Craniocervical junction abnormalities

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Craniocervical junction abnormalities are congenital or acquired abnormalities of the occipital bone, foramen magnum, or first two cervical vertebrae that decrease the space for the lower brain stem and cervical cord. These abnormalities can result in neck pain; syringomyelia; cerebellar, lower cranial nerve, and spinal cord deficits; and vertebrobasilar ischemia. Diagnosis is by magnetic resonance imaging (MRI) or computed tomography (CT). Treatment often involves reduction, followed by stabilization via surgery or an external device.
Neural tissue is flexible and susceptible to compression. 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 lt; 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 gt; 135#176; ), seen on lateral skull imaging: Shortness of the neck and usually causes no symptoms unless it is accompanied by basilar invagination