Dynamic bilateral vertebral artery occlusion owing to os odontoideum mimicking basilar artery occlusion: illustrative case.

backgroundAcute vertebrobasilar occlusion is most commonly thromboembolic and requires emergency endovascular therapy (EVT). However, upper cervical instability may rarely cause neck motion-dependent dynamic vertebral artery (VA) occlusion. Os odontoideum is a recognized cause of atlantoaxial instability that has been associated with bow hunter syndrome; however, acute bilateral VA occlusion mimicking basilar artery occlusion (BAO) is exceedingly uncommon. OBSERVATIONS: A 73-year-old man with recurrent posterior circulation infarctions collapsed while golfing and rapidly progressed to coma with respiratory failure and quadriparesis. CT angiography demonstrated bilateral VA occlusion at the vertebrobasilar junction, and EVT was initiated for presumed BAO. Angiography revealed left VA occlusion at C1-2. After micro-guidewire passage, contrast stagnation at the lesion demonstrated to-and-fro flow extending to the left posterior inferior cerebellar artery. Full stent expansion with preserved aspiration backflow and repeated failure to retrieve the thrombus suggested a dynamic, nonthrombotic mechanism. Bone-window CT angiography identified os odontoideum with displacement and extension-dependent bilateral VA compromise. Durable VA patency was restored by cervical immobilization followed by occipitocervical fusion. LESSONS: Persistent to-and-fro flow during EVT should raise the suspicion of dynamic occlusion. Disproportionate neurological decline, including hypoventilation, warrants evaluation for cervical instability requiring surgical stabilization. https://thejns.org/doi/10.3171/CASE26267.