Retro-Odontoid Pseudotumor without Atlantoaxial Subluxation or Rheumatic Arthritis.

We present a case of retro-odontoid pseudotumor (ROP) without rheumatoid arthritis (RA) or atlantoaxial subluxation (AAS). A 76-year-old woman presented with paresthesia and weakness of both lower and upper extremities. She had no laboratory or physical findings of RA. Dynamic X-ray showed no AAS and magnetic resonance imaging (MRI) showed a retro-odontoid mass compressing the spinal cord. Transdural mass debulking and biopsy were performed via minimal left suboccipital craniectomy and C1 hemilaminectomy. Two months after surgery, her symptoms were aggravated. Follow-up MRI visualized mass re-growth and spinal cord compression due to the mass and AAS. Posterior C1-2 fixation was performed without further decompression. Twelve months after posterior fixation, no symptoms were apparent and follow-up MRI showed complete resolution of the ROP with C1-2 bony fusion. The ROP with C1-2 instability might be completely resolved only C1-2 fusion without decompression. Furthermore, we speculated that osteoarthritis of C1-2 itself causes a partial tear or degradation of the transverse ligament, that induced formation of ROP.