Pediatric Atlanto-Occipital Assimilation With Dynamic Instability: A Case of Successful Occipitocervical Fusion.

Atlanto-occipital assimilation (AOA), also known as atlas occipitalization, is a rare congenital craniovertebral junction anomaly characterized by partial or complete fusion of the atlas with the occipital bone. It often remains asymptomatic until adulthood, and presentation in childhood is rare. We report a nine-year-old boy with partial AOA who developed early-onset neurological symptoms. He presented with a one-year history of intermittent neck pain and transient right-hand clumsiness and numbness, which were provoked by neck extension. Imaging demonstrated a C1 hemiring, partial right-sided AOA, mild basilar invagination with the odontoid tip extending 6 mm above Chamberlain's line, mild occiput-C1 instability, and dynamic C1-C2 instability. MRI showed ventral cervicomedullary indentation without associated medullary signal abnormality, and the clivo-axial angle (CXA) was reduced to approximately 117°. No other associated congenital anomalies were identified. Given the early neurological involvement, dynamic instability, and pathological clivo-axial angulation, occipitocervical fusion was performed using an occipital plate, C2 pedicle screws, and an autologous rib graft, without significant donor-site morbidity. The patient recovered uneventfully, with complete symptom resolution and preserved functional neck mobility. Six-month follow-up imaging confirmed stable instrumentation. This case highlights that neurological symptoms may occur in children with AOA even without marked static cord compression when dynamic instability and pathological clivo-axial angulation are present. Early stabilization may help prevent irreversible neurological deterioration in carefully selected pediatric patients.