He assigned a grade to the malformations beginning with Type I, the mildest form, classification today. Scientists and physicians further define Chiari malformations by the exact millimeter of brain stem that extends into the neck. Chiari type I malformations are a result of the smallest degree of herniation and are not associated with spina bifida. Chiari type II malformations are almost exclusively associated with a type of spina bifida known as a myelomeningocele, an opening of the spine and spinal cord on the lower back. III and IV malformations are very rare.
Chiari malformations can occur with other conditions.
Many individuals with Chiari malformations do not have any other associated medical problems. There are no data describing the incidence of Chiari malformations. However, malformations of the cervico-medullary junction, the connection between the brain and spine, are present in approximately 1 percent of live newborns. The most common cervico-me dullary junction malformations are Chiari malformations. Most patients with Chiari malformations are without symptoms.