Chiari malformations (CMs) are structural defects in the cerebellum, the part of the brain that controls balance. Normally the cerebellum and parts of the brain stem sit in the occiput above the foramen magnum. When part of the cerebellum is located below the foramen magnum, it is called a Chiari malformation.
CMs may develop when the bony space is smaller than normal, causing the cerebellum and brain stem to be pushed downward into the foramen magnum and into the upper spinal canal. The resulting pressure on the cerebellum and brain stem may affect functions controlled by these areas and block the flow of cerebrospinal fluid (CSF).
CM has several different causes. It can be caused by structural defects in the brain and spinal cord that occur during fetal development, whether caused by genetic mutations or lack of proper vitamins or nutrients in the maternal diet. This is called primary or congenital CM. It can also be caused later in life if spinal fluid is drained excessively from the lumbar or thoracic areas of the spine either due to injury, exposure to harmful substances, or infection. This is called acquired or secondary CM. Primary CM is much more common than secondary CM.