To understand brain aneurysms, it helps to know how they come about. Blood gets from your heart to your brain through blood vessels called arteries. Sometimes the wall of an artery inside your brain can weaken. Then, over time, the pressure of the blood flowing through the vessel can cause the wall to bulge out. That's an aneurysm. The most common type of aneurysm is the so-called “berry” aneurysm. It looks like a berry. These aneurysms form where arteries divide and branch off. If an aneurysm expands enough, it can rupture and cause bleeding (hemorrhage) into the brain. Brain hemorrhage (hemorrhagic stroke) can cause permanent brain damage. Most brain aneurysms form after age 40. About 1.5 to 5% of the U.S. population has or will develop a brain aneurysm, according to the American Stroke Association, but only a small percentage of these are at risk for rupturing. If you have one brain aneurysm, there's about a one in five chance you will have more than one. Symptoms of an Aneurysm Not all brain aneurysms are alike. One way they vary is by size. This can affect whether you have symptoms and what those symptoms might be. Small brain aneurysms rarely cause any symptoms. However, if an aneurysm grows to more than a half-inch wide, it may affect nearby tissues or nerves and cause symptoms. These symptoms may include: A headache in one part of your head A widening of one of your pupils Changes in your vision or double vision Weakness or numbness in an arm or leg Loss of speaking ability (aphasia) Not all brain aneurysms rupture. When they do, however, the bleeding into your brain causes symptoms that start very quickly. These may include: A very severe “worst ever” headache Nausea and vomiting Dizziness and confusion Stiff neck Vision loss or a change in vision Drooping eyelid Confusion and drowsiness Dizziness Weakness and numbness Pain when looking at light (photophobia) Seizure Loss of consciousness Who's at Risk? Some people are born with a brain aneurysm or a weakness in an artery that becomes an aneurysm. You also could be born with a blood vessel disease that increases your chances of developing an aneurysm. Other risk factors for an aneurysm include: Being female Being black or Hispanic Having a family history of aneurysm Having high blood pressure Having experienced brain trauma Having had a brain infection Using cocaine or amphetamine (stimulant) drugs Smoking Let your doctor know if you have any symptoms of a brain aneurysm. Seek emergency medical care if you have any sudden symptoms of a ruptured brain aneurysm. You can reduce your chances of having a brain aneurysm by: Getting screened for high blood pressure, which your healthcare provider will do at every physical Getting high blood pressure under control Using protective head gear when participating in any activity where head injuries are common Not using cocaine or amphetamines Not smoking If you have a family history of aneurysm, talk with your doctor. You may be a candidate for aneurysm screening tests.