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Basilar invagination

Case contributed by Derek Smith
Diagnosis certain


Imaged for bilateral hand sensory symptoms.

Patient Data

Age: 30 years
Gender: Female

The craniocervical junction is abnormal:

  • basilar invagination with upward displacement of the odontoid peg into the foramen magnum (with the use of the XR approximately 7 mm above Chamberlain line)
  • atlanto-occipital assimilation of the anterior arch of C1, with the posterior arch fused to the posterior C2 elements (with arcuate foramina)
  • resultant narrowing of the foramen magnum at 12 mm from the anticipated 31 mm (basion to opisthion distance)
  • as imaged, the medullocervical junction is of normal signal, with CSF maintained around the cord

Normal position of the cerebellar tonsils and allowing for limitations of the localizers, there is no hydrocephalus.

Minor noncompressive degeneration of the C2/C3, C3/C4 and C4/C5 discs but without cord compression and no foraminal stenosis. Normal lower cervical spine.

No syringomyelia in the included cord. Normal cord signal.

Normal included extraspinal tissues. No omohyoid bone.


Upward translation of the odontoid peg, measuring 12 mm above the Chamberlain line (measured from the hard palate to the midpoint of the opisthion).

Case Discussion

This craniocervical abnormality was an incidental finding. Further head imaging confirmed no hydrocephalus or signal change in the brainstem. No associated abnormalities were evident.

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