Malformations of the axis

Irregular atlantoaxial segmentation

Dens dysplasias - Ossiculum terminale

- Os odontoideum

- Hypoplasia/aplasia

Segmentation failure of


Occurs in 0.25% of the population; it only causes neurological symptoms and signs in one-quarter or one-third of this number

Very rare, except when associated with Klippel-Feil syndrome

Results from the persistence of the summit ossification center; seldom appears before the age of five years

Results from nonfusion of the epiphyseal plate and separation of the deformed odontoid process from the axial centrum. There is an increased incidence in patients with Down's syndrome, spondyloepiphysial dysplasia, and Morquio's syndrome

CT: computed tomography; MRI: magnetic resonance imaging.

Developmental and Acquired Abnormalities

These lesions may be misdiagnosed as: multiple sclerosis (31 %), syringomyelia or syringobulbia (18%), tumor of the brain stem or posterior fossa (16%), lesions of the foramen magnum or Arnold-Chiari malformation (13%), cervical fracture or dislocation or cervical disk prolapse (9%), degenerate disease of the spinal cord (6%), cerebellar degeneration (4%), hysteria (3%), or chronic lead poisoning (1 %).

The chief complaints of patients with symptomatic bony anomalies at the craniovertebral junction are: weakness of one or both legs (32%), occipital or suboccipital pain (26%), neck pain or paresthesias (13%), numbness or tingling of fingers (12%), and ataxic gait (9%). The average age of onset of symptoms in such patients is 28 years.

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