Dystopic Os Odontoideum Causing Cervical Myelopathy: A Rare Case Report and Review of Literature.

Os odontoideum (OO) was first described by Giacomini in 1886 as separation of the odontoid process from the body of the axis. Instability can consequently occurs at this level due to the failure of the transverse atlantal ligament (TAL) and this atlantoaxial instability can be a cause of progressive neurological deficits. It is considered a rare anomaly of the odontoid process. It is a disease with controversial etiology, debatable incidence, and only a partly known natural history owing to the paucity of the literature on this topic. There are insufficient demographic data about the occurrence of the disease, and most of the management is dictated by the isolated case reports and few studies which have been carried out at handful of institutes. OO is classified into two types by Fielding et al. based on the anatomic location: orthotopic and dystopic. Orthotopic OO consists of an ossicle that moves with the anterior arch of the atlas, whereas the dystopic type presents as an ossicle near the basion or one that is fused with the clivus. In one magnetic resonance imaging (MRI) study of odontoid morphology, a 0.7% (1 case of 133 patients) incidence was reported. The spectrum of the clinical presentation varies from completely asymptomatic individuals to patients presenting with features of cervical myelopathy. Here, we present a case of 35-year-old-male with dystopic OO who presented to us with features of gradually progressing cervical myelopathy without any obvious history of neck trauma. On investigations, he was found to have atlantoaxial instability with wide atlanto-dens interval. He was treated with the posterior C1-C2 stabilization and reduction of atlantoaxial instability.