AB0889 TOPOGRAPHICAL, VOLUMENTRIC AND SUBGROUP DIFFERENTIATION BETWEEN INFLAMMATORY SIJ LESIONS ON MRIS OF PATIENTS WITH AXIAL SPONDYLOARTHRITIS AND CHRONIC LOW BACK PAIN
Authors: M. Ifanti, P. Sewerin, D. Kiefer, U. Kiltz, X. Baraliakos
Journal: Annals of the Rheumatic Diseases (2024)

<h3>Background:</h3> Inflammatory lesions of the SIJ are a hallmark of disease activity in patients with axSpA and a key finding for classification according to the ASAS classification criteria. ASAS-endorsed definitions for a 'positive MRI' for inflammatory and structural lesions exist. In population studies, participants with non-SpA related back as well as healthy people have shown occurrence of inflammatory lesions fulfilling the ASAS definition for inflammatory lesions in up to 25% of cases. <h3>Objectives:</h3> Characterize the inflammatory lesions on SIJ MRIs for frequency, topography, volume and intensity in axSpA and low back pain (LBP) patients in detail. <h3>Methods:</h3> Standardized SIJ MRIs of consecutive patients with axSpA and LBP referred for clarification of axSpA were evaluated blinded to all clinical/demographic information. Inflammatory lesions were considered as defined by ASAS [1] using STIR sequences (T1-weighted sequences were available for total evaluation of lesions). Evaluation included topographic occurence of inflammatory lesions based on SIJ quadrant (upper/lower iliac/sacral), volumetric measurement including all involved slices and intensity of inflammatory signal, normalized with reference to intensity of signal in the cerebrospinal fluid of the participant. Subgroup analyses were performed for age, sex, disease activity status and laboratory inflammatory markers. Recording of lesions was done on MatLab v.R2018b. <h3>Results:</h3> 107 patients were included (53 axSpA, mean age 36.0±10.4 years, 60% males, 75% HLA-B27 positive, mean (SD) ASDAS 2.9±0.9, mean CRP 0.8±1.0 and 54 LBP, mean age 41.5±11.5 years, 39% male, 53% HLA-B27, mean CRP 0.9±3.2). Overall, significantly more lesions were found in axSpA vs. LBP in all SIJ quadrants. The range of lesions in the different quadrants was 54-84 in axSpA and 26-32 in LBP (p&lt;0.003 in all quadrants). The lower iliac and the lower sacral quadrants were most frequently affected in both groups. Similarly, the total volume of lesions was significantly higher in axSpA (12.640mm<sup>3</sup>) as compared to LBP (1.630mm<sup>3</sup>), p≤0.001, mainly due to the lower quadrants. No statistical difference was found for the upper quadrants in both iliac and sacral bone. No differences were found in the comparison of normalized signal intensity between groups. In a more detailed analysis, significantly more lesions were found for all quadrants in patients with high pain levels (NRS ≥4/10) between patients with axSpA vs. LBP, while the total volume of lesions was significantly higher in patients with axSpA in both, patients with low (NRS&lt;4/10) and high pain levels, as compared to LBP. Also here, no differences were seen in the intensity of lesions. The total volume of lesions was significantly higher in male (15.624,0mm<sup>3</sup>) vs. female (8.115,7mm<sup>3</sup>) axSpA patients (p&lt;0.050), while male LBP also had significantly smaller lesions (1.824,1mm<sup>3</sup>) as compared to male axSpA patients (p&lt;0.001). Such difference was not found in females. No differences were found between axSpA patients with high vs. low BASDAI (cut-off 4/10) or ASDAS (cut-off 2.1). In the analysis of laboratory results, axSpA patients showed significantly higher total volume of inflammatory lesions in the subgroup with normal CRP levels vs. the respective patients from the control group, while this difference was not significant in the comparison of axSpA and LBP patients with increased CRP. <h3>Conclusion:</h3> Inflammatory SIJ MRI lesions fulfilling ASAS positive MRI definition exist in LBP but are more frequent in axSpA. Significant differences in morphology were found, with axSpA patients showing more lesions especially in the lower SIJ quadrants. Lesions of axSpA patients are characterized by larger volume, while the signal intensity is similar in axSpA and LBP. Male patients show larger lesions, while the lesion size is not related to occurrence of increased inflammatory markers such as CRP. <h3>REFERENCES:</h3> [1] Maksymowych WP et al, Ann Rheum Dis 2019 <h3>Acknowledgements:</h3> <b>NIL.</b> <h3>Disclosure of Interests:</h3> <b>None declared.</b>