Imaging Aspect of Basilar Invagination: A Case Report
Guezri H, Laoudiyi D, Doumiri M, Lhajoui H, Chbani K, Salam S and Ouzidane L
Published on: 2021-01-28
The case report describes basilar invagination in a 15 years old teenager, who presents a progressive left hemiparesis, evolving since 3 years. A CT scan and an MRI of the craniovertebral junction showed the tip of the odontoid process extends 15 mm above the Chamberlin line, and causes the compression of the cervicomedullary junction. We will describe the radiological aspect of basilar invagination with a review of the literature.
KeywordsBasilar invagination; Craniovertebral junction
Basilar invagination is a developmental anomaly of the craniovertebral junction (CVJ) where the odontoid process is positioned abnormally upward and backward, prolapsing into the foramen magnum . Prolapse of odontoid process may lead to the compression of the cervicomedullary junction, adjacent vascular structures, or cerebrospinal fluid spaces and may cause muscle spasm. This can result in a wide spectrum of neurologic symptoms, differing in acuity and severity depending on the clinical presentation . Our aim is to describe the imaging aspect of this rare pathology.
We report the case of a 15 years old teenager, who presents a progressive left hemiparesis, evolving since 3 years. A CT scan and an MRI of the craniovertebral junction showed the tip of the odontoid process extends 15 mm above the Chamberlin line, and causes the compression of the cervicomedullary junction, with no spinal cord signal abnormality in MRI. Extensive syringomyelia from C3 to D1 and occipitalization of the atlas were also noted (Figure 1).
Figure 1: Basilar invagination on a sagittal T2-weighted image (A), and sagittal reconstruction of a CT scan. (B): The tip of the odontoid process extends 15 mm above the Chamberlin line (green). The tip of the odontoid prolapses backward and causes the compression of the cervicomedullary junction (yellow arrow). Note the presence of syringomyelia and occipitalization of the atlas.
Basilar invagination (or basilar impression) is a developmental anomaly of the craniovertebral junction (CVJ) where the odontoid process is positioned abnormally upward and backward, prolapsing into the foramen magnum . It is often a consequence of CVJ developmental disorders, where the bone tissue remains normal, such as clivus hypoplasia, atlas hypoplasia, achondroplasia, or an incomplete ring of C1 with spreading of the lateral masses and atlanto-occipital assimilation. Chiari malformation is often associated with basilar invagination in 33 - 38% of patients . Basilar invagination can present in the form of slowly progressive or acute neurologic deterioration. In many cases, an insidious headache is the only symptom and can be a diagnostic challenge for the neurologist . Imaging makes diagnosis easy. The three reference lines used in diagnosing basilar invagination are the Chamberlain, the McGregor, and the McRae lines . These methods have good specificity and sensitivity and are easily reproducible. All three lines were established on lateral radiographs of the skull. The Chamberlain line runs from the hard palate to the opisthion (Figure 2 (A)). The tip of the odontoid process typically lies inferior to this line 1.8mm in men and 1mm in women, on average, according to a study by Cronin et al . Basilar invagination is considered if the odontoid tip extends more than 5 mm above the Chamberlain line . The McGregor line runs from the hard palate to the lowest point of occipital squama (Figure 2 (B)). Normally, the tip of the odentoid extends no more than 4.5 mm above this line and is generally considered abnormal if it projects greater than 7 mm above the McGregor line . The McRae line is defined by the line from the basion to the opisthion and is basically the anteroposterior diameter of foramen magnum measured in the midline (Figure 2 (C)). Normally, the tip of the odontoid should be below this line . CT is the best tool for imaging bone and should always be the first choice for posttraumatic changes, evaluation of the osseous structures, visualization of the relation of different bony elements, or imaging of soft tissue calcifications. Imaging from CT may assist in deciding whether the CVJ deformity is congenital or acquired, by differentiating between normal and abnormal bone. Magnetic resonance imaging (MRI) allows superior visualization of the brainstem and spinal cord compared to computer tomography and should be performed to rule out secondary parenchymal changes  (Figure 2).
Figure 2: Normal craniovertebral junction on a sagittal T2-weighted image. The Chamberlain line (A) runs form the hard palate to the opisthion. The McGregor line (B) runs from the hard palate to the lowest point of occipital squama. The McRae line (C) runs from the basion to the opisthion. The tip of the odontoid process typically lies below these lines in normal cases.
The development of computed tomography (CT) and magnetic resonance (MR) imaging has greatly expanded the ability to define craniovertebral anomalies. Multidetector CT with orthogonal reconstructions is ideal for the evaluation of bone anatomy, and MR imaging permits assessment of soft tissues, including neural structures, vascular structures, and ligaments.
- Smith JS, Shaffrey IC, Abel NF, Menezes HA, Basilar Invagination. Neurosurgery. Oxford University Press. 2010; 66: 39-47.
- Pinter NK, Jennifer MV, Mechtler L. Basilar Invagination, Basilar Impression, and Platybasia: Clinical and Imaging Aspects. Curr Pain Headache Rep. 2016; 20: 49.
- Pindrik J. Clinical Presentation of Chiari I Malformation and Syringomyelia in Children. Neurosurg Clin N Am. 2015; 4: 509-514.
- Cronin CG, Lohan DG, Mhuircheartigh JN, Meehan CP, Murphy J Roche C. CT evaluation of Chamberlain’s, McGregor’s, and McRae’s skull-base lines. Clin Radiol. 2009; 64: 64-69.
- Cronin CG, Lohan DG, Mhuircheartigh JN, Meehan CP, Murphy JM, Roche C. MRI evaluation and measurement of the normal odontoid peg position. Clin Radiol. 2007; 62: 897-903.
- Smoker WR. Craniovertebral junction- normal anatomy, craniometry, and congenital anomalies, Radiographics. 1994; 14: 255-277.
- Ross JS. Specialty Imaging: Craniovertebral Junction. Lippincott Williams Wilkins. 2013.
- Debernardi A, D’Aliberti G, Talamonti G, Villa F, Piparo M, Collice The craniovertebral junction area and the role of the ligament membranes. Neurosurgery. 2011; 68: 291-230.