The new coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection has been spreading worldwide since December 2019, resulting in enormous numbers of affected patients and substantial mortality in many countries. The proportion of children affected by COVID-19 was small compared to adults, and most affected children were asymptomatic or presented with mild symptoms [1-3]. However, some children and adolescents with Kawasaki disease (KD)-like hyperinflammatory illness were reported in Europe and the United States (US) during the peak of the COVID-19 pandemic in the spring of 2020 [4-7]. Hundreds of such cases have been reported since April 2020, and a few died of the illness [4,7]. They presented with fever, skin rash, conjunctivitis, oral mucosa changes (red fissured lips, strawberry tongue), and hand or foot edema, all of which are included in the diagnostic criteria of KD, in addition to prominent gastrointestinal symptoms (abdominal pain, vomiting, diarrhea). They also had markedly elevated inflammatory markers (erythrocyte sedimentation rate [ESR], C-reactive protein [CRP], procalcitonin, ferritin, interleukins [ILs], etc.), and neutrophilia. Some had cardiac abnormalities (left ventricular dysfunction, myocarditis, pericarditis, valvular regurgitation, coronary arterial ectasia, or aneurysm) or symptoms and signs of shock (hypotension, hypoxemia, altered consciousness). The patients tested positive for the reverse transcription-polymerase chain reaction (RT-PCR) or antibody test for COVID-19 or had a history of recent contact with COVID-19 patients. Clinicians managing such patients coined new terms for this new illness, COVID-19–associated hyperinflammatory response syndrome, pediatric inflammatory multisystem syndrome temporally associated with COVID-19 (PIMS-TS), or COVID-19–associated multisystem inflammatory syndrome in children (MIS-C). In this article, we use the term MIS-C and review its case definitions, epidemiology, pathogenesis, clinical features, treatments, and outcomes.

In May 2020, the World Health Organization, Centers for Disease Control and Prevention of the US, and Royal College of Paediatrics and Child Health (RCPCH) of the United Kingdom (UK) simultaneously published case definitions for MIS-C or PIMS-TS [8-10]. In Korea, the case definition was published in June 2020 by the Korea Disease Control and Prevention Agency (KDCA) [11]. Each case definition differs somewhat from the others, but doctors may diagnose patients with MIS-C without much difficulty using any one of them. However, the case definition by RCPCH seems less strict in requiring evidence of multiorgan involvement and SARS-CoV-2 infection (Table 1).

The principal symptoms and signs of MIS-C are similar to those of KD, which suggests a shared pathogenesis. Therefore, clinicians should suspect MIS-C, if patients present with fever, KD-like features (skin rash, conjunctivitis, oral mucosa changes, hand or foot edema), and/or gastrointestinal symptoms (abdominal pain, vomiting, diarrhea), and test positive for SARS-CoV-2 (by PCR or serology test) or have a history of recent contact with COVID-19 patients.

Hundreds of cases of children and adolescents with MIS-C have been reported first in Europe and the US since April 2020 during the COVID-19 pandemic [4-7]. They had evidence of SARS-CoV-2 infection (positive RT-PCR or serology test) or a history of recent contact with COVID-19 patients. Most reported patients with MIS-C were children and adolescents, whereas 27 adult patients with clinical features of multisystem inflammatory syndrome were reported in the US and UK, referred to as multisystem inflammatory syndrome in adults [12]. Most children affected by MIS-C were previously healthy without prior morbidities. MIS-C showed clinical features similar to KD, but their age, sex, and racial distributions differed somewhat from those of KD patients. KD usually affects infants and young children less than 5 years of age (approximately 80% are less than 5 years and about half are less than 2 years), and KD affects about 1.5 times more boys than girls [13,14]. In contrast, MIS-C equally affected children and adolescents (median age, 8.6 years; interquartile range, 7–10 years; range, 3 months–20 years) [15] and affected slightly more boys than girls [4-7,15]. The incidence of KD is markedly high in East Asian countries, including Japan, Korea, China, and Taiwan, but low in Europe and the US [13,14]. In contrast, most patients with MIS-C have been reported in Europe and the US [4-7,15], while some MIS-C cases have been reported in Korea, India, Pakistan, and Iran among Asian countries (Table 2) [15-21]. It is not yet known why the incidence of MIC-S is low in Asian countries. This may be due to the role of racial factors in the development of MIS-C, changes in the pathogenicity of SARS-CoV-2, or differences in population sizes affected by SARS-CoV-2 infection in Asia, Europe, and the US.

