What is Chiari?

Chiari (pronounced key-are-ee) malformation is a brain abnormality that causes the cerebellum, the part of the brain that controls coordination and muscle movement, to protrude into the space normally occupied by the spinal cord. Some children are born with the condition, and others develop it as they grow.

Chiari malformation sometimes happens because the space at the back of the skull, where the cerebellum sits above the spine, is too small or is unusually shaped. These "cramped quarters" squeeze the cerebellum and even part of the brain stem, which controls the nerves in the face and neck, down through the foramen magnum (a funnel-like hole below the skull through which only the spinal cord usually passes).

Chiari malformation is also sometimes called tonsillar herniation or tonsillar ectopia because the part of the cerebellum that descends into the foramen magnum is called the cerebellar tonsils.The pressure caused by Chiari malformation on the cerebellum, brain stem, and spinal cord can keep them from working correctly. It can also block the movement of cerebrospinal fluid (CSF), the liquid that surrounds the brain and spinal cord and protects them from injury. When CSF doesn't flow properly, it can build up in the brain and spinal cord and block communication of the brain's messages to the body.There are four different types of Chiari malformation (I, II, III, and IV). Type I is by far the most common in children.

Chiari malformations (CMs) 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 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 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 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.