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Multisystem Inflammatory Syndrome in Children (MIS-C) is a new clinical entity occurring in children and young adults, which is associated with the SARS-CoV-2 infection. The first cases of MIS-C were diagnosed in Poland in May 2020. Since October 2020, a significant increase in the incidence of this new disease has been observed in Poland, reflecting the increased incidence of COVID-19 in the paediatric population. MIS-C develops as a result of dysregulation of the immune system occurring 4 weeks after the SARS-CoV-2 infection. Diagnosis is based on the following criteria: a set of clinical features (including fever and signs of multiple organ damage) and elevated inflammatory markers, with exclusion of other causes. The most common complications involve the cardiovascular system: acute myocardial damage with reduced left ventricular ejection fraction, shock, and coronary artery abnormalities and arrhythmias. Mortality in Western Europe and the United States is around 1-2%. Appropriate management, including vital function support and immunomodulatory treatment, allows for a quick recovery in the vast majority of patients. This document is an updated guideline for the diagnostic and therapeutic management of children with suspected MIS-C in Poland. The most important changes concern treatment, steroid therapy, and antiplatelet therapy in particular.

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Pediatric inflammatory multisystem syndrome (PIMS) is a new entity in children, likely associated with previous coronavirus disease 19 (COVID-19) infection. Most of the reports about PIMS come from countries particularly hit by the COVID-19 pandemic. Our aim was to investigate the nature of inflammatory syndromes in Poland (country with low COVID-19 prevalence) and to perceive the emergence of PIMS in our country. On 25 May 2020, we launched a nationwide survey of inflammatory syndromes in children for retrospective (since 4 March 2020) and prospective data collection. Up to 28 July, 39 reported children met the inclusion criteria. We stratified them according to age (<5 and ≥ 5 years old) and COVID-19 status. The majority of children had clinical and laboratory features of Kawasaki disease, probably non-associated with COVID-19. However, children ≥5 years of age had PIMS characteristics, and nine children had COVID-19 confirmation. This is, to our knowledge, the first report of the PIMS register from a country with a low COVID-19 prevalence, and it proves that PIMS may emerge in any area involved in the COVID-19 pandemic. In a context of limited COVID-19 testing availability, other risk factors of PIMS, e.g., older age, should be considered in the differential diagnosis of inflammatory syndromes in children.

During the winter months of 2020/2021 a wave of multisystem inflammatory syndrome in children (MIS-C) emerged in Poland. We present the results of a nationwide register aiming to capture and characterise MIS-C with a focus on severity determinants. The first MIS-C wave in Poland was notably high, hence our analysis involved 274 children. The group was 62.8% boys, with a median age of 8.8 years. Besides one Asian, all were White. Overall, the disease course was not as severe as in previous reports, however. Pediatric intensive care treatment was required for merely 23 (8.4%) of children, who were older and exhibited a distinguished clinical picture at hospital admission. We have also identified sex-dependent differences; teenage boys more often had cardiac involvement (decreased ejection fraction in 25.9% vs. 14.7%) and fulfilled macrophage activation syndrome definition (31.0% vs. 15.2%). Among all boys, those hospitalized in pediatric intensive care unit were significantly older (median 11.2 vs. 9.1 years). Henceforth, while ethnicity and sex may affect MIS-C phenotype, management protocols might be not universally applicable, and should rather be adjusted to the specific population.