In 1891, Hans von Chiari, Professor of Morbid Anatomy at Charles University in Prague, published a report describing three types of brain malformations:

The Chiari malformation Type I (CMI) consists of hanging down (Herniation) of the cerebellar Tonsils into the upper Cervical spinal canal. In some cases the lower brain stem also hangs down into the spinal canal.

In the Type II malformation (almost always associated with spina bifida), the lower brain stem, cerebellar vermis, and tonsils hang down below the foramen magnum.

In the very rare Type III malformation the tissues of the Posterior Fossa are displaced into a soft sac (meningocele) at the back of the head and upper neck.

Dr. Chiari continued his studies and four years later published another report. In this one he made another important discovery. He found that some of the patients with the Chiari I and II malformations also had a fluid-accumulated cavity within the spinal cord called Syringomyelia.