Retro-odontoid pseudotumor in diffuse idiopathic skeletal hyperostosis.

STUDY
DESIGN: A rare case of retro-odontoid pseudotumor combined with diffuse idiopathic skeletal hyperostosis is presented.
OBJECTIVE: To discuss the pathomechanism of retro-odontoid pseudotumor in diffuse idiopathic skeletal hyperostosis. SUMMARY OF BACKGROUND DATA: Reports describing craniovertebral manifestations of diffuse idiopathic skeletal hyperostosis are quite rare. Only two cases of an atlantoaxial subluxation and one case of an odontoid fracture have been reported. Myelopathy resulting from retro-odontoid pseudotumor combined with diffuse idiopathic skeletal hyperostosis has not been reported previously.
METHODS: A 74-year-old man presented with spastic tetraparesis caused by a retro-odontoid pseudotumor combined with diffuse idiopathic skeletal hyperostosis. Transoral removal of the extradural mass combined with a dorsal atlantoaxial fusion was performed using a titanium frame with sublaminar cable wiring.
RESULTS: Yellowish amorphous material extruded from between the odontoid process and the arch of C1 when the anterior capsule had been incised. The retro-odontoid mass was very firmly attached to the hypertrophied ligaments. The mass therefore had to be sharply dissected away to expose the dura. The histologic appearance of the mass consisted of poorly cell-degenerated ligament, fibrocartilage, and fibrin. There was a focal proliferation of small vessels, but no significant inflammatory component and no evidence of neoplasia. The ligaments appeared fibrillated, disintegrated, and fragmented. After surgery, the patient's neurologic function improved.
CONCLUSIONS: This is the first reported case of a retro-odontoid pseudotumor combined with diffuse idiopathic skeletal hyperostosis. The secondary transfer of mechanical stress to the atlantoaxial segment was presented as a pathomechanism underlying the formation of this retro-odontoid pseudotumor.