Distinguishing between typical Kawasaki disease and multisystem inflammatory syndrome in children (MIS-C) associated with SARS-CoV-2

In recent months, there are increasing reports of a Kawasaki disease-like syndrome in children infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), termed 'Paediatric Multisystem Inflammatory Syndrome temporally associated with SARS-CoV-2 (PIMS-TS)' in the UK. Debate is ongoing regarding the nature of these pro-inflammatory syndromes. We herein propose that the platelet count may, at least in part, be able to help us differentiate between the two aforementioned syndromes. In a recent report, compared to a historical 'classical' Kawasaki disease (KD) cohort, patients with PIMS-TS had significantly lower platelet counts (188 vs 383 g/L, p < 0.0001). A possible explanation for this is their difference in underlying immunopathogenesis. In KD, the fundamental pathogenesis is thought to be immune complex-mediated, hence, the use of intravenous immunoglobulin (IVIg) which competes with the immunoglobulin Fc receptors (FcRs) on inflammatory cells, preventing the activation of these cells and thereby ameliorating the inflammatory response. If left untreated, these immune complexes activates the inflammatory cells (including monocytes and neutrophils), which also results in recruitment of platelets, resulting in the thrombocytosis we commonly see in KD. These immune complexes may also bind to platelets directly via FcRs on platelet membranes. In contrast, in viral-associated hyperinflammatory syndromes (e.g. PIMS-TS or MIS-C), there are mediators being secreted in the process of eradication of the virus (mainly to stimulate CD8+ cells to kill viral infected cells), which would inadvertently suppress bone marrow function and activate platelets, culminating in thrombocytopenia.