"Basil or artery occlusion is a specific type of stroke that happens in the posterior circulation. This is the blood flow that enters the brain and then in the backside of your head," said Dr. Robert Kowalski, a clinical instructor in neurology at the University of Colorado School of Medicine who does stroke research. This area of the brain is critical for cardiac function. It regulates heartbeat and breathing. "These are a terrible type of stroke because the mortality rate is close to 90% when it's not treated," Kowalski said.