil was 15.63% (8.4%-20%). Only one case required remodeling. Demographic data, procedural course, and clinical course for all three patients are presented in Tables 1 and 2.

Table 1.

Demographic and clinical summary

Patient no.	Ages (Years)	Diagnosis	Location of Aneurysm	Longest side of aneurysm (mm)	Narrowest side of aneurysm (mm)	HH	Perioperative complication	mRS 90 days
1 2 3	30s 50s 50s	SAH SAH SAH	AcomA AcomA AcomA	5.9 7.5 7.4	2.7 3.5 3.5	4 2 2	None Vasospasm None	6 0 0
SAH, subarachnoid hemorrhage; AcomA, anterior communicating artery; M, male; F, female; HH, Hunt and Hess grading; mRS, modified ranking scale.

Table 2.

Procedure and result of coil embolization

Patient no.	Framing coil	Embolization rate after framing coil (%)	Total number of coils	Adjunctive technique	Final embolization rate (%)	Embolization result
1 2 3	3-6 mm×6 cm VFC 3-6 mm×15 cm VFC 3-6 mm×15 cm VFC	8.4 20.0 18.05.00	5 5 4	Hyperglide 3 mm-15 mm None None	44.5 44.5 42.7	Complete Occlusion Complete Occlusion Complete Occlusion
VFC, versatile range fill coil.

Discussion

An oblong aneurysm shape may present special challenges for coil framing and filling during embolization. Moreover, an irregular multilobed appearance is more common in ruptured aneurysms than in unruptured ones. Many attributes of aneurysm geometry have been associated with rupture risk, including aspect ratio, undulation (including wall irregularities and multiple lobes), ellipticity, and nonsphericity ^2,6,7. Several studies have reported that oblong aneurysms are more prone to rupture than typical spherical aneurysms ^1,8. Although these reports have the limitation that the oblong shape could be the result, we believe that oblong aneurysms are more prone to rupture. Indeed irregularity may be a sequela of rupture, rather than a precursor.

Hademenos et al. used stepwise discriminant analysis to identify morphological factors and locations predictive of rupture in patients referred for endovascular treatment. They found that irregular morphology and small neck size were significant predictors of aneurysm rupture ⁹. There are two major approaches to framing an oblong aneurysm (Figure 1D,E). The first is framing with a framing coil with a helical diameter which is longer than the width of an oblong aneurysm and shorter than the height, normally using a size ratio related to the average dimension of the aneurysm (width × height/2). This approach renders the helix into either an elliptical shape or a figure of eight shape within the oblong aneurysmal sac. However, this approach may carry the potential risk of parent artery occlusion, aneurysmal sac remnants, and continuing communication with the parent vessel resulting from loose coil packing ¹⁰. Many adjunct techniques are performed for such refractory aneurysms. Another potential risk of using a coil with a native shape that is wider than the aneurysm sac in one dimension is that of increasing the wall stress, and increasing the difficulty of coil placement. Numerous studies have discussed the comparative rarity of intraprocedural rupture as well as the variability of the sequelae of a rupture. However, it is biomechanically intuitive that increased stress on the walls of an aneurysm (whether by using stiffer or oversized coils) during embolization may increase the difficulty of the procedure and the increase wall stress of the aneurysm. Thus, using helical coils with a native shape wider than the aneurysm in one dimension may be suboptimal. Another approach is a “stacked” or “snow-man” approach, stacking helical or spherical coils one upon another to fill both the dome and the neck. This approach probably results in less stress upon the aneurysm wall, but frequently requires repositioning of the microcatheter. VFCs are designed to distribute into aneurysmal spaces of complex geometry and variable width and so may simplify coil selection and inventories. They consist of two sections: a wave section and a loop section. The loop section provides stability for the coil mass and the wave structure facilitates conformation of the coil shape to that of the aneurysmal sac (Figure 1C,F). Complex loops and waves promote space seeking and coil distribution, allowing for the treatment of various aneurysm sizes and shapes with greater packing density and ease of placement (Figures 4 and 5).

Figure 4.

A photograph of the motion of a versatile filling coil (3-6 mm × 15 cm) in an aneurysm silicon model (9 × mm × 3 mm). A) The first position of a microcatheter is around the neck of aneurysm. B) The first loop of the VFC is placed at the top of aneurysm and the wave part of the VFC distributes into the aneurysm without major motion of the microcatheter. C) The second loop is placed in the midst of the aneurysm. These loop parts play the role of anchoring. The VFC distributes into the whole cavity of aneurysm. D-F) Note the microcatheter is stable without a kickback.

Figure 5.

A-C) After the initial framing coil with VFC (3-6 mm × 15 cm), a VFC (3-6 mm × 15 cm) is inserted as the second coil. The VFC fills the aneurysm, seeking space through and within the first coil. D) Finally coils fill almost entire aneurysmal cavity. The microcatheter keeps the first position consistently.

General concepts for aneurysm filling

The approach described can be applied with any coil that has more flexible features than a conventional helix or spherical shape. A “j” coil may have similar distribution characteristics, distributing within an oblong space in keeping with the native coil shaping properties.

Our aim in using this approach, with a non-helical coil, was to provide a stable initial frame that entirely filled the peripheral volume of the aneurysmal sac (adjacent to the wall) without the need for adjunct techniques. Indeed, the VFC filled the outer volume, obviating the need for meticulous re-assessment of sac size for selection of the proper coils or laborious multiple coil framing.

We suggest that the technique of using VFCs for coil occlusion of oblong aneurysms is comparable to conventional techniques in terms of safety, and may have the added benefit of reduced aneurysmal wall strain during deployment, simplification of coil distribution and microcatheter manipulation, and helping to avoid the need for adjunctive techniques such as balloons and stents.

Conclusions

There are multiple approaches for framing and filling oblong aneurysms. Our case series and in vitro experiments suggests that VFCs, which have a loop−wave tertiary structure, can easily fill an oblong space, decreasing the need for coils oversized in one dimension, “stacked” multiple coil framing, and adjunctive techniques.