Introduction: Juvenile idiopathic arthritis (JIA) is a group of rare musculoskeletal disorders with chronic inflammation of joints, typically manifesting before the age of 16 years. The assessment of disease activity remains pivotal in JIA treatment decisions, particularly during clinical remission. While musculoskeletal ultrasound (MSUS) has shown promise in detecting subclinical synovitis, longitudinal data on MSUS features in JIA remains limited. The aim of this study was to evaluate the prevalence of subclinical synovitis observed in MSUS over a follow-up period in JIA patients. Additionally, it sought to assess the consistency and correlation between clinical findings, standardized composite clinical score (JADAS10), and MSUS-detected synovitis during 9 months follow-up.Patients and methods: a prospective single-center study was conducted, enrolling all consecutive JIA patients (excluding systemic JIA) seen at the study center in one year period. At three-months intervals over a 9 months period (M0, M3, M6 and M9), patients underwent clinical examination, laboratory tests, and MSUS assessment. Data on demographic characteristics, disease profile, and treatment were collected. Patients were categorized into active disease (ACT) or remission (REM) groups based on Wallace criteria and JADAS10 scores using previously validated thresholds. The ultrasound assessments adhered to the Outcome Measures in Rheumatology Clinical Trials (OMERACT) pediatric group, covering 40 joints, were performed by two ultrasonographers at every visit. Subclinical synovitis was defined as synovitis detected exclusively by MSUS. Spearman's correlation coefficients (r_s) were used to evaluate the association between MSUS, clinical data, and outcome measures, such as active joint count (ACJ), patient's/parent's global assessment of disease activity (PaGA), physician's global assessment of disease activity (PhGA) and JADAS10.Results: subclinical synovitis was evident in 5.2% of all joints and in 80.6% of the patients at baseline. During the follow-up period, signs of subclinical synovitis decreased to 3.8% of joints, however, the proportion of affected patients remained high (67.7%), with the majority in REM group. Despite the consistent strong correlation between PaGA and PhGA throughout the study (r_s> 0.895; p < 0.001), both measures displayed moderate (r_s= 0.647; p < 0.001) to weak (r_s= 0.377; p = 0.04) correlations with MSUS findings. Notably, PaGA remained significantly correlated with MSUS at the M9 visit (r_s= 0.377, p = 0.04), while PhGA showed no correlation (p = 0.094).Conclusions: The study results indicate the persistence of subclinical inflammation detected by MSUS in a significant proportion of JIA patients, even during clinical remission. Moreover, the findings suggest that conventional measurements of JIA activity may be insufficient for assessing patients in clinical remission.

Polymyositis is a rarely reported complication of COVID-19 illness, especially in children. Molecular mimicry may be a cause of hyperactivated autoimmunity, leading to various clinical manifestations, including myopathies. Symptoms vary from mild muscle weakness to severe rhabdomyolysis. We review the literature on post-COVID myositis and report a case of severe polymyositis in a 7-year-old boy, following undefined viral infection 3 weeks before the onset of muscle pain. Patient’s condition deteriorated from physical activity-associated pain in the lower limbs to severe muscle weakness leading to dysphagia and mechanical ventilation. As antibodies against SARS-CoV-2 were detected and other possible conditions causing myositis were excluded, the diagnosis of post-COVID polymyositis was considered as the most likely. The patient was treated with high doses of methylprednisolone and cyclophosphamide, resulting in improvement. Although COVID-19 is becoming a seasonal disease, the infection itself and post-viral disorders, such as polymyositis, are still of great interest and require better investigation to ensure appropriate management for each individual. Our experience suggests that aggressive immunosuppressive therapy might be a solution for severe post-COVID-related diseases. This literature review is provided in addition to the case report presented at the 29th European Paediatric Rheumatology Congress; the abstract is available online in the Proceedings of the 29th European Paediatric Rheumatology Congress.

