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August 2010
Techniques for Cerebral Aneurysm Treatment
Current research aims to expand the tools available for endovascular treatment of this challenging disease.
By Kyle M. Fargen, MD, MPH; Brian L. Hoh, MD; and J Mocco, MD, MS
Subarachnoid hemorrhage secondary to ruptured intracranial aneurysms remains one of the most challenging disease processes in medicine. Even with recent advances in neurocritical care and interventional techniques, the morbidity and mortality rates associated with aneurysm rupture are substantial. Since the development and implementation of the detachable coil for intracranial aneurysms by Guglielmi in 1991, interventionists (with the support of industry) have continued to push the envelope in device design. With each passing year, the armamentarium of the endovascular specialist continues to diversify. Recent additions include new balloon catheters for balloon-assisted coil embolization, flow-diversion devices,1-7 new open- and closed-cell stent designs,8-13 and novel coil14-20 and embolic materials.21-23
In addition, research into the biochemical derangements after subarachnoid hemorrhage has guided our understanding and treatment of posthemorrhage vasospasm.24-30 New three-dimensional (3D) morphological analyses and flow algorithms have improved our understanding of the hemodynamic factors leading to rupture, which may help identify aneurysms that are at highest risk.31-35 This article aims to highlight some of the recent research pertaining to the endovascular treatment of aneurysms that will likely guide the future management and treatment of patients with this challenging disease.
FLOW-DIVERSION DEVICES
The necessity of total aneurysm occlusion via embolization
has been challenged recently by stent devices that target
parent vessel reconstruction (rather than embolization
of the aneurysm sac) and promote aneurysm thrombosis
by diversion of flow through the parent vessel lumen
(Figure 1). Such devices were developed for fusiform
aneurysms or other difficult-to-embolize, wide-necked
lesions.
One such device is the Pipeline embolization device (ev3 Inc., Plymouth, MN), a bimetallic braided stent designed to provide the necessary surface coverage to divert flow from the aneurysm while preserving flow into perforators or branch vessels. In the initial rabbit model, 88% of aneurysms treated showed complete or near-complete occlusion at follow-up ranging from 1 to 6 months.1
The largest series published to date in humans, consisting of 63 wide-necked or fusiform aneurysms, showed a complete occlusion rate of 95% at 12 months with a low complication rate.2 Similarly, 17 of 18 aneurysms treated with the Pipeline device were completely occluded at 6 months in a separate cohort.3 The majority of cases in the largest study were treated successfully with one device; however, multiple patients required the deployment of two or three overlapping Pipeline devices to achieve acceptable results.2 The need for multiple overlapping stents to obtain a favorable angiographic outcome is well documented.2-5,36 This fact is of particular interest given the concerns for perforator occlusion and iatrogenic infarction secondary to telescoped devices.36 The Pipeline device is currently under investigation for US Food and Drug Administration (FDA) approval.
A second flow-diverting device, the Silk stent (Balt Extrusion, Montmorency, France), has been successful in a number of case reports,6,7 although one death from posttreatment aneurysm rupture has already been reported.37
Asymmetric vascular stents containing low-porosity patches have similarly produced promising results in animal models.8,9 These devices consist of stents with varied porosity so that the area of low porosity can be positioned over the aneurysm neck while only the high-porosity regions of the stent cover the associated side branches and perforators. Human trials with these devices have yet to be completed.
Other techniques, such as the use of multiple stents to reproduce a flow-diversion device, are currently being researched. In addition, research is underway to evaluate stent porosity to determine the ideal pore size to optimize perforator preservation while promoting aneurysm thrombosis through flow diversion.38-41 Should these devices continue to achieve good results with a low complication profile, practicing interventionists will have new, easy-to-deploy tools for treating some of our previously most challenging aneurysms.
ASSISTED COIL EMBOLIZATION
Although they are by no means new to endovascular
therapies, the techniques of balloon- and stent-assisted coil
embolization continue to be the subject of important
research. Both techniques allow for higher packing density
by preventing coil herniation into the vessel lumen in widenecked
or complex aneurysms.42 New balloon catheters (Figure 2), such as the Ascent double-lumen catheter
(Micrus Endovascular Corporation, San Jose, CA), which is
compatible with 0.014-inch guidewires, have been developed
to improve balloon-assisted coil embolization by
reducing the need for additional catheters.
Stent technology has progressed markedly during the last decade, and numerous different open- and closed-cell selfexpanding stent designs are now available for use. With improvements in design and a versatile group of devices to choose from, interventionists are finding unique ways to treat aneurysms that were previously considered “uncoilable.” Additionally, the advent of catheter-based delivery of aneurysm embolization-assisting stents, first made prevalent by the Enterprise vascular reconstruction device (Codman Neurovascular, Codman, a Johnson & Johnson Company, Ratham, MA), has substantially improved stent navigability. With the newly FDA-approved Neuroform EZ stent (Boston Scientific Corporation, Natick, MA), this trend toward easier deliverability continues to progress.
