====== Basilar invagination diagnosis ======

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[[Basilar invagination]] is diagnosed by various imaging modalities such as [[plain x ray]]s, [[CT scan]]s, 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,
((Poppel MH, Jacobson HG, Du   BK, Gottlieb C. Basilar
impression and platybasia in Paget's disease. Br J
Radiol. 1953; 21:171–181))
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.
((Koenigsberg RA, Vakil N, Hong TA, Htaik T, Faerber
E, Maiorano T, Dua M, Faro S, Gonzales C. Evaluation ofplatybasiawithMRimaging.AJNRAmJNeurora- diol. 2005; 26:89–92)) 
Normal mean basal angle: 130°. Platybasia: >145° (abnormally obtuse basal angle).

===== Measurements used in BI =====

1. [[McRae’s line]]:

{{::opisthion.png?300|}}

Drawn across [[foramen magnum]] (tip of [[clivus]] ([[basion]]) to [[opisthion]])
((McRae DL. The Significance of Abnormalities of the Cervical Spine. AJR. 1960; 70:23–46)).
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
((Cronin CG, Lohan DG, Mhuircheartigh JN, Meehan CP, Murphy JM, Roche C. MRI evaluation and meas- urement of the normal odontoid peg position. Clin Radiol. 2007; 62:897–903)).

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

===== Chamberlain’s line =====


2. [[Chamberlain’s line]]
((Chamberlain WE. Basilar Impression (Platybasia); Bizarre Developmental Anomaly of Occipital Bone and Upper Cervical Spine with Striking and Mis- leading Neurologic Manifestations. Yale J Biol Med. 1939; 11:487–496)):

{{::chamberlain.png?200|}}

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
((Cronin CG, Lohan DG, Mhuircheartigh JN, Meehan CP, Murphy J, Roche C. CT evaluation of Chamber- lain's, McGregor's, and McRae's skull-base lines. Clin Radiol. 2009; 64:64–69))
and MRI
((Cronin CG, Lohan DG, Mhuircheartigh JN, Meehan CP, Murphy JM, Roche C. MRI evaluation and meas- urement of the normal odontoid peg position. Clin Radiol. 2007; 62:897–903)) 
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?200}}

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]]

{{::wackenheims_line.png?300|}}

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]]

{{::fischgold_s_digastric_line.png?300|}}

joins the digastric notches.The normal distance from this line to the middle of the atlanto-occipital joint is 10 mm (decreased in BI)
((Hinck VC, Hopkins CE, Savara BS. Diagnostic Criteria of Basilar Impression. Radiology. 1961; 76)).

6. [[Fischgold’s bimastoid line]] 

{{::fischgold_s_digastric_line.png?300|}}

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.

===== Foramen Magnum Angle =====
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
((Jian Q, Zhang B, Jian F, Bo X, Chen Z. Basilar Invagination: A Tilt of the Foramen Magnum. World Neurosurg. 2022 Aug;164:e629-e635. doi: 10.1016/j.wneu.2022.05.027. Epub 2022 May 14. Erratum in: World Neurosurg. 2022 Nov;167:255. PMID: 35577208.))

===== Basilar impression diagnosis in rheumatoid arthritis =====

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]]
((Riew KD, Hilibrand AS, Palumbo MA, Sethi N, Bohlman HH. Diagnosing basilar invagination in the rheumatoid patient. The reliability of radiographic criteria.J Bone Joint Surg.2001; 83-A:194–200)).
For this reason, other measures have been developed, including the [[Clark station]],
((Clark CR, Goetz DD, Menezes AH. Arthrodesis of the Cervical Spine in Rheumatoid Arthritis. J Bone Joint Surg. 1989; 71A:381–392)).

[[Redlund-Johnell criteria]],
((Redlund-Johnell I, Pettersson H. Radiographic measurements of the cranio-vertebral region. Designed for evaluation of abnormalities in rheumatoid arthritis. Acta Radiol Diagn (Stockh). 1984; 25:23–28))
and [[Ranawat criteria]]
((Ranawat CS, O'Leary P, Pellicci P, et al. Cervical Spine Fusion in Rheumatoid Arthritis. J Bone Joint Surg. 1979; 61A:1003–1010)).
Since even these methods will miss up to 6% of cases of BI in RA,
((Riew KD, Hilibrand AS, Palumbo MA, Sethi N, Bohlman HH. Diagnosing basilar invagination in the rheumatoid patient. The reliability of radiographic criteria.J Bone Joint Surg.2001; 83-A:194–200)),
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
((Bundschuh C, Modic MT, Kearney F, Morris R, Deal C. Rheumatoid arthritis of the cervical spine: sur- face-coil MR imaging. AJR Am J Roentgenol. 1988; 151:181–187)).

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.

===== References =====