The Chiari Malformation (also called the Arnold Chiari Malformation) is a developmental anomaly at the base of the brain which results in the downward displacement of some of the brain's structures into the spinal canal. These malformations have been subdivided to reflect the degree of displacement as well as the varying etiology of the malformations. Type I malformations consist of a downward displacement of the cerebellar tonsils (two pegs of tissue which hang off the inferior surface of the cerebellum at the base of the brain) out the inferior opening of the skull into the spinal canal. No other structures of the brain are displaced. There is frequently (30-75%) an associated accumulation of fluid within the interior of the spinal cord. Type ll malformations have greater displacement of brain structures into the spinal canal. In addition to the cerebellum's tonsils there is also displacement of the inferior vermis (bundle of tissue connecting the two halves of the cerebellum), fourth ventricle, choroid plexus and medulla. The displacement is variable but quite frequently can extend down to the mid portion of the neck. Type II malformations are typically part of a larger developmental syndrome associated with myelodysplasia (spina bifida). Type lll malformation is a special type of myelodysplasia where there is a spina bifida (division or defect in the posterior portion of the bony spine) in the neck with an associated herniation of a fluid sac into the overlying defect in tissue closure. Lower structures of the brain herniate into this fluid sac. Occasionally one hears of a Type IV malformation. This refers to a hypoplasia (incomplete development) or aplasia (lack of development) of a portion of the cerebellum and, in fact, is not a hind brain herniation at all in contrast to Types 1-III.
It is not unusual for the Chiari Malformations to have an associated accumulation of fluid within the interior of the spinal cord. This condition is termed syringomyelia or hydromyelia. The condition ultimately results in progressive formation intramedullary (referring to the interior of the spinal cord or brainstem) cyst associated with progressive neurological deficits. While there are other reasons for cysts to occur within the spinal cord (trauma, tumor), the bulk of cysts which occur in the spinal cord are due to anomalies in the anatomy at the skull base.