The time the syndrome takes to appear following the initial viral infection is debated, though it may develop between the first and second week. Epidemiological data suggest that recognition of the disease may typically be delayed by 2–6 weeks, and usually by 3–4 weeks. By the time of presention, children have often developed antibodies to SARS-CoV-2, but test negative for the virus at RT-PCR.The condition may match some or all of the diagnostic criteria for Kawasaki disease (i.e. the complete or incomplete/atypical subtypes), or for Kawasaki disease shock syndrome. It tends to affect all paediatric age groups, ranging from infancy to adolescence. It can also share clinical features with other paediatric inflammatory conditions, including toxic shock syndrome, and secondary haemophagocytic lymphohistiocytosis or macrophage activation syndrome. Coinfections with other pathogens have been recorded.Affected children always present with persistent fever. Other clinical features at presentation vary. In contrast to acute COVID-19, most children have gastrointestinal symptoms, such as diarrhoea, vomiting, and intense abdominal pain (sometimes severe enough to suggest appendicitis). Muscle pain and feelings of tiredness and general physical weakness are also very common. Some Kawasaki-like symptoms that may be present (especially in children under the age of 5) include mucosal changes around the mouth ("strawberry tongue", cracked lips, etc. ), red eyes (conjunctivitis without pus), widespread rash (consistent with leukocytoclastic vasculitis), red or swollen hands and feet, and enlarged lymph nodes. Chest or neck pain may also be present. Severe headache and altered mental state have been reported, along with various neurological disturbances.
Anti-inflammatory treatments have been used, with good responses being recorded for intravenous immunoglobulin (IVIG), with or without corticosteroids. Oxygen is often needed. Supportive care is key for treating clinical complications. Most children who receive expert hospital care survive.Knowledge of this newly described syndrome is evolving rapidly. Its clinical features may appear somewhat similar to Kawasaki disease, a rare disease of unknown origin that typically affects young children, in which blood vessels become inflamed throughout the body. It can also show features of other serious inflammatory conditions of childhood, including toxic shock and macrophage activation syndromes. Nevertheless, it appears to be a separate syndrome. Older children tend to be affected.This emerging condition has been defined slightly differently (using different names), by the World Health Organization (WHO), the Royal College of Paediatrics and Child Health (RCPCH), and the Centers for Disease Control and Prevention (CDC). Although the condition is thought to follow SARS-CoV-2 viral infection, antigen or antibody tests are not always positive. Exclusion of alternative causes, including bacterial and other infections, is essential for differential diagnosis. Some general clinical guidance has been provided by the RCPCH, the National Institutes of Health, the American College of Rheumatology, and the American Academy of Pediatrics.Clusters of new cases have been reported two to six weeks after local peaks in viral transmission. The disease is thought to be driven by a delayed biological mechanism in certain predisposed children.