Narrator: This is Science Today. A brain aneurysm is an abnormal bulging outward of one of the arteries in the brain. When one of these blood vessels rupture, it causes bleeding into the brain that can lead to death in half of those who suffer from it. Another thirty percent will be permanently disabled. Dr. Claiborne Johnston, director of the University of California, San Francisco's Stroke Center, says one to two percent of adults have an unruptured aneurysm.

Johnston : If you take a look at their blood vessels closely, you'll find one. And we know that smoking is an important risk factor for them. Being a woman is also a risk factor – not clear why; and high blood pressure appears to be a risk factor and having a family member with an aneurysm also puts you at risk for having one.

Narrator: Most unruptured aneurysms don't have symptoms, but sometimes people will develop headaches. But they're an unusual, very sudden kind of headache.

Johnston : So, like a thunderclap – boom – a headache begins. And the other is often it's often the worst headache that anybody's ever experienced up to that point. So, that's another way to trigger someone to thinking, “Hmmm, you know, maybe I ought to go in and be seen right away.”

Narrator: For Science Today, I'm Larissa Branin.