CCI and AAI Imaging: What Tests Diagnose Craniocervical Instability?

Craniocervical instability (CCI) and atlantoaxial instability (AAI) are structural conditions in which the ligaments that stabilize the junction between the skull and the upper cervical spine are lax, allowing abnormal movement that can compress the brainstem, spinal cord, and vertebral arteries. These conditions are increasingly recognized as a cause of severe dysautonomia, POTS, and a constellation of neurological symptoms that can be debilitating. Diagnosis requires specialized imaging that is not part of standard MRI or CT protocols — and knowing what to ask for is essential.

Why Standard MRI Often Misses CCI/AAI

Standard brain and cervical spine MRIs are performed with the patient lying flat and the neck in a neutral position. CCI and AAI are dynamic conditions — the instability and compression occur with movement (flexion, extension, rotation) or in the upright position. A standard supine neutral MRI may appear completely normal even in a patient with severe CCI.

The key principle is: imaging must be performed in the position and with the movements that reproduce the patient's symptoms.

The Imaging Modalities

Upright MRI (Standing or Seated MRI)

An upright MRI (available at specialized centers) allows imaging in the weight-bearing position, which can reveal compression and instability not visible on supine MRI. This is particularly important for CCI, where gravity loads the craniocervical junction and may increase compression of the brainstem and upper cervical cord.

Dynamic Flexion-Extension MRI

MRI performed with the neck in maximum flexion and maximum extension (either supine or upright) can reveal ligamentous laxity and abnormal movement at the craniocervical junction. This is the most sensitive test for CCI and AAI.

Key measurements on flexion-extension MRI:

• Grabb-Oakes measurement: Distance from the posterior cortex of the C2 body to the dural sac. Normal: ≤9 mm. Values >9 mm suggest ventral brainstem compression.
• Harris measurement (basion-dental interval, BDI): Distance from the basion (tip of the clivus) to the tip of the dens. Normal: ≤12 mm. Values >12 mm suggest occipito-atlantal instability.
• Clivo-axial angle (CXA): The angle between the clivus and the posterior surface of the C2 body. Normal: ≥135°. Values <135° suggest ventral brainstem compression.

CT Scan with Flexion-Extension Views

CT provides better bony detail than MRI and is used to assess the bony anatomy of the craniocervical junction. CT myelography (CT with intrathecal contrast) can visualize CSF flow obstruction and cord compression more clearly than standard CT.

Digital Motion X-Ray (DMX)

DMX captures real-time X-ray video of the cervical spine during movement, allowing visualization of abnormal motion at each cervical level. It is particularly useful for identifying ligamentous laxity and hypermobility at C0-C1 and C1-C2 that may not be visible on static imaging.

Upright CT (CBCT)

Cone beam CT performed in the upright position combines the bony detail of CT with weight-bearing positioning. Some specialized centers use this for CCI evaluation.

Key Measurements and What They Mean

Finding the Right Radiologist

Standard radiologists are not trained to measure CCI/AAI parameters and may report a CCI/AAI study as "normal" without performing the relevant measurements. When requesting imaging, specifically ask for:

1. Flexion-extension views (not just neutral)
2. Measurement of Grabb-Oakes, BDI, clivo-axial angle, and ADI
3. Evaluation for ligamentous laxity and abnormal motion

Radiologists with experience in CCI/AAI include those at specialized neurosurgical centers that treat EDS and connective tissue disorders. The Chiari & Syringomyelia Foundation and the Ehlers-Danlos Society maintain lists of specialists with CCI/AAI expertise.

After the Imaging

A positive CCI/AAI diagnosis on imaging requires correlation with clinical symptoms. Not all patients with borderline measurements have symptomatic instability, and not all patients with symptomatic CCI have clearly abnormal measurements. Management ranges from conservative (cervical collar, physical therapy focused on cervical stabilization, avoiding provocative movements) to surgical (craniocervical fusion) for severe cases with progressive neurological compromise.