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Medical Definition of Brain aneurysm
1. 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. (27 Sep 1997)