Etymology: Hans Chiari, Austrian pathologist, 1851-1916
a congenital anomaly in which the cerebellum and medulla oblongata, which is elongated and flattened, protrude into the spinal canal through the foramen magnum. It is classified into three types according to severity, ranging from prolapse of the cerebellar tonsils into the spinal canal without elongation of the brainstem (type I) to complete herniation of the cerebellum to form an occipital encephalocele (type III). It may be accompanied by hydrocephalus, spina bifida, syringomyelia, and mental defects. The classic form, type II, better known as Arnold-Chiari malformation, is tonsillar herniation with a myelomeningocele, a form of spina bifida. Also called Chiari's deformity. See also Arnold-Chiari malformation.