Systemic juvenile idiopathic arthritis and adult-onset Still's disease are the same disease: evidence from systematic reviews and meta-analyses informing the 2023 EULAR/PReS recommendations for the diagnosis and management of Still's disease.

Objectives: To analyse the similarity in clinical manifestations and laboratory findings between systemic juvenile idiopathic arthritis (sJIA) and adult-onset Still's disease (AOSD).

Methods: Three systematic reviews (SR) were performed. One included cohort studies comparing sJIA versus AOSD that described clinical and biological manifestations with at least 20 patients in each group (SR1). The second identified studies of biomarkers in both diseases and their diagnostic performance (SR2). The last focused on diagnostic biomarkers for macrophage activation syndrome (MAS, SR3). Medline (PubMed), Embase and Cochrane Library were systematically searched. The risk of bias was assessed with an adapted form of the Hoy scale for prevalence studies in SR1 and the Quality Assessment of Diagnostic Accuracy Studies-2 in SR2 and SR3. We performed meta-analyses of proportions for the qualitative descriptors.

Results: Eight studies were included in SR1 (n=1010 participants), 33 in SR2 and 10 in SR3. The pooled prevalence of clinical manifestations did not differ between sJIA and AOSD, except for myalgia, sore throat and weight loss, which were more frequent in AOSD than sJIA because they are likely ascertained incompletely in sJIA, especially in young children. Except for AA amyloidosis, more frequent in sJIA than AOSD, the prevalence of complications did not differ, nor did the prevalence of biological findings. Ferritin, S100 proteins and interleukin-18 (IL-18) were the most frequently used diagnostic biomarkers, with similar diagnostic performance. For MAS diagnosis, novel biomarkers such as IL-18, C-X-C motif ligand 9, adenosine deaminase 2 activity and activated T cells seemed promising.

Conclusion: Our results argue for a continuum between sJIA and AOSD.

Prospero registration number: CRD42022374240 and CRD42024534021.