Background/Purpose: MRI of the sacroiliac joints (SIJ) is part of the evaluation of patients with suspected axial spondyloarthritis (axSpA). However, sacroiliitis (SI) could be a nonspecific finding, potentially resulting from mechanical stress or degeneration. 

Furthermore, in the subgroup of patients with psoriatic arthritis with axial involvement (axPsA), the findings on MRI are still not completely clarified and do not have sufficient data comparing them with MRI findings in patients with chronic non-inflammatory back pain (NI-CBP).

Our objective was to calculate the sensitivity, specificity, positive and negative predictive values (PPV, NPV) and likelihood ratio (LR) of a positive MRI for SI according to the ASAS/OMERACT criteria for axPsA and non-psoriatic spondyloarthritis (nPsA-axSpA). We identified clinical variables that were independently associated with SI and described inflammatory spinal lesions (ISL) for each subgroup.

Methods: Total spine and SIJ MRIs performed at the University of Chile’s Clinical Hospital from adults with nPsA-axSpA, axPsA and  NI-CBP followed by the rheumatology service were retrospectively analyzed. The gold standard was diagnosis by a rheumatologist. 

For patients with nPsA-axSpA and NI-CBP, we analyzed the MRIs performed during 2015, whereas for axPsA those performed between 2015 and 2021.

MRIs were performed with a 1.5 T resonator without contrast medium. Images were analyzed by two expert radiologists blinded to the clinical data. ISL were defined according to ASAS/OMERACT definitions.

ANOVA test was used to analyze continuous data and Chi² for categorical data. Sensitivity, specificity, PPV, NPV and LR were calculated with the R program and multivariate analysis with SPSS software. 

Results: A total of 95 patients were analyzed, 29 nPsA-axSpA, 21 axPsA and 45 NI-CBP. The sensitivity of MRI SI was 83% for nPsA-axSpA, 77% for axPsA and the specificity was 91% for both subgroups. For the entire axSpA group (axPsA + nPsA-axSpA), the PPV was 91% and NPV was 80%.  The LR (+) was 8.8 and the LR (-) was  0.21.

Advanced age (OR: 1.06; CI: 1.01-1.11), BMI (OR: 1.24; CI 1.04-1.47), physical activity (OR: 3.85; CI 1.09 – 13.66), inflammatory back pain (OR: 35.11; CI 2.01 -614.36) and extra-articular manifestations (OR : 1.72, 1.02-2.9), were independently associated with SI in multivariate analysis.

In the entire sample 38 patients (40%) presented some ISL, 8 (18%) in NI-CBP, 14 (48%) in nPsA-axSpA and 16 (76%) in axPsA. The concomitant finding of spondylitis, costovertebral  and facet arthritis occurred only in axSpA group, 6 patients (28.5%) with AxPsA, 1 (3.4%) patient with nPsa-axSpa.

Conclusion: The sensitivity and specificity of SI on MRI  in our cohort was 83% for nPsA-axSpA and  77% for axPsA. Advanced age, BMI and  physical activity were associated with SI. ISL may occur in patients with NI-CBP, although the concomitance of 3 ISL was only observed in axPsA and more frequently in axPsA.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/characterization-and-comparison-of-mri-findings-in-axial-spondyloarthritis-axial-psoriatic-and-chronic-non-inflammatory-back-pain/