What Is Chiari Malformation?
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 (CM).
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.
CMs may be congenital, or present at birth, the result of structural defects in the brain and spinal cord that occur during fetal development. They also may develop later in life if spinal fluid is drained excessively from the lumbar or thoracic areas of the spine either due to injury, exposure to harmful substances or infection.
There are four types of CM:
- CM1, the most common form of CM, involves the extension of the lower part of the cerebellum into the foramen magnum without the involvement of the brainstem. (Normally, only the spinal cord passes through this opening.) CM1 typically appears in adolescence or adulthood and is the only type of CM that can be acquired.
- CM2 (classic CM or Arnold-Chiari malformation) involves the extension of both cerebellar and brainstem tissue into the foramen magnum. The nerve tissue that connects the two halves of the cerebellum may be partially complete or absent. CM2 is usually accompanied by a myelomeningocele — a form of spina bifida that occurs when the spinal canal and backbone do not close before birth, causing the spinal cord and its protective membrane to protrude through a sac-like opening in the back – which usually results in partial or complete paralysis of the area below the spinal opening.
- CM3 is characterized by serious neurological defects. The cerebellum and brainstem protrude, or herniate, through the foramen magnum into the spinal cord. Part of the brain’s fourth ventricle, a cavity that connects the upper parts of the brain and circulates CSF, may also protrude through the hole and into the spinal cord. In rare instances, the herniated cerebellar tissue can cause an occipital encephalocele, 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.
- CM4 involves an incomplete or underdeveloped cerebellum — a condition known as cerebellar hypoplasia. In this rare form of CM, the lower cerebellum is located further down the spinal canal, parts of the cerebellum are missing and portions of the skull and spinal cord may be visible.