A structural defect and vertical hypermobility (back and forth sliding) of the craniocervical junction (interface between the occipital bone and the 1st and 2nd vertebrae) which can lead to a deformation of the brain stem, upper spinal cord and cerebellum (and associated neurological symptoms) due to compression.

Etiological Co-factors: Heritable Disorders of Connective Tissue (HDCT), such as Ehlers-Danlos Syndrome (EDS).