Clinical and surgical characteristic of atlantoaxial reduction and fixation for patients with syringomyelia caused by atlantoaxial dislocation: retrospective study of 101 cases.

Background Context: Atlantoaxial dislocation (AAD) is a relatively rare condition at the craniovertebral junction. In a minority of cases, AAD may coexist with syringomyelia. There is currently a lack of large-scale case reports investigating the specific clinical and surgical characteristics of syringomyelia associated with AAD.

Purpose: This study aimed to evaluate the treatment efficacy of surgical reduction and fixation for patients with syringomyelia caused by AAD.

Study Design: Retrospective study.

Patient Sample: Patients with syringomyelia and AAD underwent surgery performed by single surgeon between 2006.1 and 2021.12.

Outcome Measures: The primary outcome was the reduction condition of syringomyelia evaluated on the follow-up MRI, classified as completely recovered, reduced and unimproved. The secondary outcome was recovery of symptoms, which were classified as improved (symptoms improved, with or without remaining other symptoms) or unimproved (no change or worsening of preoperative symptoms).

Methods: A total of 120 cases with syringomyelia and atlantoaxial instability who underwent atlantoaxial reduction and fixation surgery by single surgeon were reviewed. Out of these, 101 patients with a minimum follow-up of 6 months were included in the study. The patients' demographics, preoperative symptoms, length of syringomyelia, characteristics of craniocervical malformation, and the presence of basilar invagination (BI) and Chiari malformation were collected. Preoperative and follow-up clivus axial angle (CAA) was measured and CAA reduction angle was calculated by the postoperative CAA minus preoperative CAA. Improvement of symptoms and reduction of syringomyelia were evaluated at follow-up. Ordinal logistic regression and additional subgroup analysis were conducted to identify potential factors affecting the reduction of syringomyelia.

Results: A total of 38 males and 63 females were included in the study, with a mean age of 36.9±12.4 years old. All patients presented with assimilation of atlas, and 53 patients had congenital C2-3 fusion. Among the patients, 100 patients had concomitant type A BI, and 97 had Chiari malformation. Most patients (94) exhibited myelopathy symptoms, 40 people had cranial nerve symptoms, and 20 patients experienced cerebellar symptoms. Sixty patients underwent traction, reduction and posterior C0-2 fixation surgery, while 41 patients received traction, transoral release and reduction, and posterior C0-2 fixation surgery. The average follow-up duration was 21.6 months. After surgery, all cerebellar symptoms and cranial nerve symptoms were alleviated. Eighty (85.1%) patients showed significantly improved myelopathy symptoms. Sixty-three patients showed completely recovered syringomyelia, 30 had reduced syringomyelia and 8 had unimproved syringomyelia. Ordinal logistic regression showed that a shorter preoperative syringomyelia and larger a CAA reduction angle were favorable factors for reduction of syringomyelia.

Conclusion: Atlantoaxial dislocation can be associated with basilar invagination and lead to syringomyelia. Atlantoaxial reduction and fixation can effectively improve syringomyelia in this condition. As larger CAA reduction angle is potential favorable factor for reduction of syringomyelia, anatomical reduction should be pursued when technically feasible.