Cranio-cervical conundrum; Enigmatic complete atlanto-occipital assimilation with Chairi I malformation.

Atlanto-occipital assimilation is extremely rare congenital anomaly, characterized by partial or complete fusion of atlas and occipital bone leading to altered cranio-cervical junction anatomy. In our patient, basilar invagination resulted in acute angulation and narrowing of foramen magnum. Crowding of structures at foramen magnum was seen with tonsillar herniation and compression of brain stem. Edema signal in proximal cervical cord was also seen. Bilateral atlanto-occipital assimilation with increased atlas to dens distance seen suggestive of atlanto-axial instability. Overall findings were suggestive of Chiari I malformation with atlanto-occipital assimilation.