In tonsillar herniation, also called downward cerebellar herniation, transforaminal herniation, or “coning”, the cerebellar tonsils move downward through the foramen magnum possibly causing compression of the lower brainstem and upper cervical spinal cord as they pass through the foramen magnum.
Increased pressure on the brainstem can result in dysfunction of the centers in the brain responsible for controlling respiratory and cardiac function.
The most common signs are intractable headache, head tilt, and neck stiffness due to tonsillar impaction.
The level of consciousness may decrease and also give rise to flaccid paralysis. Blood pressure instability is also evident in these patients.