MIS-C usually develops 2–4 weeks after SARS-CoV-2 infection [4-7]. However, a few MIS-C cases were identified to develop later than 4 weeks after SARS-CoV-2 infection in epidemiological surveys. In addition, many MIS-C cases had negative PCR and positive antibody test results for SARS-CoV-2, implying that they would be in the early convalescent stage of SARS-CoV-2 infection [4-7,12]. In Korea, 2 of 3 reported cases of MIS-C seemed to develop later than 4 weeks after the diagnosis of or exposure to COVID-19, and one had negative PCR and positive antibody test results for SARS-CoV-2 [16].

The causal relationship between COVID-19 and MIS-C seemed uncertain despite many MIS-C cases developing during the COVID-19 pandemic. Because COVID-19 has been prevalent in Europe and the US, hyperinflammatory response syndromes may have developed concurrently as a complication of SARS-CoV-2 infection. In addition, MIS-C may be a group of heterogeneous diseases, including true cytokine storm syndrome (CSS) developing after SARS-CoV-2 infection, COVID-19 with severe inflammatory responses, and KD occurring concurrently with SARS-CoV-2 infection [7]. In contrast, SARS-CoV-2 infection might be a prerequisite for the development of MIS-C.

Patients diagnosed with MIS-C showed features of markedly increased inflammation with or without shock and cardiac dysfunction, which required differential diagnosis from toxic shock syndrome, KD, KD shock syndrome, or macrophage activation syndrome [4-7]. Therefore, MIS-C could be included within the spectrum of the systemic inflammatory response syndrome or CSS (or cytokine release syndrome) like KD, which might explain why it seemed similar to KD. Most MIS-C patients show clinical manifestations and laboratory findings similar to KD, so the pathogenesis of MIS-C and KD might be the same [22,23]. The pathogenesis of KD is explained by the theory of abnormal or dysregulated immune reactions. It has been postulated that one of pathogens (virus, bacteria, or fungus), toxins, or environmental agents may trigger abnormal or dysregulated immune reactions in genetically susceptible children, leading to the release of large amounts of inflammatory cytokines and cause KD manifesting as systemic vasculitis [13]. Therefore, KD can be included within the spectrum of CSS such as MIS-C. However, some clinicians consider MIS-C and KD different disease entities because their age and regional or racial distributions differ and MIS-C usually shows more severe clinical features than KD [5,24]. Fig. 1 shows the relationship between genetic susceptibility, triggering factors, and hyperinflammatory responses in the development of MIS-C.

There are currently no widely accepted guidelines for the treatment of MIS-C, but treatment strategies generally follow the standard therapy of KD, including intravenous immunoglobulin (IVIG) and corticosteroids [35-37]. This is because overlapping characteristics between the 2 illnesses suggested they might share a similar pathophysiology and respond to similar treatments. High-dose IVIG (1–2 g/kg) and corticosteroids (low to medium dose) are generally recommended for the treatment of MIS-C. High-dose corticosteroids (methylprednisolone pulse therapy) may be considered for treating refractory patients or patients with life-threatening complications such as shock [44,45]. Some patients required a second dose of IVIG due to persistent fever [42,46].

