The bleeding in Sen. Tim Johnson brain resulted from a condition he was born with that causes arteries and veins to grow tangled and abnormally large.

The condition, called arteriovenous malformation, or AVM, is believed to affect about 300,000 Americans, according to the National Institute of Neurological Disorders and Stroke.

The institute's Web site said only about 12 percent of the people with the condition experience symptoms, ranging in severity. The condition kills about 3,000 people a year.

This condition is a defect believed to arise during embryonic development or soon after birth. It is composed of tangled arteries and veins that can disrupt the flow of blood to and from the brain.

The most common symptoms are seizures or headaches.

The greatest danger from these malformations is bleeding. While most episodes are not detected because they are not severe enough to cause damage, larger episodes of bleeding can cause damage or even death.