The Chiari Malformations
Chiari Malformations (kee-AH-ree) are a group of disorders that affect the hindbrain. These were first described by Hans Chiari in the 19th Century. Then further expanded on by Julius Arnold who described Chiari 2 and called it Arnold Chiari Malformation after himself. Since then more types have been described in the literature. Currently, there is a lot of argument about what constitutes "Chiari". Whether it is the small posterior fossa and the herniation is a symptom. Or that Chiari is the cerebellar ectopia . As for 2021 a multidisciplinary committee of Chiari and Syringomyelia Specialists around the world created a consensus document on Chiari and Syringomyelia for Adults  and Children . Our site has been updated to reflect the information in these documents.
The normal structure of the skull is made to encase the brain, which is surrounded by protective CSF fluid, with the spinal cord exiting out the hole in the base of the skull called the foramen magnum. The cerebellum and brainstem are at the base of the brain and sit near the foramen magnum. Chiari malformation is described as a congenital condition where part of the cerebellum called the cerebellar tonsils are displaced down through the foramen magnum into the upper spinal canal due to a small posterior fossa (base of the skull).
The 2021 consensus specified herniations occurring due to the small posterior fossa should be classified as a Chiari Malformation where are other causes or "acquired" herniations should be called acquired cerebellar ectopia. This is due to there being many different ways (e.g. Intrcrannial Hypertension, Tethered Cord, CSF Leaks/Lumbar Shunts, tumours, craniocervical instability etc) that the cerebellum can become displaced which require different treatments. It was highlighted as very important to ensure the cause of the herniation and not lump them all as "Chiari".
This displacement of the cerebellar tissues can put pressure on the cerebellum, cranial nerves and sometimes parts of the brainstem. This pressure on the different parts of the cerebellum and/or brainstem may affect functions controlled by these areas. The tissue can also block the usual flow of cerebrospinal fluid (CSF) between the brain and spine. Resulting in conditions such as syringomyelia and increased intracranial pressure. It also stops the normal removal of waste, and circulation of nutrients and chemicals that occur as the CSF flows normally. It is also thought that the brain being stretched is what causes symptoms. Little is understood about WHY symptoms such as the Chiari Headache occur.
Historically there were 4 types of Chiari accepted. As of 2021, the consensus has removed Chiari 3 and 4 from the list of Chiari Malformations as it was felt these were different entities. Chiari 1.5 and 0 which have been discussed for years were introduced officially. There are discussions around Chiari 0.5, but this has not been officially accepted and only mentioned in a few research papers.
Chiari Malformation Classifications and Definitions.