The Arnold-Chiari malformation.

The Arnold-Chiari malformation is a developmental abnormality of the brainstem and cerebellum with displacement of these structures into the cervical spinal canal. Other anomalies involving the base of the skull and vertebral bodies are frequently present. In children, hydrocephalus and myelomeningocele may complicate the picture. In adults, symptoms usually do not occur until the third to fifth decade and then are often misdiagnosed as degenerative disease due to the presence of a syrinx in the cord. Treatment is focused on decompressing the spinal cord and brainstem.