Presentation
Upper limb numbness.
Patient Data
Abnormal smooth, well-corticated ossicle at the superior aspect of the hypoplastic C2 dens, consistent with os odontoideum. The separated part of the dens is not seen in its normal place, however, and is displaced anteriorly, in keeping with the dystopic type. Abnormal T2 hyperintense signal of the cervical cord opposite C1-2 level mostly represents an area of myelopathy (eliciting the "owl eyes" sign on the axial images at said level).
On CT, the os odontoideum is positioned superior to the anterior one half of the C2 vertebral body and the anterior arch of C1 is positioned anterior to the anterior C2 vertebral body - findings consistent with atlanto-axial subluxation on top of dystopic os odontoideum.
Case Discussion
Os odontoideum is an anatomic variant of the odontoid process of C2, that can be associated with atlantoaxial instability. In this case, it causes myelopathy mostly responsible for the symptoms.