Background/Purpose: Spinal mobility has been mostly investigated in cohorts of patients with established AS. However, it was scarcely studied in the early phases of the disease and how the impairment in mobility across different measures behaves as compared to normal subjects. The purpose of this study was to investigate 1) which spinal mobility measures (SMMs) are most frequently impaired and in which order; 2) which SMMs are most discriminative of activity and severity in early axial Spondyloarthritis (axSpA).

Methods: All SMM measurements of patients from the DESIR (5-year data) and SPACE (data from Leiden University Medical Center, 2.6 (1.9) years of follow-up) cohorts and with a clinical diagnosis of axSpA (level of confidence ≥7/10) were analyzed. SMMs were considered impaired when falling below pre-defined cut-offs, derived from normal individuals¹. The proportion of patients with each of the SMMs impaired was calculated, for both baseline and all observations. The BASMI, being a composite index, was not considered in the ranking of impairment of spinal mobility and only the level of impairment is presented. The same analysis was conducted in subgroups to contrast patient and disease characteristics potentially influencing spinal mobility, like treatment with biologics (ever/never), disease activity (with/without low disease activity over time, i.e., ASDAS<2.1 in ≥2/3 of visits) and the presence of baseline syndesmophytes (yes/no).

Results: We included 328 (54% males, mean (SD) age of 32 (8) years) and 148 (64% females, mean (SD) age of 30 (9) years) patients from the DESIR and SPACE cohorts, respectively. No strict and fixed order of impairment in SMMs was seen in both cohorts. Considering patients in whom all SMMs were assessed, in DESIR, the most frequently impaired SMM (below 2.5^th percentile) was mSchober (42%), followed by Lateral Spinal Flexion (LSF; 37%), Tragus-to-wall (16%), Cervical rotation (16%) and Chest expansion (11%). In SPACE, the order of impairment was: LSF (36%), mSchober (14%), Chest expansion (13%), Cervical rotation (11%) and Tragus-to-wall (3%). LSF and mSchober captured the majority of patients with ≥ 1 SMM impaired (86% and 78% for DESIR and SPACE, respectively). LSF and BASMI best discriminated between subgroups of patients, with higher impairment in patients ever treated with biologics, with higher disease activity and presence of baseline syndesmophytes (Table 1, data from DESIR). Similar results were obtained in the SPACE cohort.

Conclusion: LSF and mSchober are the most impaired SMMs, together allowing the identification of the majority of patients with impaired spinal mobility in early axSpA. LSF and BASMI discriminate best between subgroups of patients, reflecting a worse spinal mobility in patients with more active and severe disease.

¹Ramiro S et al. Ann Rheum Dis. 2015 Jun;74(6):1218-24.