Atlantoaxial dislocation presenting as sudden onset of quadriplegia in Down's syndrome.

A sudden onset of quadriplegia and dyspnea occurred in a 4-year-old girl with Down's syndrome, who had been noted to have atlantoaxial dislocation. She underwent cervical immobilization initially with a halo-vest and subsequently by posterior fusion, resulting in significant functional recovery. The catastrophic myelopathy that occurred in this patient emphasizes the necessity of treatment even for asymptomatic atlantoaxial instability in some individuals with Down's syndrome.