Craniocervical junction abnormalities can cause or contribute to cervical spinal cord or brain stem compression; some abnormalities and their clinical consequences include the following: Fusion of the atlas (C1) and occipital bone: Spinal cord compression if the anteroposterior diameter of the foramen magnum behind the odontoid process is < 19 mm Basilar invagination (upward bulging of the occipital condyles): Protrusion of the odontoid process through the foramen magnum, typically shortening the neck and causing compression that can affect the cerebellum, brain stem, lower cranial nerves, and spinal cord Atlantoaxial subluxation or dislocation (displacement of the atlas, usually anteriorly in relation to the axis but sometimes posteriorly, laterally, vertically, or rotationally): Acute or chronic spinal cord compression Klippel-Feil malformation (fusion of the upper cervical vertebrae or of the atlas to the occiput): Deformity and limited motion of the neck but usually no neurologic consequences, but sometimes compression of the cervical cord after minor trauma Platybasia (flattening of the skull base so that the angle formed by the intersection of the clival and anterior fossa planes is > 135 °), seen on lateral skull imaging: Shortness of the neck and usually causes no symptoms unless it is accompanied by basilar invagination Etiology Craniocervical junction abnormalities, of which there are many, can be congenital or acquired.