Anomalous V3 Segment Aneurysm Associated with Congenital Atlantoaxial Dislocation: Case Report and Discussing the Challenges in Management

Background An anomalous vertebral artery is not a deterrent for posterior C1-C2 joint manipulation and reduction of atlantoaxial dislocation (AAD). However, presence of an incidental aneurysm in the aberrant segment of artery with concurrent AAD adds to the surgical challenge. Case description A 30-year-old woman presented with neck pain and spastic quadriparesis. Her imaging revealed atlantoaxial dislocation and bony segmentation defects. Three-dimensional computed tomography angiography showed bilateral anomalous vertebral arteries (V3 segment) and an incidental aneurysm on the arterial segment that crossed the right C1-C2 joint posteriorly. Because the artery bearing the aneurysm was nondominant, it was ligated and successful C1-C2 posterior reduction and fusion could be performed. Conclusion The association of an incidental aneurysm with an anomalous vertebral artery in congenital AAD is unusual. The etiology could be an underlying collagen defect or repeated shearing trauma to the vessel wall due to C1-C2 instability. It would be less risky to proceed with endovascular embolization followed by occipitocervical fusion without opening the joints in case the aneurysm is present on the dominant aberrant V3 segment. Ventral decompression can be supplemented for irreducible AAD. On the contrary, if the aneurysm is present on the nondominant aberrant V3 segment, the C1-2 joint can be opened and manipulated following an initial endovascular treatment of the aneurysm. If the circumstances demand, the nondominant artery can be ligated and sacrificed, although there is a small risk of formation of stump aneurysm.