RESUMO
Os odontoideum is a separate ossicle from the odontoid process from the body of the axis by a variable transverse gap. A boy with cerebral palsy probably due to prematurity and kernicterus, was treated with botulinum toxin for continuous dystonic movements at the age of 3.5 years. Although botulinum toxin appeared to be remarkably effective for relaxing hypertonia, abnormal frequent anterior flexion of the neck remained. Because of feeding difficulty and frequent aspiration episodes, additional botulinum toxin therapy was discontinued. His condition seemed to be stable and he could walk with support at age 7. However, at age 8, he presented with decreased movement of the extremities and bilateral ankle clonus. Radiographic examination of the cervical spine revealed cystic lesion and os odontoideum. With cervical posterior fixation, the patient made a good recovery. Although athetoid cerebral palsy displays an increased risk of cervical myelopathy, os odontoideum is rare in early childhood. The frequent dynamic stress of the neck due to an unbalanced, persistently contracted state and sudden collapse, possibly attributed to botulinum toxin therapy, might have led to atlantoaxial instability and os odontoideum.