We describe the case of a symptomatic early dystopic os odontoideum in a very young child. Os odontoideum is a rare congenital anomaly of C2, first described by Giacomini in 1886 ^{Reference Giacomini1} and characterized by a smooth, independent ossicle separated from the base of an abnormal odontoid process and without osseous connection to the body of C2. ^{Reference Fielding, Hensinger and Hawkins2,Reference Jumah, Alkhdour, Mansour, He, Hroub and Adeeb3} The normal odontoid process develops from two ossification centers that fuse in the midline during the seventh fetal month. The odontoid process fuses with the body of C2 (cartilaginous synchondrosis) by 3–6 years of age. ^{Reference Lustrin, Karakas, Ortiz, Cinnamon, Castillo and Vaheesan4}

A 2-year-old girl was evaluated with the concern of lack of development of gait. The child was born through cesarean section, after an uneventful pregnancy, except for the late development of preeclampsia. Parents reported no problems after birth and early growth and development appeared regular. Milestones appeared timely. At 5 months, she started to crawl, but never began to walk. The language developed normally.

On examination, there was a limitation in the range of motion of the cervical spine, especially during flexion and right rotation. There were no pathological reflexes and other neurological assessments appeared normal. Imaging evaluation was performed with X-ray, CT, and MRI of the cervical spine.

CT scan with 3D reconstructions demonstrated the presence of a small, rounded, well-corticated ossicle posterior to the anterior arch of the atlas, separated from the body of C2, suggesting an os odontoideum (Figure 1). This was located more cranial and left lateral than expected. In addition, significant rotatory atlantoaxial subluxation was noticed.

Figure 1: CT of the cervical spine with 2D (up) and 3D (down) reconstructions shows a dystopic os odontoideum (arrows). C1–C2 rotatory subluxation is also appreciable.

With a rotation of approximately 32^o, the subluxation was defined as grade I, according to Fielding–Hawkins. ^{Reference Fielding and Hawkins5} C1 appeared partially left-lateralized compared to the body of C2, while the os odontoideum remained at a regular distance to the anterior arch of the atlas (<3 mm).

MRI demonstrated the presence of myelopathy at the level of the dystopic os odontoideum – the body of C2 (Figure 2).

Figure 2: MRI demonstrated the presence of myelopathy at the level of the dystopic os odontoideum (circle).

Since the presence of any neurological symptom or deficit represents an indication for surgical intervention, ^{Reference Giacomini1} the patient was taken to surgery. A posterior instrumented stabilization of C1 and C2 with a single bar (rigid fixation of C1 lateral masses and C2 right laminar screw) was performed. Immediately after surgical treatment, the child developed spontaneous movements of the lower extremities. On the clinical follow-up of 2 months after surgery, the patient was able to stand unassisted and to walk with help.

We present the case of a very young child with a symptomatic atlantoaxial instability due to a dystopic os odontoideum ^{Reference Goel, Patil, Shah, Dandpat, Rai and Ranjan6} that prevented gait development. An os odontoideum may significantly compromise the atlantoaxial stability, ^{Reference Watanabe, Toyama and Fujimura7} with a huge spectrum of clinical presentations. Usually, a minor or major trauma during adolescence and early adulthood could reveal its presence. ^{Reference Klimo, Kan, Rao, Apfelbaum and Brockmeyer8,Reference Spinnato, Renò, Girolami, de Pasqua and Gasbarrini9} Few scientific studies concern this topic in pediatric population, ^{Reference Watanabe, Toyama and Fujimura7,Reference Helenius, Bauer, Verhofste, Sponseller, Krengel and Hedequist10} but the literature lacks reports about very young children with significant manifestations. In our case, the considerable improvement after surgical fixation suggests that the os odontoideum determined an atloaxial instability and caused symptoms.