AAI can occur in 3 directions: rotation, flexion-extension, and distraction leading to excessive displacement, which can result in subluxation or even dislocation.
Rotatory subluxation is most common in pediatric patients with congenital abnormalities and presents with neck pain and frequently a classic "cock-robin" head position (head is tilted, rotated, and slightly flexed). Rotatory subluxation can also occur after minor or major trauma or local infection (eg, Grisel syndrome).
Anterior atlantoaxial subluxations are more common in adults and result from any of the pathologies listed above. Anterior subluxations result in neurologic deficits or death in one-third of patients. Patients with rheumatoid arthritis (RA) are at most risk for dislocation in the anteroposterior (AP) plane but can develop rotational AAI as well.
Posterior atlantoaxial subluxations are rare and often occur from erosion of the odontoid process of the axis.
M24.80 – Other specific joint derangements of unspecified joint, not elsewhere classified
202820009 – Atlantoaxial Instability
Differential Diagnosis & Pitfalls
- Cervical fracture without instability
- Cervical degenerative disk disease
- Cervical disk herniation
- Cervical stenosis
- Spinal cord infarction
- Spinal epidural hematoma (see spinal cord compression)
- Spinal cord tumor / extradural tumor (see spinal cord compression)
- Spinal cord abscess (see paraspinal abscess)
- Cervical dystonia
- Occipital headache