Giant intracranial aneurysms account for about 5 percent of all aneurysms, and are arbitrarily defined as having a diameter greater than 25 mm. These lesions, if left untreated, have an extremely poor natural history, with higher rates of morbidity and mortality than smaller saccular aneurysms. A retrospective review of untreatable giant aneurysms revealed an 80% risk of death or severe disability within 5 years of presentation. In prospective natural history studies, patients with unruptured, untreated giant aneurysms had a 5 year cumulative hemorrhage rate of 40-50%.

In contrast to patients with smaller aneurysms, 65 to 85% of patients with giant aneurysms present with symptoms related to mass effect. The presence of specific focal neurologic deficits, cranial neuropathies, and visual field defects are directly related to the size and location of the specific lesion. Subarachnoid hemorrhages occur in 25 to 35 percent of patients on presentation. Thrombus formation within these large aneurysms may result in ischemic events from parent vessel thrombosis, perforator vessel occlusion and embolic events.