A chiari malformation is where the back of the brain (called the cerebellum) goes down below the base of the skull and into the upper spinal canal. Children may be born with this malformation or can develop it later in childhood. In many instances, a fluid collection is found in the spinal cord because of the chiari malformation. This fluid collection is called a syrinx..

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There are four types of Chiari malformation:
• Chiari I is the most common type
• Chiari II, often called the Arnold-Chiari Malformation, is mostly found in children with spina bifida
• Chiari III and IV are very rare and may be life threatening