Atlanto-occipital Dissociation in the Setting of Relatively Normal Radiologic Findings.

Background: Craniocervical junction (CCJ) dislocations are often fatal. Atlanto-occipital dissociation can be challenging to diagnose, especially in patients who present with absent or subtle radiologic signs.

Case Description: A neurologically intact 37-year-old patient presented to the hospital following a high-speed motor vehicle accident. Initial computed tomography scans showed normal CCJ anatomy, but magnetic resonance imaging (MRI) of the CCJ was performed to further evaluate perimesencephalic subarachnoid hemorrhage. MRI revealed partial disruption of the anterior atlantoaxial membrane and tectorial membrane as well as complete disruption of the posterior atlanto-occipital membrane, ligamentum flavum, and apical ligament, signifying atlanto-occipital dissociation. Halo spinal immobilization was performed in preparation for stabilization with posterior occipitocervical fusion; however, the CCJ distracted widely during surgery owing to the accident-related dislocation, signifying an unstable fracture. Posterolateral fusion was performed, and the distraction injury was corrected via posterior surgical instrumentation.

Conclusions: Normal occiput-C1 craniometric parameters in the setting of unexplained perimesencephalic subarachnoid hemorrhage does not eliminate the possibility of missed or delayed diagnosis of traumatic atlanto-occipital dissociation injuries. Cervical MRI without contrast should be considered in patients with vertebral artery dissection or perimesencephalic subarachnoid hemorrhage after a blunt injury with neck pain. When MRI shows evidence of disruption of ≥2 atlanto-occipital ligaments, surgical stabilization should be considered, as these are clinically very unstable injuries.