Arnold-Chiari Malformation(Archived)

Arnold-Chiari malformation, 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. 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 is 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 malformation is characterized by brainstem herniation and a protruding cerebellum, in addition to the herniated cerebellar tonsils and vermis, resulting from an open distal spinal dysraphism/myelomeningocele. Chiari III involves the herniation of the hindbrain (cerebellum with or without the brainstem) into a low-occipital or high-cervical meningoencephalocele. Chiari IV is now considered obsolete. Before 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 reported, yet controversial, classifications, including Chiari 0, Chiari 1.5, and Chiari V. Chiari 0 is characterized by syringomyelia without hindbrain herniation. At the same time, Chiari 1.5 is considered a progression of Chiari I, characterized by 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.