The Arnold-Chiari malformation is a deformity of the hindbrain in which there is displacement of parts of the cerebellum and brain stem through the foramen magnum into the upper part of the vertebral canal.
It was first observed by Arnold1 in 1894 in a newborn infant with lumbosacral spina bifida and a large Myelocyste. A portion of the cerebellum extended downward through the foramen magnum as a tonguelike process overlying the dorsal aspect of the spinal cord as low as the midcervical level. The lower portion of the elongated fourth ventricle was contained within the vertebral canal, and the vermis of the cerebellum was poorly developed.
In an analysis of 63 cases of hydrocephalus, Chiari,2 in 1895, described in great detail the deformities of the hindbrain and classified the changes under three types according to the degree of the displacement.
1. Elongation of the tonsils and medial parts
OGRYZLO MA. THE ARNOLD-CHIARI MALFORMATION. Arch NeurPsych. 1942;48(1):30-46. doi:10.1001/archneurpsyc.1942.02290070040003