Os odontoideum is a rare anomaly of the second cervical vertebrae, which is a separated ossicle from the body of the axis. This abnormality of the odontoid process can result in an atlanto-axial instability & subsequent cervical cord compression. In spite of the high prevalence of atlanto-axial instability due to the ligamentous laxity of atlanto-axial joint, the frequency of neurologic symptoms is relatively low in the patient of the Down syndrome. The asymptomatic atlanto-axial instability by trauma may progress to Os odontoideum in Down syndrome population and may cause cervical cord compression. Therefore, early recognition and an appropriate management of patients with atlanto-axial instability can significantly reduce the morbidity and mortality.

We report a case of Os odontoideum in a child with Down syndrome, who had an atlanto-axial instability which resulted in the cervical cord compression.