The Chiari malformation (CM) was first identified by Dr. Hans Chiari in 1891. It is an uncommon, complex, neurologic deformity which is present at birth. The brainstem normally sits in a funnel-like cavity just above the spinal cord. The problem occurs when the posterior fossa is not formed properly. Instead of the brainstem sitting in its proper space, the cerebellar tonsils are displaced downward into the funnel, thus causing pressure on this part of the brain and spinal cord (Figure-1).
There can also obstruction of the normal flow of CSF through this area because the tonsils of the cerebellum are “stuffed” into the funnel. Sometimes the brainstem appears atrophic (or shrunken), or elongated (stretched). This downward displacement of the cerebellar tonsils is called the Chiari malformation. Although there is a form of Chiari malformation which occurs in children with meningomyelocele, the Chiari I malformation more frequently causes problems in adults.
The exact cause of the Chiari malformation is unknown. It has been suggested that the Chiari malformation occurs during early embryo development of the brainstem and spinal cord. An abnormally small posterior fossa forces the brainstem downward. Persons with Chiari I malformation often have no symptoms until the third or fourth decade of life.The Chiari I malformation occurs more often in women than men. There is no particular ethnic or geographic incidence of this disorder. The Chiari malformation does seem to run in some families, but a specific gene has not been identified.
Some people with Chiari I malformation also have a syrinx (se-ri-nx) or syringomyelia (Figure 2).