Non-traumatic atlantoaxial dislocation co-existing with hypoplastic and high riding vertebral arteries: a case report and review of literature

Introduction: Non-traumatic atlantoaxial dislocation (AAD) is an uncommon condition characterized by abnormal displacement between the C1 and C2 vertebrae, resulting in upper cervical spine instability with the possibility of causing mortal injury. Case presentation: We report a 63-year-old woman presented with chronic mechanical occipitocervical pain and mild upper-extremity weakness lasting more than 20 years. Her vital signs and systematic examination were unremarkable. Neurological examination revealed mild weakness of upper-extremity weakness (grade 4/5), hyperreflexia, impaired fine motor control, hand numbness, and a positive dynamic Hoffmann sign. Preoperative CT-angiography demonstrated left vertebral artery hypoplasia and right high-riding vertebral artery associated with atlantoaxial dislocation. The patient underwent C1–C2 fusion using a hybrid construct: a C1 lateral mass screw and C2 translaminar screw on the HRVA side, and a transarticular screw on the contralateral side. Intraoperative neuromonitoring was unavailable; therefore, the procedure was guided carefully using real-time C-arm fluoroscopy. Postoperatively, her neurological symptoms resolved completely. Clinical discussion: Anatomical variants of vertebral artery significantly increase the risk of iatrogenic vascular injury during atlantoaxial instrumentation. Surgical intervention in this setting requires adequate meticulous preoperative imaging assessment and surgical planning as well as experienced surgeons to achieve a safe and successful fixation. Conclusion: Atlantoaxial fixation in non-traumatic atlantoaxial dislocation with high-riding and hypoplastic vertebral artery carries a high risk of vascular injury. Comprehensive vascular assessment and individualized planning are essential to achieving safe instrumentation.