Background/Purpose: The use of spinal mobility measures (SMMs), at the individual level and in the follow-up of patients with early axSpA has not yet been studied. The purpose of our study was to investigate the longitudinal use of SMMs and the relation with mobility curves of healthy individuals in patients with early axSpA.

Methods: All SMMs 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 analysed. All available SMMs were plotted for each patient in function of age, and together with the percentile curves derived for healthy volunteers, the mobility curves¹. A subgroup analysis was performed in patients with low disease activity over time (ASDAS<2.1 in ≥2/3 of visits), in order to control for the influence of disease activity on spinal mobility. Intra- and inter-observer reliability were analyzed using Intraclass Correlation Coefficients (ICC) and the Smallest Detectable Change (SDC) in the SPACE cohort.

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. The mean number of observations with assessment of SMMs was 7 (1.1) and 4 (1.7) per patient in DESIR and SPACE cohorts, respectively. A high variability in SMMs within the same patient over time was observed, with very discrepant values for the same SMM from visit to visit, even when restricting the analysis to patients with low disease activity over time. Figure 1 shows the results for 10-cm Schober’s test and Lateral Spinal Flexion (LSF) in the DESIR cohort. The results were strikingly similar for all the SMMs and in both cohorts. The reliability of SMMs was only “fair” to “good” (inter-reader ICC (2,1): 0.55-0.84; intra-reader ICC (2,1): 0.49-0.72). The obtained SDCs reflect that large variations in SMMs are needed to capture a true change beyond measurement error (e.g. 1.4 cm for 10-cm Schober’s test; 5.1 cm for LSF; 2.2 cm for Chest expansion and 12.2 degrees for Cervical rotation).

Conclusion: There is a high variation of SMMs from visit to visit, which impairs the use of spinal mobility measures, at the individual level in the follow-up of patients with axSpA. It should be tested if reliability can be improved to reduce at least part of the variability.

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