Atlanto-axial subluxation
Updates to Article Attributes
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was changed:
Atlanto-axial subluxation (AAS) is a disorder of C1-C2 causing impairment in rotation of the neck. The anterior facet of C1 is fixed on the facet of C2. It may be associated with dislocation of the lateral mass of C1 on C2.
Pathology
Aetiology
- congenital
- arthritides
-
acquired
- trauma
- retropharyngeal abscess / Grisel syndrome
- surgery
- URTI
Mechanism
There are several ways in which a subluxation can occur:
- antero-posterior subluxation
- rotatory subluxation known as atlantoaxial rotatory fixation (AARF): characterised four different types 3
- type I: the atlas is rotated on the odontoid with no anterior displacement
- type II: the atlas is rotated on one lateral articular process with 3 to 5 mm of anterior displacement
- type III: comprises a rotation of the atlas on both lateral articular processes with anterior displacement greater than 5 mm
- type IV: characterised by rotation and posterior displacement of the atlas
- vertical subluxation
- lateral subluxation 4
Radiographic features
Plain film (C spine)radiograph
In a non traumatic setting flexion and extension views may be performed. The expected distance between anterior arch of C1 and the dens in the fully flexed position should be <3~5
In a vertical subluxation the dens is often above the McGregor line by over 8 mm in men and 9.7 mm in women
CT
On CT, C1 is not oriented in line with the head. The head may be pointed anteriorly, C1 is turned. If this is a fixed defect, C2 is rotated in conjunction with C1.
Predisposing factors
Congenital
os odontoideum-
Down syndrome(20%) Morquio syndromespondyloepiphyseal dysplasiaosteogenesis imperfectaMarfan diseaseneurofibromatosis type 1 (NF1)
Arthritides
rheumatoid arthritispsoriatic arthritisReiter syndrome (reactive arthritis)ankylosing spondylitissystemic lupus erythematosus (SLE)
Acquired
trauma-
retropharyngeal abscess/Grisel syndrome surgeryURTI
Differential diagnosis
Possible differential considerations on imaging include
-<p><strong>Atlanto-axial subluxation (AAS)</strong> is a disorder of C1-C2 causing impairment in rotation of the neck. The anterior facet of C1 is fixed on the facet of C2. It may be associated with dislocation of the lateral mass of C1 on C2.</p><p>There are several ways in which a subluxation can occur:</p><ul>-<li>antero-posterior subluxation</li>-<li>rotatory subluxation known as <strong>atlantoaxial rotatory fixation (AARF)</strong>: characterised four different types <sup>3</sup><ul>-<li>-<strong>type I:</strong> the atlas is rotated on the odontoid with no anterior displacement</li>-<li>-<strong>type II:</strong> the atlas is rotated on one lateral articular process with 3 to 5 mm of anterior displacement</li>-<li>-<strong>type III:</strong> comprises a rotation of the atlas on both lateral articular processes with anterior displacement greater than 5 mm</li>- +<p><strong>Atlanto-axial subluxation (AAS)</strong> is a disorder of C1-C2 causing impairment in rotation of the neck. The anterior facet of C1 is fixed on the facet of C2. It may be associated with dislocation of the lateral mass of C1 on C2.</p><h4>Pathology</h4><h5>Aetiology</h5><ul>
-<strong>type IV:</strong> characterised by rotation and posterior displacement of the atlas</li>-</ul>-</li>-<li>vertical subluxation</li>-<li>lateral subluxation <sup>4</sup>-</li>-</ul><h4>Radiographic features</h4><h5>Plain film (C spine)</h5><p>In a non traumatic setting flexion and extension views may be performed. The expected distance between anterior arch of C1 and the dens in the fully flexed position should be <<strong>3</strong> mm in an adult (~<strong>5</strong> mm in a child).</p><p>In a vertical subluxation the dens is often above the <a href="/articles/mcgregor-line-1">McGregor line</a> by over <strong>8</strong> mm in men and <strong>9.7 </strong>mm in women</p><h5>CT</h5><p>On CT, C1 is not oriented in line with the head. The head may be pointed anteriorly, C1 is turned. If this is a fixed defect, C2 is rotated in conjunction with C1.</p><h4>Predisposing factors</h4><h5>Congenital</h5><ul>- +<strong>congenital</strong><ul>
-<a href="/articles/down-syndrome">Down syndrome</a> (20%)</li>- +<a href="/articles/down-syndrome">Down syndrome</a> (20%)</li>
-</ul><h5>Arthritides</h5><ul>- +</ul>
- +</li>
- +<li>
- +<strong>arthritides</strong><ul>
-</ul><h5>Acquired</h5><ul>- +</ul>
- +</li>
- +<li>
- +<strong>acquired</strong><ul>
-<a href="/articles/retropharyngeal-abscess">retropharyngeal abscess</a>/<a href="/articles/grisel-syndrome-2">Grisel syndrome</a>- +<a href="/articles/retropharyngeal-abscess">retropharyngeal abscess</a> / <a href="/articles/grisel-syndrome-2">Grisel syndrome</a>
-</ul><h4>Differential diagnosis</h4><p>Possible differential considerations on imaging include</p><ul><li><a href="/articles/odontoid-fracture">odontoid fracture</a></li></ul>- +</ul>
- +</li>
- +</ul><h5>Mechanism</h5><p>There are several ways in which a subluxation can occur:</p><ul>
- +<li>antero-posterior subluxation</li>
- +<li>rotatory subluxation known as <strong>atlantoaxial rotatory fixation (AARF)</strong>: characterised four different types <sup>3</sup><ul>
- +<li>
- +<strong>type I:</strong> the atlas is rotated on the odontoid with no anterior displacement</li>
- +<li>
- +<strong>type II:</strong> the atlas is rotated on one lateral articular process with 3 to 5 mm of anterior displacement</li>
- +<li>
- +<strong>type III:</strong> comprises a rotation of the atlas on both lateral articular processes with anterior displacement greater than 5 mm</li>
- +<li>
- +<strong>type IV:</strong> characterised by rotation and posterior displacement of the atlas</li>
- +</ul>
- +</li>
- +<li>vertical subluxation</li>
- +<li>lateral subluxation <sup>4</sup>
- +</li>
- +</ul><h4>Radiographic features</h4><h5>Plain radiograph</h5><p>In a non traumatic setting flexion and extension views may be performed. The expected distance between anterior arch of C1 and the dens in the fully flexed position should be <3 mm in an adult (~5 mm in a child).</p><p>In a vertical subluxation the dens is often above the <a href="/articles/mcgregor-line-1">McGregor line</a> by over 8 mm in men and 9.7 mm in women</p><h5>CT</h5><p>On CT, C1 is not oriented in line with the head. The head may be pointed anteriorly, C1 is turned. If this is a fixed defect, C2 is rotated in conjunction with C1.</p><h4>Differential diagnosis</h4><p>Possible differential considerations on imaging include</p><ul><li><a href="/articles/odontoid-fracture">odontoid fracture</a></li></ul>
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