Case of Atlantoaxial Dislocation with Assimilated C1, Absent Posterior C2, Butterfly C3, and Fused Subaxial Cervical Spine: Management Dilemma with Multiple Segmentation and Formation Defects.

Segmentation defects are often seen with congenital atlantoaxial dislocation (AAD) though an associated absence of posterior arch of C2 and butterfly C3 is rare. Apart from rarity, the combination of formation and segmentation defects adds to the management dilemma. We report a case of AAD with assimilated atlas, absent C2 posterior arch, C3 butterfly vertebra with floating posterior elements, and fused C4-C6. The child was managed by C1-C2 fusion alone with immediate symptomatic improvement. The presence of formation defects such as adjacent butterfly vertebra and absent posterior elements does not alter the management of AAD. Fusing the C1-C2 joints appears to be a balanced approach.