Capillary leak syndrome was associated with more severe multisystem inflammatory syndrome in children during the COVID‐19 pandemic

Aim This population‐based study investigated the occurrence of capillary leak syndrome (CLS) in children with multisystem inflammatory syndrome in children (MIS‐C), associated with COVID‐19. We also examined associations between CLS and MIS‐C disease severity. Methods All eligible individuals aged 0–18 years, who were diagnosed with MIS‐C in Skåne, southern Sweden, from 1 April 2020 to 31 July 2021, were studied. They were all included in the Pediatric Rheumatology Quality Register and clinical and laboratory data were compared between patients with and without CLS. Results We included 31 patients (61% male) with MIS‐C in the study. The median age at diagnosis was 10.6 years (range 1.99–17.15) and 45% developed CLS. All six patients who required intensive care had CLS. Patients with CLS also had a higher incidence of reduced cardiac function, measured as low ejection fraction. The CLS group exhibited significantly higher C‐reactive protein values ( p < 0.001) and N‐terminal pro‐B‐type natriuretic peptide levels ( p < 0.001), as well as lower platelet counts ( p = 0.03), during the first week of treatment. Individuals with CLS also received more intense immunosuppression. Conclusion CLS was a common complication of MIS‐C in our study and these patients had a more severe disease course that required more intensive treatment.