Atlantoaxial Instability in Mucopolysaccharidoses: Surgical Indications and Recommendations

BACKGROUND: Cervical spine atlantoaxial instability (AAI) is frequently encountered in patients with mucopolysaccharidoses (MPS). The value of clinical examination, radiographic measurements, and MRI findings in determining the management of MPS AAI is unknown. The goal of this study is to compare these tools and report the utility and significance of each in surgical decision-making. METHODS: Retrospective case-control series from a single center's skeletal dysplasia (SD) division, 2007-2023, included MPS patients who had documented history, examination, and flexion-extension (F-E) cervical spine radiographs and MRI. Radiographic measurements included dens morphology, anterior atlanto-dens interval (AADI), and posterior atlanto-dens interval (PADI). MRI measurements included cervical stenosis, C1 SAC, and the presence of myelomalacia. Factors important were quantified by sensitivity, specificity, and odds ratio (OR). Criteria cutoffs were defined by receiver operating characteristic (ROC) curve analysis, and a scoring rubric was generated by internal validation. RESULTS: Of all SD patients, 35 had MPS and met all inclusion criteria. The most common diagnosis was Morquio syndrome (77%, 27/35). Patients were majority 54% female and 80% white. About half (18, 51%) underwent surgery at an average age of 10.6-years-old. Measurement analysis followed by ROC evaluation defined 7 surgical cutoffs as follows: (1) paresthesias in the history, (2 and 3) weakness and/or positive Babinski on exam, (4) os or aplastic odontoid morphology, (5) ≥3 mm AADI instability, (6) myelomalacia on MRI, and (7) ≥115% C1-C2 stenosis. Stenosis and myelomalacia were the most impactful by OR. Os/aplastic odontoid morphology and positive Babinski were both 100% specific. Validation found that 100% of patients with 0 criteria were able to be managed nonoperatively. Conversely, patients with 3 or more criteria present were overwhelmingly indicated for surgery in 93% of cases. DISCUSSION: This is the largest case series describing AAI in MPS patients. This study identified the above 7 criteria utilizing MPS-specific thresholds. These thresholds allow for the qualification of pathologic findings against other MPS patients, weight factor importance, and provide guidance for how many factors should be present to indicate MPS AAI for surgical intervention.