28 Vertical Mobile and Reducible Atlantoaxial Dislocation

10.1055/b-0034-81405

28 Vertical Mobile and Reducible Atlantoaxial Dislocation

Goel, Atul, Shah, Abhidha

We identified a select group of patients having group I basilar invagination in which there was complete reduction of basilar invagination on extension of the head without the need of any cervical traction. We labeled this group of patients as having “vertical mobile and reducible” atlantoaxial dislocation.1 Considering that the imaging characteristics with the head in a flexed position are of group I basilar invagination, the term mobile and reducible basilar invagination can also be used to classify the clinical condition. It is critical to differentiate this group of patients from other group I basilar invagination cases that have “fixed” atlantoaxial dislocation, as the treatment of the two clinical entities is discrete.

Clinical Features

Based on our understanding of the presence of mobile and reducible vertical dislocation, as well as the use of dynamic computed tomography (CT), we identified 8 out of 64 patients with group I basilar invagination seen between January 2006 and March 2008 as having vertical mobile and reducible dislocation. Pain at the nape of the neck, spasm of the neck muscles, and a short neck were the main symptoms.1 Patients ranged in age from 8 to 54 years (mean 24 years). There were five male and three female patients. A short neck and torticollis were clinical findings in all patients and were present since early childhood. In six patients, there was a history of relatively moderate degree of trauma at the time of onset of major neurological symptoms. Torticollis exaggerated after the injury in all cases. The history of trauma ranged from 15 days to 4 years (mean 6 months) prior to diagnosis and treatment. All patients had different degrees of neck pain, neck muscle spasm, and spastic quadriparesis. Sensory symptoms were relatively mild and predominantly included bilateral upper and lower extremity paresthesias and kinesthetic sensation deficits.1

Radiological Features

The Wackenheim clival line and McRae line of the foramen magnum were used to evaluate the basilar invagination or the superior migration of the tip of the odontoid process in relationship to the clivus or to the anterior arch of the atlas.2,3 To grade the degree of vertical dislocation, we used our previously described vertical atlantoaxial instability index.4 Most patients were adolescent or middle-aged. The dislocation was generally more severe in younger patients. Because the compression of the cervicomedullary cord is not as acute as is seen in horizontal atlantoaxial dislocation, the symptoms are relatively mild and long-standing.

Vertical Instability

Vertical instability of the atlantoaxial joint has been identified and more often linked with cranial settling associated with rheumatoid arthritis. It has also been linked by some authors to basilar invagination secondary to congenital anomaly in the region. Reduction of basilar invagination on institution of cervical traction has been observed by us and by others.5–7 Such a reduction suggests the presence of vertical instability. We had earlier identified a relatively inclined profile of the facets of the atlas and axis to be the primary cause of basilar invagination. Progressive “slip” of the facet of C2 over C1 could be the cause of spondylolisthesis, a phenomenon that eventually results in basilar invagination. In all cases with vertical mobile and reducible dislocation, the atlantoaxial joint was markedly inclined, an anomaly that appeared to be primarily responsible for incompetence of the lateral masses and abnormal vertical mobility. Although several authors consider congenital basilar invagination as a “fixed” anomaly, others have identified it as the result of vertical craniocervical instability.3,5–9 Essentially, it appears that cases with group I basilar invagination can be subclassified as having either fixed atlantoaxial dislocation (where the basilar invagination does not disappear on neck movements) or vertical mobile and reducible atlantoaxial dislocation (where the basilar invagination disappears on extension of the neck).

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