A 20 yo male with gait ataxia and progressive lower limb weakness, came for CV junction screening.
There is abnormal increase in Atlanto Dental distance ( also called predental space) the distance between posterior margin of anterior arch of C1 and anterior surface of odontoid process is 8mm in this case (normal values 3mm or less in adults and 5mm or less in children). Thus posterior subluxation of odontoid process in relation to anterior arch of C1 is reducing the available AP bony canal diameter at C1-2 with significant compression over cord. Impingement will be more significant on flexion. Focal abnormal intramedullary T2 hyperintensity attributed to focal cord odema which is extending up to C3-4 disc.
Laxity leads to Atlantoaxial instability and is defined by an increase in the pre dentate space greater then 3 mm in adults and 5 mm in children.