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Abstract
Arnold-Chiari, or simply Chiari malformation, is the name given to a group of deformities of the posterior fossa and hindbrain (cerebellum, pons, and medulla oblongata). Issues range from cerebellar tonsillar herniation through the foramen magnum to the absence of the cerebellum with or without other associated intracranial or extracranial defects such as hydrocephalus, syrinx, encephalocele, or spinal dysraphism. Classification Chiari malformations are classified based on their morphology and severity of anatomic defects, typically through imaging (or autopsy). Chiari I is the least severe and often found incidentally. It is characterized by one or both pointed (not rounded) cerebellar tonsils that project 5 mm below the foramen magnum, measured by a line drawn from the basion to the opisthion (McRae Line). Chiari II consists of brainstem herniation and a towering cerebellum in addition to the herniated cerebellar tonsils and vermis due to an open distal spinal dysraphism/myelomeningocele. Chiari III involves herniation of the hindbrain (cerebellum with or without the brainstem) into a low occipital or high cervical meningoencephalocele. Chiari IV is now considered obsolete. Prior to becoming an obsolete diagnosis, it was already a more controversial and rare variant that demonstrated severe cerebellar hypoplasia, similar to primary cerebellar agenesis. Previously some stated that myelomeningocele could be present, while others argued that the presence of myelomeningocele should then be classified as a Chiari II with a vanishing cerebellum. There are other controversial reported classifications, including Chiari 0, Chiari 1.5, and Chiari V. Chiari 0 is characterized by syringomyelia without hindbrain herniation while Chiari 1.5 is felt to be the progression of Chiari I with increased cerebellar tonsillar descent and some involvement of the brainstem. Chiari V, the most severe variant, represents cerebellar agenesis with occipital lobe descent and herniation through the foramen magnum.

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