Most Chiari malformations are congenital problems that affect the lower part of the brain, near to the passage at the base of the skull through which the brain stem connects with the spinal cord. Often, Chiari malformations arise because the space within the brain that normally houses the cerebellum is too small.

Chiari malformations are usually classified as Type 1, Type 2 and Type 3. Mild forms of Chiari malformation produce no symptoms. More severe forms produce symptoms ranging from dizziness through to severe cognitive impairment and paralysis, which arise because the flow of cerebrospinal fluid becomes restricted. Cerebrospinal fluid usually protects the cerebrum from damage and when it is in short supply, the brain injures easily.

Chiari malformations and the cerebellum

The cerebellum is mainly associated with the control of balance, but it is also involved in a range of other functions. These include:

  • Motor activity: the cerebellum ‘fine tunes’ movement.
  • Diverse cognitive functions: these include our attention span, visual-spatial reasoning (reading a map), learning, language and memory.
  • Emotions: the cerebellum is important in determining the way we react emotionally to day-to-day events.

Damage to the cerebellum does not usually produce the same devastating effects as damage to the cerebral cortex but it does lead to communication problems, dyslexia and mental problems.

Type 1 Chiari malformation

This type is the most common and the least severe and may go undetected. Symptoms, when they exist, include headaches, pain, tinnitus, nausea, dizziness, irregular breathing during sleep, trouble with swallowing, and impaired coordination.
Syringomyelia, the presence of a cavity or a cyst inside the spinal cord, sometimes occurs in a type 1 Chiari malformation. Syringomyelia can cause pain or even paralysis but symptoms usually develop later in life. Type 1 Chiari malformation is sometimes detected only by chance during an investigation for an unrelated health problem. Although most type 1 malformations are congenital, some develop as a result of another condition; disorders of connective tissue, such as Ehlers–Danlos Syndrome, may lead to acquired type 1 Chiari malformation.

Type 2 Chiari malformation

Type 2 is a more severe form that is usually associated with myelomeningocele. Myelomeningocele is a kind of spina bifida in which a short section of the spinal cord protrudes from the spine and is enclosed in a visible sac outside the body at birth. Problems associated with myelomeningocele include severe learning difficulties, impaired perception and clumsiness.
Myelomeningocele can also lead to complete or partial paralysis of areas of the body below the spinal abnormality.
Type 2 Chiari malformation can also lead to:

  • Microcephaly: underdevelopment of the brain, and small head size
  • Hydrocephaly: fluid on the brain
  • Asymmetric brain development
  • Deformities, particularly of the hips, legs and feet
  • Lower back pain

Type 3 Chiari malformation

The most severe type of Chiari malformation usually leads to encephalocele, in which areas of the brain protrude through holes in the skull and are contained in a sac or sacs lying outside the skull at birth. In some cases of type 3 Chiari malformation, the sac lies at the back of the head, and contains areas of the cerebellum and the brain stem.

Children affected do not develop at the normal pace and can have severe learning difficulties and paralysis. It is rare but, tragically, most of those affected die in infancy.

Other types - Type 0 and Type 4

Two other classifications of Chiari malformation are sometimes used. In Type 0 Chiari malformation, symptoms of Type 1 malformation are present but there are no obvious distortions to the base of the brain.

Type 4 Chiari malformation is exceptionally severe. It is associated with cerebellar hypoplasia, an incomplete cerebellum. It resembles spina bifida in that sections of the skull and spinal cord may be exposed. Like Type 3 malformation, it is rare, and most children born with it die prematurely, usually before the age of one.