Pseudoachondroplasia associated with os odontoideum and retro-odontoid mass: case-based update

Pseudoachondroplasia is a rare skeletal dysplasia caused by a mutation in the COMP gene. Infants with pseudoachondroplasia present with rhizomelic dwarfism. Pseudoachondroplasia can resemble achondroplasia, which also presents with a phenotype of rhizomelic dwarfism. The differentiation between these two conditions is important because while both can present with cervicomedullary compromise, the underlying pathophysiology and its management is entirely different.

We present a case of a 13-year-old female patient with pseudoachondroplasia who presented with cervical myelopathy and imaging evidence of atlantoaxial instability with an os odontoideum and a retro-odontoid cystic soft tissue mass, causing C1-2 canal stenosis and cervical cord compression. She underwent C1 decompression and C1-2 posterior cervical fixation. Post-operative imaging demonstrated improved alignment of the os odontoideum, and complete resolution of the retro-odontoid soft tissue mass. We contend that retro-odontoid masses are a marker of local C1-2 joint instability and restoration of atlanto-axial alignment with posterior fixation and fusion alone is likely to be sufficient for regression of the mass and associated compression.

Pseudoachondroplasia can present with atlantoaxial instability and cervicomedullary compromise and with retro-odontoid masses. The differentiation from achondroplasia is crucially important for surgical decision-making and management should include surgical decompression and fixation.