COAST: Coiling of Aneurysms Smaller Than 5mm With Hypersoft® and Hydrogel Coils

1.0 INTRODUCTION 1.1 Background Intracranial aneurysms (IAs) are common cerebrovascular abnormalities. The prevalence of IAs has been reported to be 0.8-2.0% of the population. [1-3] The most common presentation of IAs is subarachnoid hemorrhage (SAH), the annual incidence of which varies by geographic region from 10 to 20 per 100,000. [4 5] SAH is a devastating injury with a case-fatality rate of 51% [5]. Nearly half of its survivors are functionally incapacitated [6].

There is limited data on the natural history of small intracranial aneurysms. According to the International Study of Unruptured Intracranial Aneurysms, the risk of spontaneous aneurysm rupture is related to aneurysm size and location. ISUIA found aneurysms < 10 mm in diameter, as opposed to aneurysms 10-24 mm and >25 mm, had relative risks of rupture of 11.6 and 59, respectively. Further follow-up from this cohort showed 5-year bleeding risks of 0%, 2.6%, 14.5%, and 40% for aneurysms less than 7 mm, 7--12 mm, 13--24 mm, and 25 mm or greater, respectively. Many other authors (Juvela 2000 and Weir 2002) also suggest the risk of small aneurysm rupture is relatively low. The Sapporo SAH Study group suggests that while the overall rupture risk of small aneurysms is low, the aneurysm size ratio is a strong predictor of aneurysm rupture in small (<5mm) intracranial aneurysms [7]. However, some authors and unpublished data (MUSC) demonstrate that approximately one-third of all ruptured aneurysms are less than 4 mm in size.

Endovascular coiling of intracranial aneurysms has been shown to be safe and efficacious in the treatment of intracranial aneurysms. The International Subarachnoid Aneurysm Trial (ISAT) has shown that endovascular coiling can reduce morbidity and mortality compared to clipping of aneurysms in the setting of SAH. [8]. The goal of endovascular coiling is to prevent rupture or rebleeding by isolating an aneurysm from the normal blood circulation without narrowing the parent vessel.

A main concern of endovascular treatment is the long-term durability of treatment, that is to say that it is possible for the aneurysm to recanalize (recur) after it has been treated with coils [9]. Some factors in recanalization are incomplete initial occlusion, large aneurysm size, ruptured aneurysm, partially thrombosed aneurysm, and compaction of the coil mass within the aneurysm [9 10]. In a Study by Nguyen [11], incomplete initial aneurysm occlusion, rupture and large aneurysm size were all associated with significant recanalization. Johnston [12] concluded the degree of occlusion after initial treatment to be a strong predictor of the risk of subsequent rupture, which justified attempts to completely occlude aneurysms. Two series of small unruptured intracranial aneurysms [13 14] found recurrence rates between 5.9% and 16.9% with retreatment rates of 1.7% and 2.9%. The majority of retreatments were in small wide necked aneurysms. However, the recurrence rates of small aneurysms is much less than those of large aneurysms (71% vs 35%) [15].

The other main concern with the treatment of small aneurysms (less than 4 mm) is safety, namely the concern of intraprocedural rupture or thromboembolic events. The ATENA Study showed that the risk of intraprocedural aneurysm rupture was significantly higher in small aneurysms (3.7% for 1-6 mm vs 7% for 7-15 mm; p= 0.008). The rate of failure of EVT was significantly higher in very small unruptured aneurysms compared to larger aneurysms (13.7% vs 3.3% respectively). This is likely related to several factors. Microcatheterization of the aneurysm sac may be challenging due to aneurysm size and placing even the smallest coils, maybe challenging in small aneurysms. In the same series as above [13 14], a 10.4% overall procedural complication rate was found. There were 24 embolic events, 11 coil protrusions and 4 aneurysm ruptures, while Oishi found a 3.8% thromboembolic event rate and a 1.4% risk of aneurysm rupture. Nguyen et al reported an intraprocedural rupture rate of 11.7% in aneurysms less than 3 mm in diameter [16]. In a meta-analysis, Brinjikji et al found a procedural rupture rate of 8.3% in small aneurysms while Spiotta et al demonstrated a 13.5% rate of intraprocedural rupture in ruptured aneurysms less than 4 mm [17] Other studies have found morbidity and mortality rates that range from 0.8%-7% and 0-1.4% [13 14 18]. The introduction of the Microvention HyperSoft® 3D line of coils with sizes from 1 to 5mm may help reduce these historical risks of failure to treat and intraprocedural rupture. The complex shape of the coils may allow for stable framing of the aneurysms followed by dense packing of the aneurysm sac and neck, therefore preventing recurrence. The softness of the coils may allow for increased confidence and safety when treating these aneurysms, which may be expressed as a reduction of intraprocedural complications.