Giant intracranial aneurysm A giant intracranial aneurysm is defined as one larger than 2.5 cm in diameter. Giant aneurysms are thought to represent about 5-8% of all intracranial aneurysms. The three most common locations are:The terminal internal carotid artery (most common) Middle cerebral artery bifurcation Top of basilar arterySeventy to seventy-five percent of patients with giant aneurysm present with mass effect, usually manifest by visual defect, cranial nerve dysfunction, hemiparesis, seizures or headache. Thrombosis and stroke due to blood clot formation within the aneurysm and subsequent distant embolus, occurs in some 2-5% of patients with giant aneurysm. Size does not influence the hemorrhage rate, and approximately 25% of patients present with subarachnoid hemorrhage due to rupture.