Systemic onset juvenile idiopathic arthritis, systemic JIA, sJIA, is a disease that develops before the age of 16 years and is active for at least 6 weeks. In Europe, the prevalence of the disease varies and is approximately 0.3-0.8 cases per 100,000 children under 16 years of age. JIA is an extremely rare (orphan) disease, that is, a disease with a prevalence of no more than 10 cases per 100,000 population. In most cases, pericarditis develops, with lymphadenopathy, enlargement of the liver and/or spleen, skin rash, serositis, and lymphadenitis as signs of the disease. The disease occurs most often in children aged 14 to 16 years. At the present stage, sJIA is considered as a special variant of sJIA, the pathogenesis of which has two stages. At the first (febrile, acute) stage, sJIA is mostly considered not as autoimmune but as an autoinflammatory disease, i.e. the leading role in its development is assigned to activation of the innate immune system. At the second (arthritic, chronic) stage of sJIA, characterized by the development of persistent polyarthritis, in addition to the activation of innate immunity, there is also activation of acquired immunity, which is accompanied by activation of Th17 lymphocytes, synthesizing, including IL-17. Currently, most diagnoses of sJIA are made based on the analysis of data obtained from a correct medical history and physical examination, but not on laboratory examination. Thus, an important feature of sJIA that contributes to its differentiation from other categories of sJIA is the development of various extraarticular manifestations, including intermittent fever, transient (volatile) erythematous rash, generalized lymphadenopathy, hepatomegaly and/or splenomegaly, serositis (e.g. pericarditis, and/or pleurisy, and/or peritonitis). Their presence allows differential diagnosis among other categories of JIA, but at the same time makes it much more difficult to diagnose JIA among various systemic diseases, since these symptoms are inherent not only to JIA, but also to various systemic diseases, such as sepsis, infections (for example, yersinosis or toxoplasmosis), solid tumors and oncohematology. The evaluation of laboratory diagnostic results is often difficult because the results obtained (such as general blood counts or biochemical blood tests) are often nonspecific for sJIA, which means that further differential diagnoses must be pursued. For this reason, the interval from the onset of the first signs of the disease to diagnosis is 4 to 6 weeks. Because of this, the diagnosis of juvenile arthritis with systemic onset is currently considered an "exclusion" diagnosis. Therefore, the development of a special algorithm for differential diagnosis of juvenile sJIA with systemic and rheumatic diseases is necessary.
Key words: Juvenile arthritis with systemic onset, children, immune autoaggression, diagnosis, genetically engineered biologics