Although placement of particles and other agents, including platinum coils, were subsequently attempted, it was not until the late 1980s, when detachable platinum coils were introduced, that endovascular techniques finally came of age.⁶ These coils were placed using a steel-wire delivery system via a microcatheter positioned at the aneurysmal sac. When the coils were in place, an electrical current was used to detach the platinum coil inside the aneurysm.⁷ The presence of the coil inside the aneurysm provided protection, likely from immediate hemodynamic changes and redirection of blood flow through the proper vessel. A thrombus eventually forms inside the aneurysm that protects the weaker aneurysmal neck and leads to the aneurysm being obliterated with connective tissue.⁸ Most endovascular treatments are performed under general anesthesia, although a very select group of patients can tolerate the procedure while awake.⁹ The current procedure involves a coil being placed in the aneurysmal sac with the assistance of a balloon,¹⁰ stent,¹¹ or microcatheter.¹²

Improvements in coil design have led to improved obliteration of aneurysmal sacs.¹³ Complex aneurysms are also treatable with the use of liquid embolic agents; for example, ethyl vinyl alcohol polymer in conjunction with intravascular stents also leads to obliteration of the aneurysm.¹³ In the early stages, endovascular procedures were limited to patients who were considered poor candidates for neurosurgical clipping, including: elderly patients,¹⁴ those with poor Hunt-Hess Scales (IV or V),¹⁵ posterior circulation aneurysms,¹⁶ or cavernous segment internal carotid artery aneurysms¹⁷ with active vasospasm.¹⁸ As the technology matured, with improvement in operator technique and coil design, embolization has been attempted in patients who traditionally have been good candidates for neurosurgical clipping.¹⁹ Finnish investigators compared both clipping and endovascular techniques in ruptured aneurysms, and confirmed the safety and effectiveness of coil embolization.²⁰

Mortality in both coiling and clipping was found to be comparable, with no difference in short-term clinical outcome. In a phase II trial with 57 patients in the clipping arm and 52 patients in the embolization arm, the angiographic outcome in anterior circulation aneurysm repair was better in the surgical group, while endovascular treatment had better angiographic outcome in posterior circulation patients. There was a technique-related mortality rate of 4% in the surgical group and 2% in the endovascular group.²⁰ Another recently concluded, larger randomized multicenter phase III trial—the International Subarachnoid Aneurysm Trial (ISAT)²¹—compared the safety and efficacy of embolization versus neurosurgical clipping. A total of 2,143 patients with ruptured aneurysms who were considered suitable for either treatment were enrolled. There were 1,070 patients enrolled in the clipping group, while the group receiving endovascular treatment by detachable platinum coils had 1,073 patients. Clinical outcomes were assessed at both two months and one year, with interim updates on re-bleeds and death. Out of 801 patients allocated to the endovascular arm, 190 (23.7%) became dependent or died at one year compared with 243 (30.6%) of the 793 allocated to the neurosurgical clipping arm (p=0.0019). Exmination of the risk of re-bleeding at one year showed that from endovascular treatment to be two per 1,276 patients, while risk of re-bleeding from clipping was 0 per 1,081 patients. The conclusion was that one-year outcomes in disability-free survival were significantly better with endovascular coiling.²¹ Further follow-up of these patients will hopefully add more information about the durability of coiling versus clipping.²² Figure 1 summarizes the results of the ISAT trial.

The outcome of endovascular treatment of unruptured aneurysms has improved following the introduction of platinum detachable coils. In a 2005 article, Taha et al. showed that endovascular embolization of cerebral aneurysms is a safe alternative to surgical clipping in the treatment of both ruptured and unruptured cerebral aneurysms; however, long-term outcome needs to be evaluated.²³ A retrospective comparative analysis of endovascular versus surgical treatment of unruptured aneurysms was published by Johnston et al. in 1999.24 In this review of data from 60 universities, factors such as age, sex, race, mode of admission, and year of treatment were all considered. When they looked at rates of in-hospital mortality, adverse outcomes (discharge to rehabilitation centers versus nursing homes) and length of stay were all greater for clipping compared with endovascular treatment.²⁴ More than one study examining noncontrolled patient series has been published that supports the safety and effectiveness of cerebral aneurysm embolization.^25–27 New coil designs, including 2-D and 3-D configurations, have led to improved radiographic obliteration of aneurysms.^28,29