<p><strong>BACKGROUND: </strong>The objective of this work was to describe magnetic resonance imaging (MRI) changes over time in inflammatory and structural lesions at the sacroiliac joint (SIJ) in children with spondyloarthritis (SpA) exposed and unexposed to tumor necrosis factor inhibitor (TNFi).</p>

<p><strong>METHODS: </strong>This was a retrospective, multicenter study of SpA patients with suspected or confirmed sacroiliitis who underwent at ≥2 pelvic MRI scans. Images were reviewed independently by 3 radiologists and scored for inflammatory and structural changes using the Spondyloarthritis Research Consortium of Canada (SPARCC) SIJ inflammation score (SIS) and structural score (SSS). Longitudinal, quantitative changes in patient MRI scans were measured using descriptive statistics and stratified by TNFi exposure. We used an average treatment effects (ATE) regression model to explore the average effect of TNFi exposure over time on inflammatory and structural lesions, adjusting for baseline lesion scores.</p>

<p><strong>RESULTS: </strong>Forty-six subjects were evaluated using the SIS (n&nbsp;= 45) and SSS (n&nbsp;= 18). Median age at baseline imaging was 13.6 years, 63% were male and 71% were white. Twenty-three subjects (50%) were TNFi exposed between MRI studies. The median change in SIS in TNFi exposed and unexposed subjects with a baseline SIS ≥0 was - 20.7 and - 14.3, respectively (p&nbsp;= 0.09). Eleven (85%) TNFi exposed and 8 (89%) unexposed subjects with a baseline SIS ≥0 met the SIS minimal clinically important difference (MCID; ≥2.5). Using the ATE model adjusted for baseline SIS, the average effect of TNFi on SIS in patients with a baseline SIS ≥2 was - 14.5 (p&nbsp;&lt; 0.01). Unadjusted erosion change score was significantly worse in TNFi unexposed versus exposed subjects (p&nbsp;= 0.03) but in the ATE model the effect of TNFi was not significant.</p>

<p><strong>CONCLUSION: </strong>This study quantitatively describes how lesions in the SIJs on MRI change over time in patients exposed to TNFi versus unexposed. Follow-up imaging in TNFi exposed patients showed greater improvement than the unexposed group by most metrics, some of which reached statistical significance. Surprisingly, a majority of TNFi unexposed children with a baseline SIS≥2 met the SIS MCID. Additional studies assessing the short and long-term effects of TNFi on inflammatory and structural changes in juvenile SpA are needed.</p>

<p><strong>OBJECTIVE: </strong>To describe characteristics of children with enthesitis-related arthritis (ERA) and juvenile psoriatic arthritis (JPsA) enrolled in the Childhood Arthritis and Rheumatology Research Alliance (CARRA) Registry.</p>

<p><strong>METHODS: </strong>All children with ERA and JPsA were identified. Demographics, clinical characteristics, and treatments were described. Those with and without sacroiliitis were compared. In those with sacroiliitis, the first visit with clinically active sacroiliitis (which came first in 72% of cases) was compared to the first visit without.</p>

<p><strong>RESULTS: </strong>Nine hundred two children with ERA or JPsA were identified. Children with ERA were older at diagnosis (10.8 vs. 8.2 years, p&lt;0.01) and more likely male (56% vs. 38%, p&lt;0.01). Polyarticular involvement was reported in 57% and 72% of those with ERA and JPsA. HLA-B27 was positive in 38% and 12% of those tested with ERA and JPsA. At least one biologic was taken by 72% and 64% of those with ERA and JPsA. Sacroiliitis (diagnosed clinically and/or by imaging) was reported in 28% (40% ERA and 12% JPsA). Of these, 54% were female, 36% were HLA-B27 positive, and 81% took at least one biologic. In children with sacroiliitis, the physician global, parent/patient global, and cJADAS 10 were all significantly worse at the first visit with clinically active sacroiliitis versus the first visit without active sacroiliitis.</p>

<p><strong>CONCLUSION: </strong>In this registry, there are over 900 children with ERA or JPsA. There was high biologic use in this population, especially in those with sacroiliitis. Further, there was equal sex representation in those with sacroiliitis.</p>