Aneurysm is not a real tumor, as we know from its Latin or English name Aneurysm, it has nothing to do with tumor, but we just conventionally call it an aneurysm. In common parlance, an aneurysm is a bulge in the wall of an artery, which is somewhat like a blown up balloon with thin walls, and will rupture and bleed if it cannot withstand the pressure and rush of arterial blood flow. Because most of the larger intracranial arteries are located in the subarachnoid space, and aneurysms tend to occur in these larger arteries, aneurysm rupture often causes spontaneous subarachnoid hemorrhage (SAH), which can be combined with intracerebral and intraventricular hemorrhage in severe cases. The mortality rate for the first rupture of an aneurysm is 40-45% and doubles for the second rupture within a short period of time, making it one of the most dangerous diseases in neurosurgery.