Atlantoaxial instability in Chiari formation- an analysis based on static and dynamic head imaging in 35 patients

Background

The study evaluates dynamic imaging in cases with Chiari formation to identify and qualify the nature of atlantoaxial instability.

Materials and Methods

During the period June 2023 to February 2024, 35 consecutive and selected cases of Chiari formation surgically treated by atlantoaxial fixation and subjected to preoperative static and dynamic head imaging were analyzed. There were 19 males and 16 females and their ages ranged from 23 to 62 years (average 41 years). Apart from conventional imaging with plain radiographs, magnetic resonance imaging (MRI) and computerized tomography (CT), all patients underwent dynamic head flexion–extension, head right-left rotation and lateral right-left head tilt CT imaging. They also underwent an open mouth motion X-ray of the craniovertebral junction. As per the defined parameter, facetal mal-alignment of more than 5 mm on any dynamic imaging was considered to indicate atlantoaxial instability. On rotatory and lateral head tilt imaging, atlantoaxial instability was subdivided according to its presence on one or both sides. Depending on the degree of facetal mal-alignment, atlantoaxial instability was classified in 3 grades. Vertical atlantoaxial instability was assessed on dynamic head flexion–extension imaging.

Results

On static imaging, all patients were identified to have central or axial atlantoaxial dislocation (CAAD) as per previously described parameters. Dynamic head rotation imaging confirmed atlantoaxial instability on one side in 6 patients and on both sides in 29 patients. Lateral head tilt confirmed atlantoaxial instability on one side in 9 cases and on both sides in 26 cases. Dynamic head flexion–extension imaging identified vertical atlantoaxial instability in 3 cases.

Conclusions

Dynamic imaging helps in identifying atlantoaxial instability in cases with Chiari formation even when the other validated parameters are within the range of ‘normal’. Such investigation can influence the surgical management.