Several large series have indicated excellent midterm or long-term results with stent-assisted coiling.10-12 Of concern is the risk of in-stent stenosis or thrombosis, which occurs in approximately 3% to 6% of cases after deployment of previous Neuroform iterations (Boston Scientific Corporation)13 or Enterprise stents (unpublished data, 2010) for treating intracranial aneurysms. Given this concern, placement of an intracranial stent usually necessitates dual-antiplatelet therapy. There is current research being done to evaluate the risk of in-stent stenosis or thrombosis in patients undergoing stent-assisted coiling as well as the optimal length of time needed for dual-antiplatelet therapy. In addition, novel stent delivery systems and designs are currently being evaluated.
THERAPIES TO PREVENT RECANALIZATION
AFTER EMBOLIZATION
Of significant concern to practicing interventionists is the
considerable risk of aneurysm recanalization after coil
embolization, which has been estimated to occur in anywhere
from 5% to 38% of coiled aneurysms.43-45 The first
detachable coils developed for deployment within cerebral
aneurysms were composed of bare platinum. To improve
recanalization rates, research has shifted toward the use of
other compounds to produce a more robust inflammatory
reaction that will enable early fibrosis of the aneurysm sac
and therefore result in improved recanalization rates.
The most prominent developments in this arena are the use of bioactive and hydrogel coils, which are platinum coils variously constructed with either a bioabsorbable polymeric or a hydrogel that expands on contact with blood. Early data on the HydroCoil (MicroVention, Inc., Tustin, CA), a platinum-based coil coated with hydrogel, suggest decreased recanalization rates with comparable complication rates compared to bare-platinum coils.14,15 Bioactive coils, the most prominent of which is Cerecyte (polyglycol acid-loaded coils) (Micrus Endovascular Corporation), have shown similar results.16-18 However, recent data presented at the American Society of Neuroradiology 2010 annual meeting from the Cerecyte coil trial, a randomized trial of 500 patients comparing bare-platinum and Cerecyte coils, indicate excellent aneurysm occlusion rates of approximately 85% in both groups and therefore little superiority to Cerecyte over bare platinum.
Focus has also shifted toward the testing of coils with complex designs instead of standard helical coils. New data have suggested that complex-shaped coils may reduce recanalization rates by allowing improved packing density,19 and testing of these coils has demonstrated satisfactory midterm occlusion results.20
However, long-term success rates with both novel coil materials and coil designs are currently unavailable. Unfortunately, comparative studies between such coils have not yet provided the necessary evidence to show superiority among any of these products.46 Cerebrovascular interventionists eagerly await the results of trials underway that will hopefully provide these much-needed answers.
Finally, active research is also exploring alternative avenues to prevent delayed recanalization after embolization. Onyx HD-500 (ev3 Inc.), a liquid embolic agent, has been used to treat intracranial aneurysms since obtaining FDA approval in 2007. Several large series (> 70 aneurysms) have shown 80% to 90% occlusion rates at 12 to 18 months and comparable complication rates to coil embolization.21-23 Other liquid agents are currently being evaluated in animals for feasibility and efficacy,47 which may translate to human trials should continued success be achieved. Novel endovascular therapies that use radiofrequency ablation and in situ radiation are currently being pursued and have shown efficacy in nonhuman models; however, their effectiveness in human subjects has yet to be elucidated.48-50
ANEURYSM MORPHOLOGY
AND HEMODYNAMICS
Although somewhat antiquated, most interventionists
and neurosurgeons currently use size and location as the
primary determinants of whether to treat unruptured
intracranial aneurysms. Unfortunately, findings from the
International Study of Unruptured Intracranial Aneurysms
stating that small (< 7 mm) aneurysms have a near-zero risk
of hemorrhage annually do not correlate with other studies
suggesting many ruptured intracranial aneurysms are, in
fact, small. In addition, certain aneurysm locations (anterior
communicating artery) harbor a disproportionately high
number of ruptured small aneurysms compared to other sites. These contradictions have led to a push by researchers
to investigate other factors, aside from merely the largest
diameter and the location, which may better explain an
individual aneurysm's risk of rupture.
