Repeat Pediatric Trisomy 21 Radiographic Exam: Does Atlantoaxial Instability Develop Over Time?

Background: Atlantoaxial instability (AAI) is common in pediatric patients with Trisomy 21 and can lead to spinal cord injury during sports, trauma, or anesthetized neck manipulation. Children with Trisomy 21 therefore commonly undergo radiographic cervical spine screening, but recommendations on age and timing vary. The purpose of this study was to determine if instability develops over time. Methods: We performed a retrospective review for all pediatric Trisomy 21 patients receiving at least 2 cervical spine radiographic series between 2008 and 2020 at our institution. Atlantodens interval (ADI) and space available for the cord at C1 (SAC) were measured; bony abnormalities such as os odontoidium, and age and time between radiographs were noted. AAI was determined by ADI >= 6 mm or SAC <= 14 mm based on our groups' prior study. Those who developed instability were compared with those who did not. Results: A total of 437 cervical spine radiographic series from 192 patients were evaluated, with 160 included. Mean age at first radiograph was 7.4 +/- 4.4 years, average ADI was 3.1 mm (+/- 1.2), and SAC was 18.1 mm (+/- 2.6). The average time between first and last radiographs was 4.3 years (+/- 1.8), with average final ADI 3.2 mm (+/- 1.4) and SAC 18.9 mm (+/- 2.9). Seven patients (4%) had instability: 4 were unstable on their initial studies and 3 (1.6%) on subsequent imaging. Os odontoideum was found in 5 (71%) unstable spines and 3 (2%) stable spines (P<0.0001); only 1 patient that became unstable on subsequent radiograph did not have an os. There was no specific age cut-off or surveillance time period after which one could be determined no longer at risk. Conclusions: Trisomy 21 patients have a 4.4% overall rate of AAI in our series with a 1.6% rate of progression to instability over similar to 4 years. Given this nearly 1 in 23 risk of instability, we recommend initial surveillance radiograph for all children over 3 years with Trisomy 21; repeat asymptomatic surveillance should continue in those with os odontoideum given their high instability risk.