In the meantime, different names and provisional case definitions are being used around the world. The initial case definitions released by the World Health Organization (WHO), the Royal College of Paediatrics and Child Health (RCPCH) and Centers for Disease Control and Prevention (CDC) all include involvement of more than one organ system, along with fever and elevated inflammatory markers. Criteria that vary among these three definitions include the ways in which involvement of different organs is defined, the duration of fever, and how exposure to COVID-19 is assessed. The preliminary WHO case definition is for "multisystem inflammatory syndrome (MIS) in children and adolescents temporally related to COVID-19" (box). The WHO has established a platform for standardized, anonymized clinical data, along with a dedicated case report form, and underlines the "urgent need for collection of standardized data describing clinical presentations, severity, outcomes, and epidemiology." Diagnostic guidance by the RCPCH proposes a broader case definition (for PIMS-TS), which was also endorsed by an expert panel convened by the American College of Cardiology. Key clinical criteria set out in the RCPHC case definition are: persistent fever, inflammation (indicated by neutrophilia, high CRP levels and low lymphocyte count), and evidence of organ dysfunction (shock; cardiac, respiratory, renal, gastrointestinal, or neurological disorder), coupled with additional clinical features, including laboratory, imaging and ECG findings. Coronary artery abnormalities, such as dilatation, may be apparent at echocardiography and ECG (or contrast CT of the chest). Biomarkers supporting the diagnosis include abnormal fibrinogen levels, high D-dimers (possible coagulopathy), high troponin, low albumin, and high ferritin.
Based on available reports, the fatality rate among diagnosed cases appears to have been about 1.7% (notably higher than the rate of 0.07% recorded among children with Kawasaki disease in Japan). A rapid risk assessment conducted by the European Centre for Disease Prevention and Control (ECDC) concluded that the overall risk to children in the European Union (EU), European Economic Area (EEA) and the UK "is considered low, based on a very low probability of [the disease] in children and a high impact of such disease. "Clusters of cases of the newly described condition have been recorded 3–4 weeks after peaks in SARS-CoV-2 viral transmission through various local communities. Such observations have been seen to support the concept that SARS-CoV-2 infection may be capable of triggering a severe form of a Kawasaki-like disease. Frequent presentation without prominent respiratory symptoms in children who do not appear to have ongoing SARS-CoV-2 infection but who have already developed antibodies suggests that the disease may be driven by a delayed, post-infectious mechanism.The median age of onset appears to be at least 7 years (compared with 2 years for Kawasaki disease, which primarily affects children under the age of 5). Male children seem to be more frequently affected (broadly in line with Kawasaki disease, where the male to female ratio is about 1.5 to 1). Many affected children appear not to have underlying health conditions, such as asthma or autoimmune disorders, and there have been relatively few reports of known congenital heart disease or preexisting cardiovascular disease.