Early observations and pilot data that first suggested a new direction
For decades, microsurgical clipping was the only definitive treatment for cerebral aneurysms, established by Yasargil and others as the gold standard from the 1960s onward. The development of Guglielmi detachable coils (GDC) in the early 1990s introduced an endovascular alternative that could occlude aneurysms from within the vessel lumen without open craniotomy. Early case series demonstrated technical feasibility and acceptable complication rates, prompting calls for randomized comparison. The neurosurgical and neurointerventional communities recognized that only head-to-head trials could determine whether this less invasive approach could match or exceed the outcomes of surgical clipping.
Landmark RCTs and pivotal trials that established the evidence base
The International Subarachnoid Aneurysm Trial (ISAT) was the pivotal trial that transformed cerebral aneurysm management. This landmark RCT randomized 2,143 patients with ruptured aneurysms suitable for either treatment to endovascular coiling or surgical clipping. Published in the Lancet in 2002, ISAT demonstrated that coiling reduced the risk of death or dependency at 1 year by 23.9% relative to clipping (absolute risk reduction 7.4%). The trial was stopped early by the data safety monitoring committee due to clear superiority of coiling. This single trial fundamentally altered neurosurgical practice worldwide, establishing endovascular coiling as the preferred first-line treatment for ruptured aneurysms amenable to both approaches.
Follow-up studies, subgroup analyses, and real-world validation
Long-term ISAT follow-up at 10 years confirmed sustained survival benefit with coiling, though rebleeding rates were slightly higher in coiled patients (a finding that did not negate the overall benefit). The Barrow Ruptured Aneurysm Trial (BRAT) provided additional RCT evidence with 6-year follow-up showing similar outcomes between coiling and clipping for anterior circulation aneurysms, while posterior circulation aneurysms strongly favored coiling. For unruptured aneurysms, the International Study of Unruptured Intracranial Aneurysms (ISUIA) provided crucial natural history data that informed treatment thresholds. The development of flow diverter devices (Pipeline Embolization Device, approved 2011) opened a new treatment paradigm for large and wide-necked aneurysms previously difficult to coil, with the PUFS trial demonstrating 86% complete occlusion at 5 years.
Integration into clinical practice guidelines and recommendations
The AHA/ASA guidelines for the management of aneurysmal subarachnoid hemorrhage (2012, updated 2023) recommend that both coiling and clipping should be considered for ruptured aneurysms, with endovascular treatment preferred when both options are technically feasible and equivalent. The guidelines emphasize that treatment decisions should be made by multidisciplinary teams including vascular neurosurgeons and neurointerventionalists. For unruptured aneurysms, the guidelines recommend individualized assessment incorporating aneurysm size, location, morphology, patient age, and comorbidities.
AHA/ASA
For ruptured aneurysms amenable to both treatments, endovascular coiling is reasonable as first-line treatment (Class IIa, Level B)
European Stroke Organisation
Endovascular treatment should be considered as first option when both treatment modalities are feasible
Now
Current standard of care and ongoing research directions
Current practice reflects a dramatic shift from the pre-ISAT era: endovascular approaches are now the dominant treatment modality, accounting for approximately 70% of aneurysm treatments at most centers. Surgical clipping remains essential for complex aneurysms not amenable to endovascular treatment, including many MCA aneurysms and those with wide necks or branch vessel incorporation. Flow diverter technology continues to evolve with intrasaccular devices (WEB device) and newer generation flow diverters. Active areas of research include the role of treatment for small unruptured aneurysms, optimal management of recurrent aneurysms after coiling, and the long-term durability of flow diversion. The ongoing debate between surgical and endovascular communities has given way to collaborative multidisciplinary decision-making at most cerebrovascular centers.
Is coiling always better than clipping for ruptured aneurysms?+
ISAT showed coiling superiority in aneurysms suitable for both treatments, but many complex aneurysms are not amenable to coiling. Posterior circulation aneurysms strongly favor coiling (BRAT data), while some anterior circulation aneurysms — particularly MCA aneurysms with branch vessels — may be better treated surgically. The decision should involve multidisciplinary discussion.
What about retreatment rates after coiling?+
Coiled aneurysms have higher rates of recurrence and retreatment (approximately 10-15%) compared to clipped aneurysms. However, retreatment is typically straightforward endovascularly, and the initial procedural advantage of coiling outweighs the retreatment risk in most analyses. Long-term ISAT follow-up confirms sustained net benefit despite higher rebleeding rates.
What are flow diverters and when are they used?+
Flow diverters are stent-like devices deployed across the aneurysm neck that redirect blood flow away from the aneurysm sac, promoting gradual thrombosis. They are primarily used for large, wide-necked, or fusiform aneurysms that are difficult to treat with standard coiling. The Pipeline Embolization Device is the most studied, with the PUFS trial showing 86% complete occlusion at 5 years.
