The proband was a forty-four-year-old man with left hemiparesis for five years who had progressive weakness and numbness in his right upper extremity for two months prior to presentation. He reported an unsteady gait requiring an assistive device to walk, and he lacked fine-motor coordination in his hands. He reported no trauma of his head and neck. Physical examination revealed limited cervical spine flexion and extension. His gait was wide-based, spastic, and unsteady. Sensory deficits in the upper extremities were present. His left upper and lower extremities had muscle weakness of grade 3 or 4 of 5. He had symmetrical hyperreflexia, as well as positive Hoffmann and Babinski signs bilaterally. Radiographs of the cervical spine showed atlantoaxial instability from a dystopic os odontoideum with associated cervicomedullary compression (Fig. 1). Reconstructed computer tomography (CT) studies demonstrated the ossicle fused to the clivus (Fig. 1, b) and the transverse radiolucent gap located above the level of the superior articular facets of the axis (Fig. 1, d). He underwent posterior atlantoaxial fusion with use of C1 lateral mass and C2 pedicle screws. Solid fusion was present after six months. At the time of the latest follow-up at ten months, his unsteady gait had fully recovered, but there was no improvement in the upper extremities.
The proband was a forty-four-year-old man with left hemiparesis. a: Lateral radiograph of the cervical spine. b: The sagittal reconstruction CT scan demonstrates the round ossicle fused to the clivus. Note the sign of subdental synchondrosis (arrow). The transverse gap between the ossicle and C2 was located above the level of the superior articular facets of the axis. c: The axial CT image. d: Sagittal T1-weighted MRI of the cervical spine demonstrates the subdental synchondrosis (arrow). e: Postoperative lateral radiograph of the cervical spine demonstrates anatomic reduction of C1 and C2. f: Sagittal CT view of the upper cervical spine made six months after surgery demonstrates healed fusion between the C1 and C2 laminae.
The mother of the proband was seventy-two years old and had had an unsteady and spastic gait for about twenty years. After her son's diagnosis of os odontoideum, she underwent magnetic resonance imaging (MRI), with the finding of an os odontoideum fused to the clivus (Fig. 2). The location and morphology of the ossicle were similar to those of the proband. However, she had more lordosis in the lower cervical spine than the proband.
Sagittal T1-weighted (a) and T2-weighted (b) MRI scans of the cervical spine of the mother of the proband demonstrate an os odontoideum fused to the clivus (yellow and blue arrows).
The daughter of the proband was an asymptomatic sixteen-year-old high-school student. A cervical spine MRI demonstrated a dystopic os odontoideum fused to the clivus (Fig. 3), in addition to failure of segmentation of C6 and C7.
Sagittal T1-weighted (a) and T2-weighted (b) MRI scans of the cervical spine of the daughter of the proband, showing associated C6-C7 fusion (open arrow in Fig. 3, a) and ossicle (white arrow in Fig. 3, b).
None of the three patients recalled any prior substantial head and neck trauma. The younger brother of the proband had a well-developed odontoid on cervical radiographs.