When pressure increases around the cerebellum, one or both cerebellar tonsils may herniate downward through the opening in the base of the skull (the foramen magnum). The medulla also gets compressed in this downward herniation; tiny blood vessels going to the brainstem get stretched and damaged resulting in bleeding and stroking of the brainstem. Breathing and circulation centers are controlled by this area of the brainstem and are injured, death usually occurs.