Chiari malformation (CM) is a rare neurological condition in which the back of a person’s brain extends into the upper spinal canal (the channel formed by bones of the spine). In some cases, a portion of the brainstem is also involved. This malformation may cause increased pressure on the cerebellum (back of the brain) and brainstem, which can affect bodily functions controlled by these areas of the brain.

Malformation can occur during fetal development or later in life. Normally, the cerebellum and a portion of the brainstem rest in an indented space at the lower rear area of the skull. When this space is smaller than normal (e.g., due to a birth defect), it may force the cerebellum and brainstem to be pushed downward through an opening at the base of the skull (foramen magnum) and into the upper spinal canal.

There are four types of CM. Types I and II are more common than types III and IV, which are rare. Symptoms vary widely among the types, with type 1 having the mildest symptoms. Some patients with milder forms of CM do not experience any symptoms. Other patients may experience swallowing or gagging difficulties, abnormal hand coordination, difficulties with fine motor skills, neck pain and strange sensations in the arms and legs. Some types of CM can be fatal.

People with CM may also experience several complications, including enlargement of the skull (hydrocephalus). Neural tube defects (e.g., spina bifida) are conditions in which the brain and spinal cord (bundle of nerves enclosed within the spinal canal) fail to properly develop. These defects are associated with CM type II and can lead to paralysis.

In most cases, CM is present at birth, although people may not experience symptoms until they reach their 30s or 40s. CM is often not discovered until a physician identifies it while diagnosing another condition. Imaging tests such as magnetic resonance imaging (MRI) may reveal the presence of CM. In some cases, CM may be diagnosed during the prenatal period using an ultrasound.