Complex developmental abnormality of the atlas mimicking a Jefferson fracture: Diagnostic tips and tricks.

Congenital atlas abnormalities are rare - often asymptomatic - findings, not requiring any specific treatment. They are frequently discovered, by chance, in trauma patients, in the course of the radiological work flow at the Emergency Department. In these cases they may represent a diagnostic challenge, since physicians are expected to differentiate them from complex C1 fractures (isolated Jefferson's fractures or associated with Anderson and d'Alonzo's fractures) requiring surgical treatment. Although difficult to identify, a correct diagnosis is mandatory in order to optimize the patient's treatment. In this article we report a case of congenital atlas abnormality, and discuss the tips and tricks to make a correct differential diagnosis through the most appropriate clinical and radiological work flow.

INTRODUCTION

Congenital atlas abnormalities are present in less than 5% of the population: posterior clefts are the most frequent abnormality; anterior and combined anteroposterior clefts are rarer.[1-5] Often asymptomatic, they may remain unknown for the whole life-span.[67] They are, therefore, frequently suspected only during the radiological evaluation of trauma patients.[489] Differentiating clefts from fractures is mandatory to start a specific treatment. Unfortunately, conventional radiological imaging sometimes cannot exclude congenital abnormalities when fractures are suspected. This is mainly the case when a bone displacement of a Jefferson fracture has to be differentiated by a congenital anteroposterior cleft, with lateral displacement of the two hemiarches.

CASE REPORT

The case of a 74 year-old man, who suffered a severe direct head and cervical spine traumatic injury is presented. On admission the patient was fully awake, he complained of mild neck pain, and upon clinical examination, presented stiffness of the posterior cervical musculature. The remainder of neurological examination was negative. He suffered from hypertension and benign prostatic hypertrophy and disclosed a β-thalassemic trait. The patient firmly denied any history of previous traumas in the cervical spine region nor any surgical operations.

A head scan showed evidence of post-traumatic subarachnoid hemorrhage, along with a small right frontal cerebral contusion (volume < 0.7 cm3), without mass effect. The plain cervical X-rays, despite their technical limits, aroused the suspicion of a C1 fracture; therefore, the patient was immobilized in a Philadelphia collar and transferred to our neurosurgical ward. A cervical computed tomography (CT) scan, not only demonstrated an anterior small defect of the arch on its left side, with tiny bony fragments in the middle of the defect, but also a posterior rim defect at the level of the C1 tuberculum [Figure 1]. There was no major swelling of the paravertebral soft tissue. The absence of irregular, coarse, bony fragments (which were common in a traumatic case), and the presence of minimum sclerosis around the edge of the bony defect made us wonder about the real pathogenesis of the bony defect. A non-traumatic origin was sustained by cervical magnetic resonance imaging (MRI), which excluded the presence of blood and edema near the bony defect on T2 Weighted imaging [Figure 2]. Furthermore, a flexion-extension cervical X-ray [Figure 3] definitely excluded the presence of abnormal movements. The Philadelphia collar was therefore removed, the patient reassured, and discharged 24 hours later, as no radiological evolution was detected on the control CT scan. After 12 months the patient was fine with no complaints.

Figure 1

CT spine showing, sagittal (a) and axial (b-d) views of the bone defect in the anterior arch of C1

Figure 2

Spinal T2 Weighted MR Imaging: Sagittal (a) and axial (b) views do not show any evidence of hyperintensity suggestive of a recent trauma, concurrent with the C1 defect

Figure 3

Flexion (a) and extension (b) cervical X-rays do not show any evidence of abnormal movements

DISCUSSION

Atlas abnormalities are often discovered incidentally, during a radiological evaluation performed on trauma patients.[1-46-9] Physicians should be aware of the craniocervical junction congenital abnormalities, because in a traumatic patient, radiological findings are not necessarily related to a fracture, but may indicate a congenital situation. It is, therefore, mandatory to differentiate between these two conditions, in order to treat the patients specifically.[1379]