Chronic recurrent multifocal osteomyelitis (CRMO) is a rare auto-inflammatory bone disorder that primarily affects young girls, with a mean age of 10 years at onset. Generally, it is a self-limited disease. However, recent data indicate that more than 50% of patients have a chronic persistent disease and about 20% a recurring course of this condition. Also, there are more cases reported with associated auto-inflammatory and autoimmune diseases. In this case report, we present a rare case of sporadic CRMO in which the patient eventually developed C-ANCA (cytoplasmic anti-neutrophil cytoplasmic antibodies)-associated renal vasculitis and hyperparathyroidism. A 14 year old female patient was brought to the emergency department with a sudden onset of left leg pain and oedema. After physical evaluation and initial investigation, she was diagnosed with femoral and pelvic deep vein thrombosis. While searching for possible thrombosis causes, osteomyelitis of the left leg was identified. Additional CT and MRI scans hinted at the CRMO diagnosis. Due to the multifocal lesions of CRMO, endocrinological evaluation of calcium metabolism was done. The results showed signs of hyperparathyroidism with severe hypocalcaemia. Moreover, when kidney damage occurred and progressed, a kidney biopsy was performed, revealing a C-ANCA associated renal vasculitis. Treatment was started with cyclophosphamide and prednisolone according to the renal vasculitis management protocol. Severe metabolic disturbances and hyperparathyroidism were treated with alfacalcidol, calcium and magnesium supplements. Secondary glomerulonephritis (GN) associated hypertension was treated with ACE (angiotenzine converting enzyme) inhibitors. Anticoagulants were prescribed for deep vein thrombosis. After 1.5 years of treatment, the patient is free of complaints. All microelement and parathormone levels are within normal range. Kidney function is now normal. To date, there are no clinical or diagnostic signs of deep vein thrombosis. This case report presents a complex immunodysregulatory disorder with both auto-inflammatory and autoimmune processes. We hypothesize that the long lasting active inflammation of CRMO may induce an autoimmune response and result in concomitant diseases like C-ANCA-associated vasculitis in our patient. Any potential specific pathogenic relationships between these two rare pathologies may need to be further studied. Furthermore, there is a lack of specific biomarkers for CRMO and more studies are necessary to identify CRMO’s characteristic patterns and how to best monitor disease progression.

Biomarker search for juvenile idiopathic arthritis (JIA) diagnosis and monitoring remain the focus of research worldwide. Several microRNAs (miRNAs) have been identified as relevant in different rheumatic conditions; however, studies in JIA remain limited. Our study aimed to explore the potential of serum and urine-derived miRNAs for JIA diagnostics and longitudinal JIA monitoring. In this single-center, prospective study, three selected miRNAs (miR-16, -146a and -155) were tested in serial serum and urine samples collected from 31 JIA patients and 22 healthy controls (HC) via quantitative reverse transcription polymerase chain reaction (RT‒qPCR). The diagnostic performance of variables for distinguishing JIA patients from HCs was assessed by determining the area under the receiver operating characteristic (ROC) curve (AUC). The prediction of remission was evaluated using Cox regression and Kaplan-Meier analyses. A p-value < 0.05 was considered statistically significant. Lower miR-16 and higher miR-155 levels were detected in serum of JIA patients’ vs. HC (p < 0.01), whereas the level of miR-146a was lower in urine of JIA patients (p = 0.032). In ROC analysis, miR-16 and miR-155 distinguished JIA patients from HC when analyzed in serum (AUC 0.81, 95% CI 0.70–0.93, p < 0.001 and AUC 0.73, 95% CI 0.59–0.87, p = 0.005, respectively), and miR-146a– in urine (AUC 0.68, 95% CI 0.53–0.82, p = 0.030). During 12 months follow-up period increasing miR-16 (p = 0.021) and decreasing miR-155 (p = 0.009) levels were observed in serum samples. Kaplan-Meier survival analysis revealed that a high level of miR-146a in serum significantly predicts JIA remission (HR = 2.2, 95% CI 0.7–6.9, p = 0.040). This study highlights the utility of miRNAs in JIA diagnosis, monitoring and prognosis and demonstrates the feasibility of using urine as a noninvasive source of miRNAs in children with non-systemic JIA.

Introduction: Multisystem inflammatory syndrome in children (MIS-C) has been reported as one of the cytokine storm syndromes associated with COVID-19. Despite the several proposed diagnostic criteria, MIS-C remains a diagnostic and clinical challenge. Recent studies have demonstrated that platelets (PLTs) play a crucial role in COVID-19 infection and its prognosis. This study aimed to investigate the clinical importance of PLT count and PLT indices in predicting MIS-C severity in children.Patients and methods: We conducted a retrospective single-center study at our university hospital. A total of 43 patients diagnosed with MIS-C during a 2-year period (from October 2020 to October 2022) were included in the study. MIS-C severity was evaluated according to the composite severity score.Results: Half of the patients were treated in the pediatric intensive care unit. No single clinical sign was associated with a severe condition, except for shock (p = 0.041). All the routine biomarkers, such as complete blood count (CBC) and C-reactive protein (CRP), used for MIS-C diagnosis were significant in predicting MIS-C severity. Single PLT parameters, such as mean PLT volume, plateletcrit, or PLT distribution width, did not differ between the severity groups. However, we found that a combination of PLT count and the previously mentioned PLT indices had the potential to predict MIS-C severity.Conclusions: Our study emphasizes the importance of PLT in MIS-C pathogenesis and severity. It revealed that together with routine biomarkers (e.g., CBC and CRP), it could highly improve the prediction of MIS-C severity.