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 . There is still no consensus on the definition of "Chiari".
Normally all the brain tissue resides within the skull, with the spinal cord exiting out the hole in the base of the skull called the foramen magnum. The cerebellum and brainstem sit just above the foramen magnum. Chiari malformation is described as when part of the cerebellum called the cerebellar tonsils are displaced down through the foramen magnum into the upper spinal canal .
The are several established and theoretical ways this displacement of brain tissue can occur. It can be congenital due to a misshapen or small skull forcing the cerebellar tonsils out. While most incidences of Chiari is considered congenital, some are acquired later in life. These are currently called "acquired chiari" while a consensus is being reached with the definition of Chiari. Other causes can include issues such as tethered cord pulling the cerebellar tonsils down, excessive spinal fluid draining and/or high intracranial pressure.
This displacement of the cerebellar tissues can put pressure on the cerebellum 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.