An unusual cause of vertebrobasilar insufficiency in a case of atlantoaxial dislocation with anomalous vertebral artery

Background Posterior circulation stroke resulting from atlantoaxial dislocation (AAD), although uncommon, is a well-described entity. The normally coursed V3 segment of the vertebral artery (VA) is likely to be stretched because of C1-C2 dislocation and further compromised by the C1-C2 translational mobility. The persistent first intersegmental artery (PFIA), an anomalous variant does not course through the C1 transverse foramen, but rather crosses the posterior C1-C2 joints and is unlikely to be affected by the C1-C2 dislocation. Therefore, a patient with AAD and anomalous VA presenting with stroke should be evaluated for other etiologies of VA compromise. Case Description We report a patient of AAD with PFIA who presented with posterior circulation stroke. Careful radiological evaluation revealed a loose body (LB) adjacent to the medial aspect of the left C1-C2 facet compressing the anomalous VA. Intraoperatively, there was a large LB on the posteromedial border of the joint, compressing the VA. The anomalous VA was mobilized, and the offending element removed followed by fixation of the C1-C2. Conclusions One should be aware of such an etiology of arterial compromise in cases of AAD with coexistent anomalous VA. An underlying LB or large osteophytes resulting from instability may be the offending cause, and needs to be dealt with, as fusion alone may not benefit the patient.