Background: The vertebral artery (VA) may have an anomalous course in patients with congenital atlantoaxial dislocation. In such cases, the artery crosses the C1-C2 joint posteriorly. These cases need thorough evaluation of the VA course to avoid its injury during posterior reduction and fusion of C1-C2. We describe for the first time an unusual subisthmic course of VA in a case of congenital irreducible atlantoaxial dislocation (AAD). The surgical implication and possible embryology of such an anomaly have been highlighted.

Methods: A 25-year-male diagnosed with congenital AAD presented with spastic quadriparesis. Radiology showed irreducible AAD with oblique C1-C2 facet joints. The computed tomography angiogram showed the right VA traversing beneath the isthmus of atlas. The joint was opened, drilled, manipulated, and fused with a C1 lateral mass and C2 pedicle screw.

Results: The C2 isthmus was defined, and the C2 pedicle screw was inserted to prevent injury to the VA. The patient gradually improved and was doing well at his 6-month follow-up.

Conclusions: The subisthmic variant of the VA may be the result of persistent C2 intersegmental artery. The presence of a subisthmic VA increases the chance of injury while inserting transarticular or C2 pars screw and warrants insertion of direct pedicle screw.