Chiari malformations are structural defects in the cerebellum, the part of the brain that controls balance. When the indented bony space at the lower rear of the skull is smaller than normal, the cerebellum and brainstem can be pushed downward. The resulting pressure on the cerebellum can block the flow of cerebrospinal fluid (the liquid that surrounds and protects the brain and spinal cord) and can cause a range of symptoms including dizziness, muscle weakness, numbness, vision problems, headache, and problems with balance and coordination. The exact cause is still unknown. In some cases of Chiari Malformation type 1, genetic factors may be involved. Treatment may require surgery.

There are four types of Chiari malformations classified according to the degree of severity, with type 1 being the most common and least severe:

Chiari malformation type 1
Chiari malformation type 2
Chiari malformation type 3
Chiari malformation type 4

Some researchers add a type 0 to the classification, which is characterized by no or very little herniation of the cerebellum.