Arnold Chiari Malformation is a neurological disorder where part of the brain, the cerebellum (or more specifically the cerebellar tonsils), descends out of the skull into the spinal area.  This results in compression of parts of the brain and spinal cord, and disrupts the normal Flow of cerebrospinal fluid (a clear fluid which bathes the brain and spinal cord).

Chiari malformations are structural defects in the cerebellum, the part of the brain that controls balance. Normally the cerebellum and parts of the brainstem sit in an Indented space at the lower rear of the skull, above the foramen magnum (a funnel-like opening to the spinal canal). When part of the cerebellum is located below the foramen magnum, it is called a Chiari malformation. CMs may develop when the bony space is smaller than normal causing the cerebellum and brainstem to be pushed downward into the foramen magnum and into the upper spinal canal. The resulting pressure on the cerebellum and brainstem may affect functions controlled by these areas and block the flow of cerebrospinal fluid (CSF) — the clear liquid that surrounds and cushions the brain and spinal cord — to and from the brain.