Diagnostic Challenge of a Tumor-mimicking Spinal Pseudogout Lesion: Case Report and Literature Review

Retro-odontoid pseudotumor (ROP) is a non-neoplastic soft-tissue mass occurring around the C2 odontoid process and is most often associated with rheumatoid atlantoaxial instability. In rare instances, calcium pyrophosphate dihydrate (CPPD) crystal deposition (pseudogout) in the C1-C2 ligaments - a presentation known as "crowned dens syndrome" - can produce a periodontoid mass. Such CPPD-induced lesions usually present with acute neck pain and stiffness, with calcifications visible around the odontoid process on imaging. However, these masses can occasionally enlarge and compress the cervical spinal cord, mimicking a neoplasm and posing a diagnostic challenge.

We describe a 77-year-old man with a 3-month history of neck pain radiating to the occiput, progressive hand numbness, and gait disturbance. Magnetic resonance imaging revealed an upper cervical intra-/extradural mass compressing the spinal cord. No atlantoaxial instability was evident on radiographs, and no calcification of a retro-odontoid mass was seen on computed tomography. A presumptive diagnosis of a spinal tumor (e.g., meningioma or nerve sheath tumor) was made. The patient underwent posterior decompression and resection of the lesion. Intraoperative frozen-section pathology revealed birefringent calcium pyrophosphate crystals, consistent with a tophaceous pseudogout. Postoperatively, the patient's myelopathic symptoms improved significantly. By one month, he was ambulatory with a cane and had recovered normal motor strength with rehabilitation.

This case highlights CPPD crystal deposition that can produce a ROP leading to cervical myelopathy even in the absence of radiographically visible calcifications. Awareness of this entity is crucial to avoid misdiagnosis. In an elderly patient with an upper cervical mass, considering CPPD in the differential diagnosis can guide appropriate management and prevent unnecessary interventions.