Background/Purpose: The current concept of axial Spondyloarhtritis (axSpA) considers non-radiographic axSpA (nr-axSpA) and Ankylosing Spondylitis (AS) as two stages of one disease. There are limited data available regarding rate of radiographic progression of the sacroiliac joints (SIJ) and the rate of transition from nr-axSpA to AS. The aim of the current study was to investigate radiographic progression in the SIJ in an early axSpA cohort.

Methods: Patients included in the study came from an early SpA cohort, from three centers of the ESPERANZA programme, a Spanish nationwide health management programme designed to provide excellence in care for early SpA. One of the inclusion criteria was symptom duration between 3 and 24 months. Every patient had a complete examination at baseline that included ESR, CRP, ASDAS-CRP, ASDAS-ESR, BASDAI, HLA-B27, pelvis x-Rays and Magnetic resonance (MRI) of SIJ. Pelvis x-Rays at baseline and at follow up were performed for every patient. These x-Rays were centrally digitized, anonymised and the SIJ were scored independently by two trained readers, according to the grading system of the modified New York criteria (mNYc). The readers scored images of both time points randomly and were blinded for all clinical data. MRI of SIJ were assessed independently and blinded by two trained readers,according to ASAS definition for bone marrow oedema (BME). Descriptive analysis was performed using SPSS program v.21.0. Cohen´s kappa coefficients were also calculated to assess agreement between the two readers (mNYc for x-Rays and ASAS definition for a positive MRI).

Results: Forty four patients (52% male) were included. Mean age was 34.4 years (range: 21-45) and 52.3% were HLA-B27 positive. Mean time since symptoms onset until baseline visit was 10.4 months (range: 3-24). Mean (SD) follow up period (between baseline x-Ray and last follow up x-Ray) was 3.6 (1.4) years (median 4 years). All patients fulfilled ASAS criteria for axSpA and 16 of them had AS according to mNYc at baseline, in the opinion of both readers. Interobserver agreement for the x-Ray reading at baseline visit (44 x-Rays) was at the limit between moderate and good (kappa 0.62), but it increased to very good agreement (kappa 0.90) at the x-Rays reading corresponding to the follow-up visit. Regarding MRI, data from 27 images were available for analysis and the kappa value for the ASAS definition of BME was 0.76 at baseline. Progression of sacroiliitis over follow up period by at least one grade at one side was found in 43.2% (19/44), in the opinion of both readers. Among the 28 nr-axSpA patients at baseline, 3 of them (11%) progressed to AS according to NYmc in the opinion of both readers. We also analyzed the possible association between all variables collected at baseline with radiographic progression but we did not find any statistically significant association for any of them.

Conclusion: Progression of radiographic sacroiliitis by at least one grade after a mean time of follow-up of 3.6 years occurs in almost half of the patients. The rate of progression from nrax-SpA to AS was 11%. The inter-reader reliability of pelvis x-Ray improves as disease progresses. Regarding to these results, pelvis x-Ray has a limited utility at early stages of axSpA.