What is Chiari malformation (CM)?

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Chiari malformation (CM) is a congenital malformation in which the bottom of the brain (cerebellum) is crowded in the skull cavity, and forces the lower tips of the cerebellar hemispheres (tonsils) into the hole at the bottom of the skull (foramen magnum). This causes blockage of cerebrospinal fluid (CSF) flow into the spinal canal. 
A Chiari malformation is a structural defect in the base of the skull that creates a number of neurological symptoms, says Michael Seiff, MD, a neurosurgeon at Sunrise Hospital. In this video, he describes those symptoms that are often misdiagnosed.

Chiari malformations (CM) are structural defects in the cerebellum, the part of the brain that controls balance. Normally, the cerebellum and parts of the brain stem sit in an indented space at the lower end 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.

CM may develop when the bony space is smaller than normal, causing the cerebellum and brain stem to be pushed downward into the foramen magnum and the upper spinal canal. The resulting pressure on the cerebellum and brain stem may affect the 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.

This answer is based on souorce information from the National Institute of Neurological Disorders and Stroke.

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