Aneurysm Saccular aneurysms are thin-walled blisters protruding from specific arterial bifurcation sites, most commonly on the arteries of the circle of Willis at the base of the brain. Aneurysms, presumed to be the result of developmental defects in the media and elastica, gradually enlarge. A much less frequent type of aneurysm is due to infection caused by an embolus from the heart in a patient with subacute bacterial endocarditis. This embolus usually lodges in a distal branch of the middle cerebral artery, and an aneurysm in this location is unlikely to be due to any other cause. The most common site for saccular aneurysms are the junction of the anterior cerebral and anterior communicating arteries, the junction of the internal carotid and posterior communicating arteries, the first major branch of the middle cerebral artery, the bifurcation of the internal carotid artery into the anterior and middle cerebral arteries, and the bifurcation of the basilar artery.
Most aneurysms are asymptomatic until they rupture.
Occasionally pressure on a cranial nerve is the first manifestation. Rupture of the aneurysm usually causes a sudden severe headache. The patient often complains of the most severe headache they have ever had. The hemorrhage is usually into the subarachnoid space but may also be in the brain tissue. In smaller hemorrhages there may be only headache, photophobia, and stiff neck.