The Arnold Chiari malformation

Abstract

The Arnold Chiari malformation is a herniation of the lower brain stem and cerebellum caudally into the vertebral canal. The malformation occurs in three types of which Type ii, in which there is herniation of the cerebellar tonsils, vermis, and fourth ventricle, is the most common. The Arnold Chiari malformation is not commonly associated with a normal spinal cord, but several cases of the malformation in association with a normal spinal cord have been reported. The association of the Arnold Chiari malformation with an abnormal spinal cord led to the introduction of the traction theory in which it was postulated that traction on the inferior brain stem and cerebellum from lower spinal cord levels in effect pulled the cerebellum through the foramen magnum. However, recent work, both clinical and experimental, suggested that pressure from above rather than traction from below is involved in producing the Arnold Chiari malformation. The traction produced by the tethering of the lower spinal cord levels does not affect the cervical spinal cord or medulla. In light of the recent work, the theory of pressure from above appears the most tenable; however, the traction theory is still widely accepted as the mechanism of the Arnold Chiari malformation.