Atlanto-Axial Subluxation in Children
The ADI is the most widely accepted measure of atlanto-axial subluxation. This is measured from the posterior inferior border of the atlantal archto the nearest anterior surface of the dens. The measurement is taken from good-quality lateral cervical spine x-rays in flexion and extension. It is important to ensure that there is no rotation as this invalidates the measurement.
- Decreasing ADI with age: The acceptable interval will vary with age. Less than 5 mm is acceptable for children younger than 8 years of age, while <3 mm is used in older children and adolescents.
- ADI changes with flexion: The ADI can increase by up to 2 mm with flexion in the normal individual.
- Normal elevation in ADI: An increase in the ADI may be seen in children and adults with Down syndrome, Morquio syndrome, juvenile rheumatoid arthritis, and Griselsyndrome.
Posterior Canal Width or Posterior ADI
- Portion of canal housing cord: The PADI is the distance from the posterior surface of the odontoid peg to the anterior surface of the posterior arch of the atlas. This is the space available for the spinal cord. While some have argued it is a more important measure of atlanto-axial subluxation, it is less commonly used in clinical practice. A value of <13 mm is considered abnormal.
Atlanto-Axial Rotatory Fixation in Children
- Normal cervical rotation: The normal cervical range of motion is 80 degrees of rotation to each side with 40 degrees of this range occurring at C1-2 joint.
- Causes: Clinical torticollis may be secondary to infection, tumor, sternocleidomastoid pathology, or plagiocephaly, as well as atlanto-axial subluxation.
- “Cock-robin” pose: This is the common presentation of a rotary subluxation with a combination of head tilt to one side with rotation towards the other side.
- CT scan of cervical spine: A dynamic CT of the C1-2 complex is performed with the head first rotated maximally to the left, then right, and finally with the head in neutral position. In true atlanto-axial rotatory subluxation, the subluxation will persist in all three positions. It is important to appreciate that a CT of a normal spine may be indistinguishable from an atlanto-axial subluxation in one position, but as the position of the head is changed, a subluxation will persist with atlanto-axial subluxation, while a normal spine will normalize its alignment.
3D CT reconstruction of atlanto-axial rotatory fixation(anterior view left, posterior view right): There is rotatory fixation with the left lateral mass of C1 subluxed anteriorly in front of C2.