Is Chiari malformation nature's protective “air-bag”? Is its presence diagnostic of atlantoaxial instability?

Several authors have evaluated the subject of Chiari malformation since its first description by Cleland in 1883.[1] A number of hypothesis and speculations have been proposed as to the cause and effects of Chiari malformation.[2-6] A congenital anomaly is generally agreed to be the cause. Smaller posterior fossa volume that in unable to contain the cerebellar mass that herniates into the spinal canal has been the most accepted pathogenic phenomenon. Structural defects caused by genetic mutations, or lack of proper vitamins or nutrients in the maternal diet are also considered to be the probable cause. The cerebellar tonsils herniate into the spinal canal through the foramen magnum and appear to cause compression of the critical neural structures.