Atlantoaxial subluxation: Flexion and extension

60 years old with rheumatoid arthritis and anterior atlantodental subluxation. Sagittal T2-weighted magnetic resonance image in neck flexion (A) and extension (B) demonstrate increase in anterior atlantodental interval (double-headed arrows) and narrowing of the central canal with cervical extension and note decrease in T2-hyperintense cerebrospinal fluid (arrows), both caudal to cerebellar tonsil (asterisk) and posterior to cervicomedullary junction with neck extension.