There are four types of Chiari malformation (CM) that are generally recognized:

• Type I. Occurs when the lower part of the cerebellum (back of the brain), known as the cerebral tonsils, extends into the foramen magnum (the opening to the spinal canal). The spinal canal is the channel formed by the bones of the spine. The brainstem (lower part of the brain) is not involved. This is the most common form of CM.
  
  CM type I occurs during embryonic development and usually does not cause symptoms. When symptoms occur, headaches are common due to obstruction of the foramen magnum. CM type I may be identified during imaging tests, although in many cases, it is not diagnosed until adolescence or adulthood when it is discovered accidentally. Spinal curvature often occurs with this type of CM.
  
  
• Type II. Also known as classic CM or Arnold-Chiari malformation. In this case, the cerebellum and a portion of the brainstem are displaced into the spinal canal. People with CM type II may not have some or all of the nerve tissue that connects the two halves of the cerebellum. They may also develop myelomeningocele (a type of spina bifida in which nerves of the spinal cord protrude through the spinal canal) – which can cause paralysis below the spinal canal opening. In addition, hydrocephalus (excessive buildup of CSF in the brain) is most often associated with this type of CM. Some patients with CM type II die within two years of birth.
  
  
• Type III. The most serious form of CM, it occurs when the cerebellum and brainstem protrude through the foramen magnum and into the spinal canal. Although rare, the protruding tissue sometimes causes a pouch-like structure to appear on the back of the head or neck (occipital encephalocele). The covering of the brain and spinal cord also can protrude through an abnormal opening in the back of the skull, resulting in significant neurological problems, and life-threatening complications may develop.
  
  
• Type IV. A rare form of CM in which the cerebellum is incomplete or underdeveloped (cerebellar hypoplasia). In patients with CM type IV, the cerebellum may extend farther down into the spinal canal, parts of the cerebellum may be missing, and portions of the skull and/or spinal cord may be visible. This form of CM is typically fatal during infancy.