Narrator: This is Science Today. Aneurysms are blood vessels that balloon out because of weakened vessel walls. Although unruptured cerebral aneurysms don't always present with symptoms, one classic warning sign is a headache - but according to Dr. Clay Johnston of the University of California, San Francisco, these aren't normal headaches.

Johnston: It generally is very sudden onset headache and very different in quality. Usually much more severe than an ordinary headache. Patients usually don't confuse this with a normal headache. They realize that something different has happened. That would be a sign that they had a sudden increase in the size of an aneurysm or a leaking from the aneurysm.

Narrator: Ruptured aneurysms increase with age and it's most common in women, but there are other risk factors including smoking, drinking or having hypertension.

Johnston: And people with family histories tend to be more likely to have one. So there's some predisposition that you're born with, probably and superimposed on that are the things you do in your life and your other diseases, like hypertension.

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