Recent improvements in 3D and volumetric imaging analysis, as well as blood flow algorithms, have advanced our understanding of aneurysm morphology and hemodynamics and have provided the means to reproduce and test these factors. Focal regions of low wall-shear stress on the inner-aneurysm wall, focused flow impingement zones, complex flow, and multiple vortices have been identified as potentially hazardous flow characteristics that are more commonly associated with ruptured aneurysms compared to those that are unruptured.31,32 Elevated aneurysm-toparent vessel size ratio has recently been associated with ruptured aneurysms in several studies33,34 because such anatomy may be responsible for generating dangerous flow patterns.35Work is currently underway in numerous centers to further explore the relationships of blood flow and aneurysm morphology with the ultimate goal of providing a calculated risk prediction for any given unruptured aneurysm. Interventionists readily await a large-scale trial to provide this necessary information and to finally quiet the questions generated by the International Study of Unruptured Intracranial Aneurysms trial data.
THERAPIES TO PREVENT AND TREAT
VASOSPASM AFTER SUBARACHNOID
HEMORRHAGE
Vasospasm remains a significant cause of morbidity and
mortality after subarachnoid hemorrhage. Radiographic
vasospasm occurs in roughly 30% to 70% of patients whereas
symptomatic vasospasm occurs in approximately 20% to
30% of patients.51 In most centers, patients with perfusion
deficits, neurologic deficits, angiographically narrowed vessels
on imaging, or high flow on transcranial Doppler are
usually initiated on “triple H” therapy (hypervolemia,
hemodilution, and hypertension) and/or treated with intraarterial
angioplasty or verapamil to temporarily relieve arterial
spasm.
Methods to prevent and treat vasospasm have been a major focus of research lately, particularly in the study of the role of biochemical factors in inducing arterial spasm and in therapies for relieving vasospasm in animal models. Thus far, vasospasm prevention research has been disappointing, especially with the failure of tirilazad mesylate to show any benefit in clinical outcome over calcium channel blockers. 24,25 Novel therapies are currently being tested in human subjects, such as intraventricular tissue plasminogen activator with lumbar drainage, which in one series reduced the need for intra-arterial intervention as well as the need for shunting.26 A randomized trial evaluating the effect of intra venous human albumin after subarachnoid hemorrhage has also begun enrolling patients.27 The use of other intra-arterial antispasmodic agents, such as milrinone, has also been evaluated and has been met with initial success.28
However, the most promising endothelial receptor antagonist at this time is clazosentan, which showed a trend in reducing vasospasm-associated mortality and significantly reduced radiographic vasospasm in the phase II CONSCIOUS- 1 trial.30 The CONSCIOUS-2 trial, a phase III trial evaluating clazosentan in patients undergoing surgical clipping, 29 has completed enrollment. The CONSCIOUS-3 trial, evaluating patients undergoing coil embolization, is still in the enrollment phase. Given the enormous investment of both dollars and researchers into reducing the morbidity associated with vasospasm, we hope to see significant advancements in the prevention and treatment of arterial spasm after subarachnoid hemorrhage soon.
CONCLUSIONS
With each passing year, the armamentarium of the
endovascular specialist continues to diversify. Current
research in the endovascular treatment of aneurysms is
focused on new flow-diversion devices that may be of integral
importance in treating wide-necked or fusiform
aneurysms. Furthermore, improvements in stent-assisted
coiling techniques and devices have made this technique
commonplace. New coil materials and designs are currently
being tested to evaluate for improved occlusion and
recanalization. Improvements in vasospasm prevention and
treatment are actively being sought across the world. The
development of complex fluid dynamic models and volumetric
imaging is allowing the determination of high-risk
predictive factors. The ability to provide accurate predictions
of rupture risk for each individual aneurysm based on
morphological and hemodynamic testing is within our
reach. It is truly an exciting time to practice in this challenging
field of medicine
The University of Florida receives an educational grant and consulting fees from Codman Neurovascular.
Kyle M. Fargen, MD, MPH, is a resident with the Department of Neurosurgery, University of Florida College of Medicine in Gainesville, Florida. He has disclosed that he holds no financial interest in any product or manufacturer mentioned herein.
Brian L. Hoh, MD, is Assistant Professor with the Department of Neurosurgery, University of Florida College of Medicine in Gainesville, Florida. He has disclosed that he receives honoraria from Codman Neurovascular and Actelion Inc., as well as nonfinancial research support from Micrus Endovascular.
J Mocco, MD, MS, is Assistant Professor with the Department of Neurosurgery, University of Florida College of Medicine in Gainesville, Florida. He has disclosed that he serves on the Clinical Events Committee Adjudication Board for the CONSCIOUS-2 and CONSCIOUS-3 International Studies (Actelion, Inc.). He is a consultant for Nfocus and Lazarus Effect and has received honoraria from Edge Therapeutics. Dr. Mocco may be reached at j.mocco@neurosurgery.ufl.edu.
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