Chiari Malformations (Adult and Pediatric)

What are Chiari malformations (CMs)? They are structural differences in the part of the brain that controls balance – the cerebellum. Normally the cerebellum and parts of the brain stem 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.

How does a Chiari malformation develop? It may occur when the bony space is smaller than normal. This can cause the cerebellum and brain stem to be pushed downward into the foramen magnum and into the upper spinal canal. The resulting pressure on the cerebellum and brain stem may affect functions controlled by these areas. It may also block the flow to and from the brain of the clear liquid (cerebrospinal fluid [CSF]) that surrounds and cushions the brain and spinal cord.

Chiari malformations are classified by the severity of the disorder and the parts of the brain that protrude into the spinal canal. There are four types of Chiari malformations including:

Type I: This occurs when the lower part of the cerebellum (cerebellar tonsils) extends () into the foramen magnum, without involving the brain stem. Normally, only the spinal cord passes through this opening. Type I—which may not cause symptoms—is the most common form of CM. It is usually first noticed -- often by accident -- in teens or adults during an examination for another condition. Type I is the only type of CM that is not evident at birth.

Type II: Classic CM involves the extension of both cerebellar and brain stem tissue into the foramen magnum. The term Arnold-Chiari malformation (named after two pioneering researchers) is specific to Type II malformations. The nerve tissue that connects the two halves of the cerebellum (cerebellar vermis) may be only partially complete or absent. This type of CM is usually found along with a form of spina bifida that occurs when the spinal canal and backbone do not close before birth (myelomeningocele.) Spina bifida causes the spinal cord and its protective membrane to protrude through a sac-like opening in the back.  A myelomeningocele usually results in partial or complete paralysis of the area below the spinal opening.

Type III:  In the most serious form of CM, the cerebellum and brain stem protrude (herniate) through the foramen magnum and into the spinal cord. Part of the brain’s fourth ventricle which connects with the upper parts of the brain and circulates cerebrospinal fluid, may also protrude through the hole and into the spinal cord. In rare cases, the herniated cerebellar tissue can enter a pouch-like structure that protrudes out of the back of the head or the neck and contains brain matter. The covering of the brain or spinal cord can also protrude through an abnormal opening in the back or skull. This type of CM causes severe neurological challenges.

Type IV: This refers to an incomplete or underdeveloped cerebellum (cerebellar hypoplasia). In this rare form of CM, the lower part of the cerebellum is located in a normal position, but parts are missing and portions of the skull and spinal cord may be visible.

Type 0: There may be another form of CM disorder in which there is no protrusion of the cerebellum through the foramen magnum but headaches and other symptoms of CM are present. Some scientists debate whether this is technically a Chiari Malformation at all.

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Established in 2001, the Chiari Institute of Northwell Health Neuroscience Institute is the world's first comprehensive, multidisciplinary center for the management of patients suffering from Chiari malformation.

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