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Purpose: 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. Results: 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. Conclusion: 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. [ABSTRACT FROM AUTHOR]