May 9, 2011
36-year-old man with unknown past medical history, following a motorcycle accident.
Atlanto-Occipital Dislocation (AOD)
- AOD injuries are seen in high-speed motor vehicle accidents.
- They are typically associated with high morbidity and mortality rate due to injuries involving the brainstem, spinal cord, and vertebral arteries.
- Mechanism: Hyperextension, distraction of the cervical spine with resultant complete disruption of the stabilizing ligaments at the craniocervical junction.
- Helmet protects against intra-cranial injuries. However, it adds to the weight of the head and therefore can increase the centripetal force, which can sometimes predispose to AOD.
- Children are more susceptible due to the increased head to body ratio, small horizontal occipital condyles, and inherent instability of the neck.
- Imaging: On CT, Certain lines and ratios should be remembered. Basion-dental interval (BDI): from tip of clivus to tip of dens. If BDI > 5 mm in adults and > 10 mm in children, consider AOD. Powers ratio = BC/AO where, B: basion, C: anterior cortex of posterior arch of C1, A: anterior arch of C1, O: opisthion (posterior margin of foramen magnum). Lines thus drawn, connecting B to C, and A to O can be then used to obtain the Powers ratio. A ratio > 1 is worrisome for AOD. Atlanto-occipital interval > 5 mm is also worrisome of AOD. On MR, rupture of the ligaments, and pseudomeningocele can be seen. Brainstem, cord injury, vertebral artery dissections, and pre-vertebral soft tissue swelling can be seen.
