The Change of Asymptomatic Atlantoaxial Instability With Down Syndrome From Infant to Adolescent in Japanese Population: Minimum 10 Years Follow-Up

Background: Although 10% to 60% of patients with Down syndrome (DS) develop atlantoaxial instability (AAI), clarifying the course of asymptomatic AAI may prevent unnecessary clinical interactions and investigations. This study investigates the radiographic changes observed in asymptomatic AAI associated with DS in Japanese children as they grow from infancy to adolescence over a minimum of 10 years. Methods: A retrospective analysis of cervical radiographs acquired from asymptomatic patients with DS in both infancy and adolescence was carried out. Radiographic evaluation included measuring the atlantodental interval (ADI) and the space available for the cord (SAC). In neutral lateral cervical radiographs, AAI was defined as ADI >6 mm and SAC <14 mm. Results: Two hundred thirty-nine patients were included. The mean follow-up was 12.8 years. ADI was 2.3 mm at initial evaluation and 2.7 mm at final evaluation (P<0.01) in a neutral position, 3.1 and 3.3 mm in flexion (P=0.18), and 1.7 and 2.1 mm in extension (P<0.01), respectively. SAC was 15.8 mm at initial evaluation and 20.9 mm at final evaluation (P<0.01) in neutral position, 15.6 and 20.7 mm in flexion (P<0.01), and 16.8 and 21.0 mm in extension (P<0.01), respectively. Forty-five patients (18.8%) showed evidence of AAI at the initial evaluation with 4 patients meeting the threshold for AAI at the final evaluation, one of which had os odontoideum. In contrast, of the 194 patients who did not have AAI at the initial evaluation, 3 (1.3%) developed AAI at the final evaluation. One of these with a normal den developed AAI at 13 years. Conclusions: As 1 patient with a normal dens developed AAI at 13 years, we recommend screening for AAI with X-ray in infancy and adolescence regardless of the presence or absence of an os odontoideum. Evidence level:Level IV.