Congenital absence of the posterior arch of the atlas is rare. Its detection in the setting of acute trauma may be confusing and mimic a fracture. This case serves to highlight the adjunctive use of computed tomography in equivocal cases. Most patients can be managed conservatively, however operative treatment should be reserved for patients in whom atlanto-axial instability has been shown.
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A 26 year old woman was referred to our service complaining of neck pain after being involved in a low velocity road traffic accident in which she was struck from behind. On examination she had no neurological signs and standard cervical spine radiographs showed features suggestive of a fracture of the posterior arch of C-1 (fig 1). However, computed tomography identified this as a congenital defect in which the posterior arches were absent and the anterior arches had failed to fuse in the midline (fig 2). Magnetic resonance imaging confirmed the above findings, showing fibrous bands in the place of the posterior arch. It also excluded any soft tissue injury, haematoma, or disc prolapse. Lateral flexion/extension plain radiographs of the cervical spine outruled atlanto-axial instability. The patient was treated conservatively and her neck pain resolved spontaneously within 48 hours. She remains well at follow up.
Congenital defects of the ring of the atlas may be asymptomatic or may present with neurological symptoms secondary to atlanto-axial instability and resultant cord compromise.1 They may also be detected on plain radiographs after trauma as in this case. This case serves to emphasise that not all plain film cervical abnormalities, even in the setting of acute trauma, are traumatic in origin and the important adjunctive role of computed tomography in accurately delineating equivocal cases.2 It also emphasises that treatment of these anomalies should be dependent on the presence or absence of atlanto-axial instability, with or without neurological symptoms.3 Most patients can be managed conservatively, however a small number of patients with instability will require operative stabilisation. Instability can be shown on lateral flexion/extension plain radiographs or magnetic resonance imaging of the cervical spine. The second method has the advantage of assessing any signal change of the cord during the manoeuvre and should be the investigation of choice.
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