Basilar invagination diagnosis

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Basilar invagination is diagnosed by various imaging modalities such as plain x rays, CT scans, and MRI.

Quantitated by measuring the basal angle, which on plain x-rays, measured the angle between lines drawn from the nasion to center of sella and then to the anterior foramen magnum, ¹⁾ but on MRI was felt to be better represented by the angle between a line drawn along the floor of the anterior fossa to the dorsum sellae and a second line drawn along the posterior clivus. ²⁾ Normal mean basal angle: 130°. Platybasia: >145° (abnormally obtuse basal angle).

1. McRae’s line:

Drawn across foramen magnum (tip of clivus (basion) to opisthion) ³⁾. The mean position of the odontoid tip below the line is 5 mm (± 1.8 mm SD) on CT and 4.6 mm (± 2.6 mm SD) on MRI ⁴⁾.

No part of odontoid should be above this line (the most accurate measure for BI)

2. Chamberlain’s line ⁵⁾:

Less than 3 mm or half of dens should be above this line, with 6 mm being definitely pathologic. Seldom used because the opisthion is often hard to see on plain film and may also be invaginated. On CT ⁶⁾ and MRI ⁷⁾ the normal odontoid tip is 1.4 mm (± 2.4) below the line

see Chamberlain’s line for Basilar invagination diagnosis

3. McGregor’s line:

http://www.ebmconsult.com/content/images/Xrays/McGregor_Line.png

It refers to a line connecting posterior edge of the hard palate to the most caudal point of the occipital curve. If the tip of the dens lies more than 4.5 mm above this line it is indicative of basilar invagination.

4. Wackenheims line

Normally the tip of the dens is ventral and tangential to this line. In basilar invagination odontoid process transects this line.

5. Fischgold’s digastric line

joins the digastric notches.The normal distance from this line to the middle of the atlanto-occipital joint is 10 mm (decreased in BI) ⁸⁾.

6. Fischgold’s bimastoid line

joins tips of mastoid processes. The odontoid tip averages 2 mm above this line (range: 3 mm below to 10 mm above) and this line should cross the atlanto-occipital joint.

The foramen magnum angle (FMA), which is formed by the Chamberlain's line and McRae's line, has not been fully studied. The study aimed to investigate the FMA and its relationship with other craniocervical parameters.

Participants were divided into control, type A BI, and type B BI groups. Parameters included Chamberlain line violation, atlantodental interval, clivus height, clivus anteroposterior dimension, FMA, basal angle, clivo-axial angle, head and neck flexion angle, Boogard's angle, and subaxial cervical spine lordosis angle. A comparison of these parameters among the 3 groups and correlation analysis between FMA and other parameters were performed. The significance level was set at P < 0.05.

A total of 111 controls, 111 type A BI patients, and 62 type B BI patients were enrolled. The FMAs in the control, type A BI, and type B BI groups were 6.21° (3.67°, 8.71°), 22.16° ± 6.61°, and 22.39° (17.27°, 31.08°), respectively. Correlation analysis revealed correlations between the FMA and other variables. In the 2 BI subgroups, FMA was significantly correlated with Chamberlain line violation, clivus height, clivus anteroposterior dimension, basal angle, clivo-axial angle, and Boogard's angle.

The FMA in patients with BI was approximately 22° and approximately 6° in controls, indicating that the foramen magnum in BI had a greater tilt. As a pathological condition, FMA can reflect the degree of BI. Clivus hypogenesis is a reason for the excessive tilt of the FM ⁹⁾

Erosion of the tip of the odontoid, commonly seen in rheumatoid arthritis (RA), obviates use of any measurement that is based on the location of the tip of the odontoid ¹⁰⁾. For this reason, other measures have been developed, including the Clark station, ¹¹⁾.

Redlund-Johnell criteria, ¹²⁾ and Ranawat criteria ¹³⁾. Since even these methods will miss up to 6% of cases of BI in RA, ¹⁴⁾, it is recommended that suspicious cases be investigated further (e.g.with CT and/or MRI).

MRI: optimal for demonstrating brainstem impingement, poor for showing bone.

Cervicomedullary angle: the angle between a line drawn through the long axis of the medulla on a sagittal MRI and a line drawn through the cervical spinal cord. The normal CMA is 135 – 170 ° . A CMA < 135 ° correlates with signs of cervicomedullary compression, myelopathy or C2 radiculopathy ¹⁵⁾.

CT: primarily done to assess bony anatomy (erosion, fractures…).

CTA should be performed when surgery is contemplated, to show detail of VA anatomy.

Myelography (water soluble) with CT: also good for delineating bony pathology.