PMC6349990_01
(a-c) Axial planes of the subluxated atlanto-axial joint with the malposition of the odontoid process in the native CT. (d) Angiographic CT of the head and neck. No hints of stenosis or dissection of the cerebrum-supplying arteries are observed here.
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Immediately upon arrival at the ED, we conducted an angiographic computed tomography (CT) of the cerebrum and neck (Figure 1).
A CT scan of the odontoid process in the sagittal plane (left). Striking is the missing posterior arch of C1. One can assume a subluxated position in the atlanto-axial joint. The right image shows the AAD. The red line indicates an ADI of 4.5 mm (physiologically < 3 mm). The dotted blue line depicts the basion-axial interval, which extends 15.5 mm (physiologically < 12 mm in females). The green line depicts the McGregor line. A superior odontoid migration of 15 mm can be seen, and as a result, the patient suffers from a basilar invagination. The odontoid process migrated 8.8 mm above the McRae line (yellow).
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This pathological finding indicates a basilar invagination, such that these anatomical circumstances led to a narrowing of the foramen magnum and thus to craniocervical instability (Figure 2).
MRI of the cervical spinal column shows a chronic subluxation in the atlanto-axial joint with an impression and constriction of the foramen magnum. The arrow depicts an older deformity of the medulla. No perifocal edema is observed, which could result from a chronic compression. It is located dorsal of the odontoid process. No hints of a ligament injury can be seen, and in particular, the transverse ligament is also free of damage.
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It showed a deformity of the medulla at the level of the dens axis (Figure 3).
The left sketch depicts the physiological anatomy of the craniocervical junction in the sagittal plane. In the right sketch, the congenital malformation is shown with its AAD. The occipital bone slips closer to the odontoid process due to the hypogenetic posterior arch of the atlas. Furthermore, the odontoid process appears more cranial, indicating the basilar invagination. In conjunction with this scenario, the vertebral canal is narrower in its diameter than is physiologically normal, and becomes even narrower when the neck is flexed.. 1a = C1 anterior arch, 1b = C1 posterior arch, 2a = odontoid process, 2b = C2 Processus spinosus, 3a = Os occipitate, Pars basilaris, 3b = Os occipitale, occiput.
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Movements in the atlanto-occipital joint further minimized the leftover space and suggested an incarceration of non-bony structures, including the medulla (Figure 4).
Dynamic X-rays images showing the cervical spine in active extension (left) and in active flexion (right), which is as far as possible for this patient. One can observe a congenital fusion of C6 and C7 and the missing posterior arch of C1. The ADI is indicated by the red line. A mobility of 4 mm in the ADI was observed, which indicates an AAD.
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Dynamic X-ray images were taken in flexion and extension (Figure 5).