Intracranial aneurysms are relatively common in the United States, with a reported incidence of 0.2 to 10 percent of the population, or 10 million to 15 million persons. Most intracranial aneurysms do not rupture; therefore, the need for surgical management is controversial, because little is known about the natural history of these lesions and the risks associated with their repair. Investigators for the International Study of Unruptured Intracranial Aneurysms evaluated the risks associated with repair of unruptured aneurysms and also explained the natural history of this condition.

The study consisted of retrospective and prospective components. The purpose of the retrospective component was to describe the natural history of unruptured intracranial aneurysms. Inclusion criteria for this group encompassed patients who had at least one intracranial aneurysm, regardless of symptoms. Patients were also eligible if they had had a previous aneurysm at another site that spontaneously ruptured or was treated surgically. Patients with a traumatic or mycotic aneurysm or one that measured less than 2 mm were excluded from the study.

The purpose of the prospective component was to assess treatment-related morbidity and mortality in patients with newly diagnosed unruptured intracranial aneurysm. Inclusion criteria for this group were similar to those of the retrospective group, except that investigators decided whether to include patients with or without planned surgical or endovascular intervention in this component of the study.

All patients underwent cerebral angiography to identify the size, presence and location of the aneurysms. Follow-up was conducted with a variety of methods, including annual questionnaires, telephone interviews and chart reviews. End points of the study included the incidence of intracranial hemorrhage, stroke and death. In addition, evidence of surgery-related complications, such as neurologic deficits and mortality, was recorded for patients in the prospective group.

A total of 1,449 patients from centers in the United States, Canada and Europe made up the retrospective group. Mean patient age was 52 years, and approximately 75 percent of the patients were women. In this group, 1,085 patients had single aneurysms, and 364 had multiple lesions. Signs and symptoms that led to the diagnosis were, in order of frequency, headaches, ischemic cerebrovascular disease, cranial nerve deficits, aneurysmal mass effect, ill-defined spells, seizures, and subdural or intracerebral hemorrhage. Principal risk factors in this group included a history of hypertension and its treatment, and smoking. Thirty-two patients experienced confirmed ruptures during the 10-year follow-up. The two significant predictors of rupture were a lesion size greater than 10 mm and a location at the basilar tip or in the posterior cerebral distribution. In addition, patients with a history of rupture were about 11 times more likely to experience subsequent rupture than patients who had not had a previous rupture.

A total of 1,172 patients were enrolled in the prospective group. The clinical presentations that resulted in the diagnosis of aneurysms in these patients were similar to those in the retrospective group, as were the principal risk factors. Intracranial surgery was performed on 996 of these patients. Overall surgical morbidity and mortality were approximately 16 percent at 30 days and 15 percent at one year after surgery. These figures included a total of 34 deaths, of which 30 were related to the surgery. Age was the only independent predictor of a poor surgical outcome, with patients over 64 years of age having the worst outcome.

The authors conclude that for patients with no history of subarachnoid hemorrhage, the likelihood of rupture of a cerebral aneurysm less than 10 mm in size is extremely small. Patients with a history of subarachnoid hemorrhage were 11 times more likely to experience a rupture. Moreover, the risks of morbidity and mortality from surgical intervention exceed the risk of rupture in patients with small aneurysms even 7.5 years after diagnosis, with or without planned surgical or endovascular intervention.