Retro-odontoid pseudotumours, also known as periodontoid pseudotumours, are non-neoplastic soft tissue masses adjacent to the odontoid process (dens) of C2, which can cause cervicomedullary compression.
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Epidemiology
The prevalence of retro-odontoid soft tissue thickening, particularly with mineralisation presumed to represent calcium pyrophosphate deposition, increases significantly with age 1.
Clinical presentation
These are often asymptomatic. Acute inflammation in these masses (as in crowned dens syndrome) can manifest as neck pain or headache. As a chronic process, mass effect on the cervical spine can manifest as myelopathy including sensory and motor deficits.
Pathology
Pseudotumours can arise by various mechanisms and aetiologies 2,3:
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rheumatoid arthritis (RA): in which case it may be termed pannus
trauma
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atlantoaxial hypermobility compensating for subaxial ankylosis
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deposition diseases
amyloid arthropathy associated with haemodialysis
dens fracture callus
migrated disc herniation
Radiographic features
The entity is defined by soft tissue thickening at the atlantoaxial junction around the expected location of the transverse atlantic ligament, posterior to the dens.
CT
Mineralisation within the pseudotumour may be present in calcium pyrophosphate deposition disease (chondrocalcinosis, linear/arciform), hydroxyapatite deposition disease (cloudlike), gout (faintly), fracture callus, and ossification of posterior longitudinal ligament (by definition).
Joint or pressure erosion may be present in RA, CPPD, gout, and tenosynovial giant cell tumour.
MRI
Signal characteristics vary by aetiology. In general, there are no reliable imaging features that distinguish rheumatoid arthritis-related pannus from non-rheumatoid retro-odontoid pseudotumour.
T1: usually low
T2: variable, often heterogeneous
Differential diagnosis
Consider:
retro-odontoid synovial cyst
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