Unruptured intracranial aneurysms (UIA) constitute a significant public health problem in the United States which is growing in magnitude. The economic and social implications of optimizing clinical practice in this area are striking given the considerable and escalating frequency with which UIAs are now detected in the population. The prevention of unnecessary death and disability related to UIA depends to a large degree upon a better understanding of the natural history of these lesions as well as the short and long-term benefits and risks associated with their repair.

The International Study of Unruptured Intracranial Aneurysms (ISUIA) Study Group undertook an initial epidemiological project on September 1, 1991, to obtain a better understanding of the natural history of unruptured intracranial aneurysms (UIAs) and the risks associated with surgical and endovascular repair of UIAs to help define the optimal management of patients with these lesions. Over the next 7½ years, 5,500 patients were entered into the study and followed until 2001 as outlined in the research protocol. As planned, the overall patient group from Phase I of the study included approximately 1,500 retrospectively identified patients who were unoperated and approximately 4,000 prospectively entered patients including approximately 1,700 who were conservatively managed, 1,900 who had surgical treatment and 450 who underwent initial endovascular ISUIA I and II was published in The Lancet in July 2003.

The current phase is a continuation prospective component of the International Study of Unruptured Intracranial Aneurysms. The primary objectives are to define the long-term risks of UIA rupture and other UIA natural history outcomes, risk factors associated with these natural history outcomes, to ascertain the long-term outcomes associated with endovascular and surgical repair of these lesions, and to establish predictors of good and poor treatment outcomes. The first two phases of ISUIA have provided substantial and unique information regarding short-term prospective natural history and treatment outcomes and have established that short-term prospective natural history rupture rates are different than retrospective natural history rupture rates. The current phase utilizes the large ISUIA cohort of patients established over the past ten years of 4,060 cases in the prospective cohort including 1,692 cases in the unoperated cohort and 2,368 cases in the cohort which had UIA repair.