Atlas originates from three ossification centers. The first, located anteriorly, appears during the first year of age; it develops into the anterior arch and tubercle; sometimes two concurrent centers are found. The second and third are placed laterally and appear during the seventh week of intrauterine life; they extend posteromedially, fusing in the midline and forming the posterior arch. The atlas ossification is completed between three and eight years of age.[14-8]

Whenever two ossification centers develop anteriorly, but do not fuse, or the unique anterior ossification center is absent, an anterior cleft may develop.[4] Likewise, if the two lateral ossification centers do not fuse posteriorly, a posterior cleft or a wider posterior hypo/aplasia, may develop.[358]

The C1 clefts are seen in 3-5% of the population. Currarino et al. classified posterior defects — the most frequent abnormalities — into five types: Type A (failure of the posterior midline fusion) are the most frequent, representing 97% of all posterior malformations (4% of the general population). Types B–D, anterior isolated clefts and anteroposterior clefts are much more rare.[1-46-10]

The C1 clefts can be associated with Arnold-Chiari malformation, gonadal dysgenesis, Klippel-Feil, Turner and Down syndrome, and thalassemia minor.[79]

The defect may be asymptomatic or associated with headache, neck pain, stiffness, dizziness, and numbness; sometimes, major neurological deficits develop.

Congenital C1 cleft patients do not need any specific treatment, but in case of an evident degenerative instability, require surgical stabilization.

Six to ten percent of cervical fractures take place at the atlas, usually caused by axial loading at the vertex of the head. The four-point C1 fracture is called a Jefferson fracture and accounts for one-third of all atlas fractures. In this case lateral masses can displace laterally, exactly as it may occur during an anteroposterior deficiency of the atlas. CT is the best diagnostic tool in the evaluation of such vertebral injuries, due to its high sensitivity and specificity for the study of bone tissue.[238]

As many doubts may arise while dealing with such difficult cases, an assessment of the confounding factor by which an unknown C1 cleft can be misinterpreted as a fracture, is summarized here:[479]

A congenital abnormality may be unknown, until a radiological investigation is performed

Clinical findings may be very similar in C1 cleft and Jefferson fracture patients[37]

Unlike a fracture, a congenital defect is almost always seen in the midline position (97% of cases)[6]

On CT scans, a smooth bone interruption with a visible cortex suggests a congenital defect, whereas, sharp edges without cortex suggest a fracture[34]

Displacement of lateral masses is usually < 3 mm in congenital abnormalities, and > 3 mm in Jefferson fractures[38]

An hypertrophic spinous process of C2 may be seen in standard lateral cervical X-rays, if the posterior C1 arch and tubercle are absent (type E of the Currarino's classification)[710]

A congenital interruption of the C1 ring can be easily misdiagnosed as a fracture during clinical and even radiological evaluations. In Figure 4, we propose a chart, which summarizes what has been described in this article. Items are organized according to clinical and radiological findings, and the most appropriate diagnostic options are suggested to identify the most likely pathogenesis.

Figure 4

Practical chart to differentiate C1 fractures from clefts. As the arrows indicate, the red items are related more to C1 fracture and the blue items to C1 cleft; the green items, being the most unspecific findings, could be present in both situations

CONCLUSION

In an emergency setting the differential diagnosis between atlas fractures and congenital abnormalities can represent a major clinical challenge; to this regard a careful radiological evaluation is mandatory, in order to come up with the correct diagnosis and start the appropriate treatment. The clinical case reported herein is exemplificative indeed, as the initial suspect of atlas fracture, due to the deceptive history of acute neck trauma, has been eventually ruled out in favor of the correct diagnosis of a congenital bony defect. It is noteworthy that this report not only provides the Emergency Department's clinicians with a thorough analysis of the possible confounding factors and their weight in defining the initial diagnostic work flow, but also highlights the useful diagnostic tips, to rapidly and effectively differentiate those two different nosological entities, and avoid useless and risky overtreatment.