Redundant anomalous vertebral artery in a case of congenital irreducible atlantoaxial dislocation: Emphasizing on the differences from the first intersegemental artery and operative steps to prevent injury while performing C1-2 joint manipulation

Anomalous vertebral artery (VA), commonly the persistent first intersegmental artery (FIA) is often seen with congenital atlantoaxial dislocations (AAD). An unusual redundant/ectatic loop of VA passing below the C1 (upside down VA) has been described below and appears to be different from FIA. The operative technique to protect it while C1-2 joint manipulation has been described. A 35 year old male presented with progressive spastic quadriparesis after trivial trauma. Radiology showed irreducible atlantoaxial dislocation with occipitalised C1 and C2-3 fusion. The left VA was anomalous passing beneath the C1 arch with a redundant loop lying posterior to the C1-2 joint. This was unlike the persistent first intersegmental artery (FIA) and was safeguarded while dissecting the C1-2 facet. The artery was dissected and safeguarded while performing C1-2 joint manipulation. A redundant/ectatic loop lying posterior to C1-2 joint is an unusual variant of anomalous VA. Evaluation of preoperative radiology helps in diagnosing such anomalous VA. Dissection of the entire redundant loop of the anomalous artery is important in opening the C1-2 joint required for reduction and placement of spacer/ bone grafts to achieve good bony fusion. Also mobilizing the loop allows safe insertion of lateral mass screw. Care needs to be taken while fastening screws to prevent compression of the loop.