An aneurysm is an abnormal localized bulging or ballooning of the wall of a blood vessel.  Wide neck aneurysms are less likely to rupture, but are harder to treat.  A cerebral aneurysm is an abnormal bulging or ballooning of an intercerebral artery. The prevalence of cerebral aneurysms is in the range of 1-5%.[1]  According to the National Institute of Neurological Disorders and Stroke (NINDS) of NIH, the incidence of reported ruptured aneurysms is about 10 per 100,000 persons per year (about 27,000 per year in the U.S.)[2]  Approximately one third to one half of people who suffer a ruptured cerebral aneurysm die within one month and, among those who survive, approximately one half suffer significant deterioration of brain function.  The vast majority of cerebral aneurysms form in the junction of arteries known as the Circle of Willis where arteries come together and from which these arteries send branches to different areas of the brain. 

During the past three decades, there has been a general trend away from surgical clipping and toward less-invasive endovascular methods of treating aneurysms.  Also, there has been evolution in the less-invasive endovascular methods -- from balloons in the aneurysm, to coils in the aneurysm, to stents in the parent vessel before insertion of coils into the aneurysm (sometimes called "stent-assisted coiling" or "jailing"), to specialized neurological stents that reconstruct the parent vessel to address the hemodynamic conditions that contributed to formation of the aneurysm in the first place. 

However, the most common methods of treating aneurysms are still surgical clipping (placing a clamp on the aneurysm from outside the vessel) and endovascular coiling (inserting flexible coils into the aneurysm from inside the vessel).   Limitations of clipping include: risks of invasive surgery; difficulty accessing aneurysms in some areas; difficulty clipping fusiform or wide-neck aneurysms; and failure to address parent vessel hemodynamics.  Limitations of coiling include: filling only a limited percentage of the aneurysm volume; coil compaction and recanalization over time; difficulty coiling fusiform or wide-neck aneurysms; prolapse of coils into the parent vessel; difficulty clipping later if needed; and failure to address parent vessel hemodynamics.[3-8] 

For these reasons, and as confirmed by the literature, there remains a significant unmet clinical need for development of new options to treat cerebral aneurysms:

"…there is still significant room for improvement with respect to the endovascular treatment of aneurysms. The fundamental shortcoming of coiling technology is that it aims to achieve endosaccular occlusion of cerebral aneurysms without addressing the diseased parent vessel that gave rise to the aneurysm. Although this approach is very effective for many of the small, narrow-necked cerebral aneurysms that are commonly encountered in practice, the technology fails with more complex, difficult lesions. The failures of coil embolization are manifest in two major ways: (1) incomplete treatment (the inability to achieve complete angiographic occlusion of most lesions), and (2) aneurysm recanalization (interval coil compaction with time [usually over months] leading to a deterioration of the results achieved during the original treatment)."[3]

"…long-term follow-up of International Subarachnoid Aneurysm Trial patients documents higher recurrence and rehemorrhage rates after endovascular coiling."[4] 

 "Aneurysm reopening occurred in 20.8% (95% CI, 19.8% to 21.9%) and retreatment was performed in 10.3% (95% CI, 9.5% to 11.0%)."[5] 

"Recurrences were found in 33.6% of treated aneurysms that were followed up and that appeared at a mean+/-SD time of 12.31+/-11.33 months after treatment. Major recurrences presented in 20.7% and appeared at a mean of 16.49+/-15.93 months."[6] 

"Cumulative adverse outcome rates for endovascular coiling and surgical clipping were 8.8% (95% CI 7.6%-10.1%) and 17.8% (95% CI 17.2%-18.6%)." [7] 

"Initial acute angiographic results in 1036 aneurysms demonstrated total occlusion in 731 patients (70.5%), subtotal occlusion in 252 (24.3%), incomplete occlusion in 20 (1.9%), and failures in 33% (3.3%) aneurysms." [8]
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1.  Komotar et al., "Guidelines for the Surgical Treatment of Unruptured Intracranial Aneurysms," Neurosurgery, 2008, 62(1), 183-93.

2. Cerebral Aneurysm Fact Sheet, National Institute of Neurological Disorders and Stroke,
http://www.ninds.nih.gov/disorders/cerebral_aneurysm/detail_cerebral_aneurysm.htm

3. Fiorella, "Endovascular Treatment of Cerebral Aneurysms. Current Devices, Emerging Therapies, and Future Technology for the Management of Cerebral Aneurysms," Endovascular Today, June, 2008, 53-65.

4. Raja et al., "Microsurgical Clipping and Endovascular Coiling of Intracranial Aneurysms: A Critical Review of the Literature," Neurosurgery, 62(6),  June, 2008, 1187-202.

5. Ferns, S., "Coiling of Intracranial Aneurysms: A Systematic Review on Initial Occlusion and Reopening and Retreatment Rates," Stroke, Aug. 1, 2009, 40, e523-e529.

6. Guilbert et al., "Long-Term Angiographic Recurrences after Selective Endovascular Treatment of Aneurysms with Detachable Coils," Stroke, 2003, 34, 1398-1403.

7. Lee et al., "Aggregate Analysis of the Literature for Unruptured Intracranial Aneurysm Treatment," American Journal of Neuroradiology, Sep., 2005, 26(8), 1902-8. 

8. Gallas et al., "Long-Term Follow-Up of 1036 Cerebral Aneurysms Treated by Bare Coils: A Multicentric Cohort Treated between 1998 and 2003," American Journal of Neuroradiology, Nov. 1, 2009, 30,1986-1992.