In 1891, Hans Chiari (1851-1916) wrote his first paper on ectopia of cerebellar tissue.1 He described a malformation of the cerebellum and brainstem with varying descent through the foramen magnum into the cervical canal. He related the changes to congenital hydrocephalus, because he had not observed the deformities in late onset or acute hydrocephalus. The malformation is not described in Kinnier Wilson's Neurology, and presumably was long regarded as a rare curiosity until the surgery of dysraphism and hydrocephalus began. Unfortunately, some English translations of Chiari's work2 contained inaccuracies, but Koehler has corrected these.3 Chiari described three grades:

Type 1 showed “elongation of the tonsils and medial parts of the inferior lobes of the cerebellum into cone shaped projections, which accompany the medulla oblongata into the spinal canal.”

In most cases the cerebellum itself appeared normal, but in some instances there was softening or sclerosis. The fourth ventricle was normal or slightly elongated; the medulla appeared flattened. Chiari was uncertain whether these changes cause symptoms or not, but he was inclined to believe bulbar symptoms might result.

He recorded type 2 changes in a 6 month old child who had paraplegia, a paralysed bladder, and had succumbed to pneumonia. The pons descended into the spinal canal over 6 mm. And the medulla extended to the third cervical vertebra. The baby had hydrocephalus and “a cylindrical hole 6 mm in width, filled with clear serum” in the dorsal side of the spinal cord extending from the first to the seventh segment. A second cavity, a “hydromyelie” was found a few segments below.

There was also diastematomyelia, meningomyelocoele, and displacement of the conus to the level of the sacrum.

He found only one example of type 3, a malformation with absence of part of the tentorium cerebelli and prolapse of the cerebellum and fourth ventricle into the cervical canal and an associated hydromyelic cavity in communication with the fourth ventricle.

In 1896, Chiari added4 63 cases of congenital hydrocephalus of which 14 children or adults had a type 1 malformation, seven had a type 2—mainly neonates a few days old, who had varying types of spinal dysraphism. He had noticed that the severity of the hydrocephalus did not relate to the extent of the malformation and he postulated that defective growth of the skull caused raised local pressure, presumably forcing down the hindbrain. He thought that there was a graded increase, ranging from descent of cerebellar tissue within the fourth ventricle, to descent with, but dorsal to it. He also recorded two instances of hypoplasia of the cerebellum that he believed had been caused by hydrocephalus.

Hans Chiari was the son of a famous Austrian gynaecologist, JBVL Chiari, who held chairs in Prague and Vienna and described post-partum galactorrhoea with pituitary adenoma, amenorrhoea, and low follicle stimulating hormone (Chiari-Frommel syndrome), which we would now recognise as prolactinoma. Hans was professor of pathology in Prague's German University and later in Strasbourg. He wrote a classical history of pathology and the first account of chorionic carcinoma. His vast knowledge was widely admired, but if irritated by poor preparation of tissue sections by his students, he was said to rap their knuckles. He wrote extensively on topics including the pancreas and its capacity for autodigestion, and endarteritis chronica deformans of the carotid bifurcation with cerebral embolism. He died suddenly after a throat infection.

Chiari gave credit to other writers including Arnold and Cleland. John Cleland of Edinburgh (1835-1925) wrote a brief account in 1883, and his illustrations were not distinct. He described an infant with spina bifida and hydrocephalus with the cerebellar nodulus displaced into an elongated fourth ventricle, separating the cerebellar lobes. This corresponded to Chiari's type 2.5 Much of his paper was devoted to encephalocoele and spina bifida.

However, more than 50 years earlier, Jean Cruveilhier (1791-1874), physician, and Professor of Anatomy in Paris, in his two volume atlas6 clearly described and illustrated* many examples of spina bifida and hydrocephalus, usually with infection of the sac and meningitis. In a second case of meningomyelocoele he described the “Arnold-Chiari malformation” with diastematomyelia (p 2)

“ . . .the spinal cord is divided into two perfectly distinct lateral cords; each cord giving off nerves with their double roots . . .the upper part of the considerably dilated cervical region, contained both the medulla oblongata and the corresponding part of the cerebellum which was elongated and covered the fourth ventricle, itself enlarged and elongated.”

He mentioned two other cases of hindbrain malformation observed by a M Sestier.

Julius Arnold (1835–1915) was a pupil of Virchow and Friedreich and became Professor of Anatomy at Heidelberg. His paper in 1894 was about an infant with spina bifida, but without hydrocephalus. Chiari believed that it was an example of his type 2 cases, despite the absence of enlarged ventricles. The attachment of Arnold's name is probably inappropriate since his writings were mainly about the dysraphism; and, consideration of the hindbrain descent was a small and peripheral issue.

Modern authorities regard type 1 as an ectopia of the tonsils, sometimes with minor deformity of the medulla into the foramen magnum. Type 2 is almost always accompanied by hydrocephalus and a myelomeningocoele; it includes lengthening of the vermis and tonsils and in some case the elongated fourth ventricle and their displacement below the foramen magnum. The dorsal part of the medulla is shifted caudally and overrides the cord in a Z-shaped kink impacted in the tight foramen; the posterior fossa is often broad and shallow and the tentorium may be rudimentary and low; hydromyelia, syringomyelia, and a split cord are frequent. Type 3 is a cervical spina bifida, the entire cerebellum herniated through the foramen magnum forming a myelocerebellomeningocoele. Type 4 is cerebellar hypoplasia. Chiari's classification is still widely accepted by neuropathologists.7