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Neuroradiology Case of the Week

Case 131

Minchui Shin and Per-Lennart Westesson, MD, PhD, DDS

Clinical Presentation: A 75-year old male with a history of syncope was brought to the Emergency Department following a fall. At the time of examination, the patient had no cervical tenderness and no neurological findings. Plain films were taken to clear his cervical spine collar. Spine radiographs demonstrated no fractures but showed instead a non-fused posterior arch of the atlas in the midline (Figs. 1 & 2). A helical-CT of the cervical spine confirmed the presence of a bony cleft in the posterior arch of C1 (Fig. 3). There was no evidence of neural involvement in any of the examinations so an MRI was not obtained.

Radiological Findings:

Discussion: A non-fused posterior arch of the atlas is a rare entity like the rest of the congenital posterior arch anomalies [1]. Afflicted individuals are generally asymptomatic so they are only identified incidentally [2]. In a trauma setting, however, this can become problematic because it can be mistaken for a fracture. So it is wise to become familiar with this abnormality to avoid the wrong diagnosis and management [3,4].
     The embryological origin of this entity can be traced back to the seventh week of gestation. During this time, there are two chondrification centers in the lateral masses from which the two laminae of the posterior arch extend dorsomedially to meet at the center. By the time of birth these laminae are connected by cartilages, and by the age of 5 they are completely ossified and fused forming a complete posterior arch [5-8]. When this last process is disrupted this will present as a non-ossified and non-fused posterior arch just like in our patient.
     However, any pathway preceding the ossification stage can go awry and this can result in a number of variations known together as the congenital absence of the posterior arch. Essentially, the non-fused posterior arch is part of the same spectrum as the congenital absence of posterior arch, but the key difference is that they happen at different stages of embryogenesis. The latter takes place during the chondrification stage and it is a failure of cartilage formation. On the other hand, the non-fusion of the posterior arch takes place during the ossification stage and it is a failure of ossification over already existing cartilages [5-8]. In short, one is an agenesis whereas the other is a simple non-union.
     Clinically, patients with any form of posterior arch abnormalities are generally asymptomatic but a few may present with neck pain and neurological deficits due to cord impingement produced by remnant of the posterior arch. This is seen only with partial agenesis malformations because an inward mobility of the posterior tubercle can compress the cord during extension of the cervical spine [9,10]. Furthermore, posterior arch abnormalities can have severe neurological deficits when associated with several other conditions such as the Arnold-Chiari malformation, gonadal dysgenesis, Klippel-Feil, Turner, and Down’s syndromes [5,11].
     As mentioned previously, the diagnosis is often incidental. They are often discovered on pain films but in most cases CT scan is used for a final diagnosis. CT can help differentiating ossified from non-ossified centers, show small defects, and quantify the degree of abnormality. However, even with the aid of CT, a definitive diagnosis cannot be reached in children under the age of ten because of normal variations of their growth [5]. Finally, an MRI evaluation is required in patients with neurological deficits.
     Treatment modality depends on presenting symptoms and radiographic findings. At this time, it is unknown whether this condition predisposes the affected asymptomatic individuals to degenerative changes over time. For now, the management of asymptomatic patients stops at recognizing this as a benign variation that needs no further intervention. Yet, when patients present with nerve compression or structural instability surgery is warranted.

References:

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