Several alternative measures are in common use for this purpose. Endovascular obliteration of the lumen of the aneurysm has become the preferred approach for most aneurysms and is strongly favored for those not easily accessible by surgery, for example, in the cavernous sinus—and for patients whose medical state precludes an operation. Among several trials that have compared surgery with endovascular placement of coils in the aneurysm, most have shown equivalence or a slight superiority of the latter. For example, the International Subarachnoid Aneurysm Trial Group, reported by Molyneux and colleagues, randomly assigned more than 2,000 patients to surgery or coil deployment; the overall rate of death or dependence at 1 year was 24 percent in the endovascular group and 31 percent in the operated group, a difference that was sustained at 2 years of follow up. A single center trial reported by Spetzler and colleagues (2013) had less restrictive entry criteria but favored endovascular treatment overall in outcome at 1 year; by 3 to 6 years the clinical outcomes of the surgical and endovascular groups were similar. Certain features of the aneurysm may dictate that surgery or endovascular treatment is not possible and the alternative is therefore favored. It is self-evident that the skill of the surgeon or interventionalist and the quality of postoperative care are major determinants of outcome.
Quite often in clinical practice, cerebral angiography, MRI, MRA, or CT performed for an unrelated reason discloses the presence of an unruptured saccular aneurysm. Or, a second or third aneurysm is found during the angiogram to assess a ruptured one. There is now a reasonably sound body of information about the natural history of these lesions. Wiebers and colleagues (1987) observed 65 patients with one or more unruptured aneurysms for 5 years or longer after their detection. The only feature of significance relative to rupture was aneurysmal size. Of the 44 aneurysms smaller than 10 mm in diameter, none had ruptured, whereas 8 of 29 aneurysms 1 cm or larger eventually did so, with a fatal outcome in 7 cases. Two large studies have attempted to refine these statistical data. In the older Cooperative Study of Intracranial Aneurysms, none of the aneurysms less than 7-mm diameter “had further trouble.” A more recent and sizable cooperative study that included 4,060 patients and gathered data prospectively for 5 years, conducted by the International Study of Unruptured Intracranial Aneurysms Investigators, found an extremely low rate of rupture, approximately 0.1 percent yearly, for aneurysms smaller than 7 mm in diameter, an annual risk of 0.5 percent for aneurysms between 7 and 10 mm, and a risk ranging from 0.6 to 3.5 percent for lesions between 13 and 24 mm (depending on location). The risk ranged up to 10 percent for aneurysms greater than 25-mm diameter. The yearly rates for rupture were higher in all categories if there had been prior bleeding from another site. The location of the lesion also had great bearing on the risk of rupture, as did increasing age; notably, vertebrobasilar and posterior cerebral aneurysms bled at a rate many times higher than the others. Such data aids in comparing the risk of surgery and endovascular treatment, which, for example, exceed the risk of bleeding within 5 years for small aneurysms located in the carotid circulation. In almost all other circumstances, there is overall benefit to obliterating the unruptured aneurysm.
A special problem pertains to clots within an aneurysm that cause transient ischemic attacks or small strokes in the vascular territory distal to the site. The frequency of this complication is not clear and it occurs at times without evident intraluminal clot on an angiogram.