Background/Purpose: Treatment of axial SpA is increasingly aimed at intervention early in the disease course before radiographic sacroiliitis has appeared and when response to treatment is greatest. If radiographs do not indicate sacroiliitis, the Assessment of SpondyloArthritis (ASAS) classification criteria indicate that bone marrow edema (BME)/osteitis on MRI is a requirement, but this may also occur non-specifically, and some patients lack MRI inflammation. The purpose of this analysis was to assess the relative importance of structural lesions on MRI in the sacroiliac joints (SIJ) of patients with non-radiographic axial SpA (nr-axSpA).

Methods: Patients had axial SpA per the ASAS classification criteria, but did not meet modified NY radiographic criteria. Patients had symptoms for >3 months and <5 years, Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) ≥4, and failed ≥2 NSAIDs. Patients were randomly assigned to etanercept 50 mg/week or placebo, then after 12 weeks, all patients received open-label etanercept 50 mg/week. Clinical and health outcomes were assessed throughout the study, and MRI of the SIJ and spine was performed by two central readers at baseline, weeks 12 and 48 to assess BME using the Spondyloarthritis Research Consortium of Canada (SPARCC) score. Additionally, a post-hoc analysis was conducted to score structural lesions using the SPARCC SIJ structural method, which assesses fat metaplasia, erosion, backfill, and ankylosis on T1-weighted spin echo (T1WSE) MRI. Two independent readers scored baseline and 48 week T1WSE MRI scans from 187 cases blinded to patients and short tau inversion recovery (STIR) MRI scans. Mean scores of the readers were used. SPARCC SIJ BME score ≥2 was used as the operational definition of positive MRI evidence of inflammation. For these analyses, all patients were combined, independent of randomization.

Results: Mean (SD) age was 32 (7.8) years, 60.5% were male, and mean (SD) duration of disease symptoms was 2.5 (1.8) years. A total of 73% of patients were human leukocyte antigen B27 (HLA-B27) positive and 81% met the ASAS MRI imaging criteria at baseline. Additionally, 61% had a structural lesion on MRI at baseline comprised of erosion (58%), backfill (23%), fat metaplasia (18%), and ankylosis (7%). Of the patients who were ASAS MRI positive, 65% had a structural lesion on MRI, compared to 43% who were ASAS MRI negative. Of the patients with SPARCC SIJ BME score ≥2 at baseline, 79% had a structural lesion on MRI compared to 35% with SPARCC SIJ BME score <2.  Relative frequencies of MRI structural lesions in patients with SPARCC SIJ BME ≥2 vs <2 were: erosions (78% vs 30%), backfill (36% vs 5%), fat (22% vs 13%), and ankylosis (7% vs 7%).

Conclusion: Structural lesions on MRI occur frequently in nr-axSpA despite the absence of radiographic sacroiliitis and even in the absence of MRI inflammation. This finding strongly reinforces the concept of nr-axSpA as an early stage of axial SpA.     

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/substantial-structural-lesions-on-mri-in-the-sacroiliac-joints-of-patients-with-non-radiographic-axial-spondyloarthritis-even-in-the-absence-of-mri-inflammation/