A 52 y.o. pedestrian was struck by a car. She was unresponsive at the scene and brought to the ED. Her imaging appears below.

Why is she unresponsive?

Our patient had an atlanto-occipital dissociation with a basion fracture. 

Tectorial membrane and alar ligaments provide the stability of the atlanto-occipital joint.  When the basion-dens interval is > 10 mm suspect disruption of the joint.  The injury is immediately fatal in 70% of cases with an additional 15% surviving to the ED.  The injury is more common in children because of the large size of their heads relative to their bodies and the more horizontal occipital condyles in children.    The injury represents 1% of all cervical spine injuries.  Rare survivors usually have a neurological deficit with damage to  cranial nerves VII to X.

The Wackenheim line can be drawn from the clivus and should intersect the dens posteriorly if ligaments are intact. 

In our patient a CT angio showed there was no flow to the brain and care was withdrawn.  She did not have an increased basion to dens distance and it is presumed that the fracture was reduced when a collar was applied. The treatment of atlanto-occipital dislocation is surgical and we happened to see a survivor of an atlanto-occipital dislocation in the same week.  He had the base of the skull fused to the vertebral column.  He had no lateral movement of his head but was neurologically intact.

Theodoere N, Aarabi B, Dhall SS, Gerb, DE, et al. The diagnosis and management of traumatic atlanto-occipital dislocation injuries. 2013 Neurosurgery 72 Supple 2: 114-26.

CooperZ, Gross JA, Lacey JM, Traven N, Mirza SK, Arbabi S. Identifying survivors with traumatic craniocervical dissociation: a retrospective study.  2010.  Journal of surgical research 160: (1 ) 3-8.