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Basilar invagination (BI) is a craniovertebral junction (CVJ) abnormality in which the odontoid process (dens) of the axis (C2 vertebra) migrates upward into the foramen magnum, compressing the brainstem, cervicomedullary junction, and/or upper cervical cord.
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| Type | Description |
|---|---|
| Congenital BI | Developmental anomaly due to occipital bone or cervical spine malformation (e.g., occipitalization of atlas, platybasia) |
| Acquired BI | Result of softening or collapse of bones at the skull base or cervical spine due to: |
| • Rheumatoid arthritis | |
| • Osteomalacia | |
| • Paget’s disease | |
| • Trauma or infection (e.g., tuberculosis) |
| Congenital | Acquired |
|---|---|
| • ‣ | |
| • ‣ | |
| • Atlanto-occipital fusion | |
| • ‣ | |
| • Chiari malformation | • ‣ (with pannus) |
| • Osteomalacia / Paget’s disease | |
| • Trauma (dens fracture, ligament injury) | |
| • Tuberculous atlantoaxial infection | |
| • Neoplastic lesions (e.g., chordoma, metastasis) |
| Symptoms | Explanation |
|---|---|
| Occipital/upper cervical pain | Compression or instability |
| Myelopathy | Spasticity, ataxia, quadriparesis |
| Bulbar symptoms | Dysphagia, hoarseness, sleep apnea |
| Torticollis | Neck tilt due to chronic instability |
| Vertigo, drop attacks | Brainstem compression |
| Modality | Imaging features |
|---|---|
| XR | • Dens projects above Chamberlain’s/McGregor’s lines |
| • Associated signs: platybasia, occipitalization of atlas | |
| CR | Best for bony details |
| • Dens position | |
| • Occipital–cervical fusion | |
| • Skull base flattening | |
| MR | • Shows brainstem/cervical cord compression |
| • Syringomyelia or Chiari I malformation may coexist | |
| • Soft tissue pannus in rheumatoid arthritis |
Radiological Criteria & Measurements
| Reference Line | Normal | Abnormal in BI |
|---|---|---|
| Chamberlain’s Line | Dens ≤3 mm above | >3 mm above suggests BI |
| McGregor’s Line | Dens ≤4.5 mm above | >4.5 mm indicates BI |
| Wackenheim’s Line | Line along clivus; should intersect or be tangent to dens | Posterior or non-intersecting dens suggests invagination |
| Boogaard angle | Skull base angle > 140° indicates platybasia (often associated) |
Vertical atlanto-axial subluxation (basilar invagination), measurement methods. (a) Lateral normal radiograph in neutral position showing the location of McGregor’s line (black) between the postero-superior aspect of the hard palate and the most caudal point of the occipital curve. Migration of the tip of the dens >4.5 mm above McGregor’s line indicates vertical subluxation. The distance indicated by the white line between McGregor’s line and the midpoint of the inferior margin of the body of axis is used to evaluate vertical subluxation according to Redlund-Johnell and Pettersson’s method. A distance less than 34 mm in men and 29 mm in women indicates vertical subluxation. (b) Sagittal CT reconstruction of a normal cervical spine showing the location of McRae’s line corresponding to the occipital foramen and the division of the axis into three equal portions used by Clark’s method for diagnosing vertical subluxation. If the anterior arc of the atlas is in level with the middle or caudal third of the axis there is slight and pronounced vertical subluxation, respectively. (c) Ranawat’s method includes determination of the distance between the centre of the second cervical pedicle and the transverse axis of the atlas. A distance less than 15 mm in males and 13 mm in females indicates vertical subluxation
Jurik, A.G. Imaging the spine in arthritis—a pictorial review. Insights Imaging 2, 177–191 (2011). https://doi.org/10.1007/s13244-010-0061-4
Radiographic criteria for basilar impression. 1 McGregor’s line between hard palate (HP) and the lowest point of occiput. Basilar impression is present if the dens protrudes >5 mm above this line. 2 Chamberlain’s line between hard palate and opisthion. Positive diagnosis if dens protrudes >2.5 mm above line. 3 McRae’s line between basion and opisthion should be above the dens. 4 Klaus index, distance between tip of dens and the tuberculum–cruciate line between tuberculum (T) and internal occipital protuberance (IP). This measures depth of the posterior fossa
Pang, D., Thompson, D.N.P. Embryology and bony malformations of the craniovertebral junction. Childs Nerv Syst 27, 523–564 (2011). https://doi.org/10.1007/s00381-010-1358-9
Basilar invagination on a sagittal T1-weighted image. The tip of the odontoid process extends 8 mm above the McGregor line (brown). The tip of the odontoid prolapses backward and causes the compression of the cervicomedullary junction. 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
Pinter, N.K., McVige, J. & Mechtler, L. Basilar Invagination, Basilar Impression, and Platybasia: Clinical and Imaging Aspects. Curr Pain Headache Rep 20, 49 (2016). https://doi.org/10.1007/s11916-016-0580-x