The cerebellum is a small part of the brain located towards the bottom back of the head. In cases of Chiari malformation, the cerebellar tonsils (sometimes in addition to the brain stem) are pushed through the hole at the base of the skill and are forced into the spinal canal, resulting in herniation. Chiari malformations are generally categorized as Chiari I and Chiari II.
Chiari I is usually void of any symptoms and is more common in adolescents and adults. During Chiari I, the cerebellar tonsils have dropped at least 4mm into the upper part of the spinal canal.
Chiari II also involves the cerebellar tonsils being pushed into the spinal canal as well as the brainstem. This can cause problems in infancy or early childhood and is often associated with spina bifida or hydrocephalus.
