Arnold Chiari malformations classification
Type I: Downward displacement of the cerebellum with elongation of the medulla such that the cerebellar tonsils egress through the foramen.
Type II: Associated with lumbosacral myelomeningocele and numerous other nervous system malformations including:
- Small posterior fossa
- Cerebellar tonsil herniation through the foramen magnum
- Elongated and thinned medulla
- Characteristic beaking appearance of the quadrigeminal plate
- Syringomyelia and occasionally syringobulbia
- Interdigitation of gyri along the interhemispheric fissure
Type III: An occipital encephalocele with protrusion of cerebellar remnants into the overlying sac.
Type IV: Isolated hypoplasia of the cerebellum not associated with other nervous system malformations.