CEREBRAL ANEURYSM - A dilated and weakened portion of a cerebral blood vessel that is prone to rupture. A cerebral aneurysm may occur as a birth defect or develop as the result of long-standing poorly controlled hypertension. Symptoms of a ruptured cerebral aneurysm include a sudden, severe "thunderclap" headache that may be associated with nausea, vomiting, and a decreased level of consciousness.