Manifestations of spinal cord compression (in order of increasing severity) from subluxation:
(1) pain
(2) torticollis
(3) abnormal gait (including a preference for sitting over standing)
(4) loss of bowel or bladder control
(5) hyperreflexia
(6) Babinski sign
(7) quadriparesis
(8) quadriplegia
Radiographs taken:
(1) neutral position
(2) extension
(3) flexion (associated with maximal atlanto-dens measurements)
Landmarks:
(1) first cervical vertebra (atlas)
(2) second cervical vertebra (axis), with the odontoid process (or dens)
(3) the clivus (along the anterior margin of the foramen magnum)
Measurements:
(1) atlanto-dens distance
(2) width of the neural canal at C1
(3) distance between the clivus and the posterior surface of the dens
atlanto-dens distance in mm =
= distance in mm between the (posterior aspect of the anterior arch of the atlas) and (the anterior surface of the odontoid process <dens> of the axis)
width of the neural canal at C1 in mm =
= distance in mm between the (posterior surface of the odontoid process of the axis) and (the anterior margin of the posterior arch in C1)
distance between the clivus and the vertical line extended from the posterior surface of the dens in mm =
= perpendicular distance in mm between (the clivus) and (a line drawn through the posterior aspect of the odontoid process <dens>)
Interpretation:
• An atlanto-dens distance that is increased (some authors use > 4.5 mm, others > 5.0 mm) indicates atlantoaxial instability.
• If the person is clinically asymptomatic, then the person is said to have asymptomatic atlantoaxial instability (AAAI).
• If neurologic signs of spinal cord compression are present, then the patient has symptomatic atlantoaxial instability (SAAI).
• An atlanto-dens distance >= 7 mm may be at significant risk for atlantoaxial instability (Pueschel et al, 1992).
Limitations:
• The current recommendation for the Special Olympics is that a patient with Down Syndrome who participates in a sport with potential for head and neck trauma should be screened by lateral cervical radiograph. Identification of instability would restrict the patient from participation in sports with potential stress on the cervical spine..
• Some authors disagree that patients with atlantoaxial instability on lateral cervical radiographs have an increased risk of spinal cord compression. They view routine screening of Down Syndrome patients as unreliable.
• White et al argue that showing a narrowing in the subarachnoid space in MRI images of the cervical spine is more predictive of risk for a compression syndrome. Patients with atlantoaxial instability on lateral X-rays would be candidates for MRI.