Some refractory patients received monoclonal antibodies to the IL-6 receptor (tocilizumab), IL-1 receptor antagonist (anakinra), monoclonal antibodies to tumor necrosis factor (infliximab), or convalescent plasma therapy. One of 3 MIS-C patients in Korea had a persistent fever after the second dose of IVIG and intravenous corticosteroid treatment, and eventually received anakinra with resolution of fever [16]. Anakinra has a short half-life and quick onset of action, allowing rapid discontinuation of therapy in cases of adverse reactions [47]. Anakinra (>4 mg/kg/day IV or subcutaneous injection) is recommended for patients whose MIS-C is refractory to IVIG and/or corticosteroid treatment. The liver function of children who have received anakinra should be monitored. Tocilizumab (<30 kg: 12 mg/kg IV; ≥30 kg: 8 mg/kg IV; max.: 800 mg) may effectively reduce mortality and the need for intensive care unit admission in patients with severe COVID-19 pneumonia and signs of hyperinflammation. However, patients treated with tocilizumab may be at higher risk of developing bacterial or fungal infections [45]. There is no consensus on which of these agents is optimal, and drug choice may depend on the clinician’s preference, cytokine test results, and drug availability.

Aspirin (for anti-inflammation or prophylaxis of thrombosis) and/or anticoagulants are commonly administered to children with MIS-C [48]. Low-dose aspirin (3–5 mg/kg per oral medication once daily) is recommended in MIS-C patients with KD-like features, coronary aneurysm, or thrombocytosis. Anticoagulation therapy with heparin, enoxaparin, or warfarin is recommended for MIS-C patients with a giant coronary aneurysm (diameter >8 mm) or a coronary artery z score >10. MIS-C Patients with documented thrombosis or ejection fraction <35% need therapeutic doses of enoxaparin for at least 2 weeks after hospital discharge [45].

Hypotension in children with MISC is often resistant to intravenous fluid therapy, so inotropes and/or vasopressors should be used appropriately in such cases. Epinephrine is recommended as the first choice in children, and norepinephrine should be used if shock persists. The use of dobutamine or dopamine is recommended in patients with myocardial dysfunction because of its inotropic effect [49-51]. Other life-sustaining treatments, such as mechanical ventilation, extracorporeal membrane oxygenation (ECMO), and a intra-aortic balloon pump, have been used in some patients admitted to the intensive care unit [35,39]. The empirical use of broad-spectrum antibiotics for possible pneumonia or sepsis has been reported in many studies [35-37,39]. Because of clinical and biochemical similarities of MIS-C with KD, the principle of therapy for KD was adapted to MIS-C and associated with rapid clinical improvement and reduced inflammatory marker levels in most patients. Table 3 summarizes the currently recommended treatments for MIS-C.

According to a systematic review, the duration of hospitalization was 4–13 days (median, 7 days), and intensive care was required in 68% of patients [35]. Inotropic support was required in 40%, mechanical ventilation was required in 15%, and ECMO was required in 2.7% [35]. The fatality rate was reportedly 1.7% in the US and 1.4% in Europe [35]. Among the studies that reported outcomes at discharge [20,39] or during follow-up [41,42], almost all patients with cardiac involvement experienced nearly full recovery of left ventricular function and normalization of cardiac inflammatory markers except for mild cardiac dysfunction observed in 9 patients at discharge in one study [43]. However, the long-term prognosis of MIS-C remains unknown. For example, long-term follow-up is necessary for coronary arterial aneurysm if present. In addition, follow-up studies and long-term cardiac surveillance are needed to monitor cardiac function and coronary arterial abnormalities. Fig. 3 summarizes the overall clinical outcomes of MIS-C.

Hundreds of cases of children and adolescents with MIS-C have been diagnosed during the COVID-19 pandemic. These patients showed clinical features similar to KD or CSS, so MIS-C may be within the spectrum of CSS. Clinicians should suspect MIS-C if patients present with fever, KD-like features (skin rash, conjunctivitis, oral mucosa changes, hand or foot edema), and/or gastrointestinal symptoms (abdominal pain, vomiting, diarrhea) and demonstrate evidence of SARS-CoV-2 infection. Despite accompanying shock or cardiac dysfunction, most MIS-C patients responded well to treatments and recovered without sequelae if adequate treatments were given. The fatality rate was approximately 1.5%–2%. As the COVID-19 pandemic might persist for a long time, we will likely see more MIS-C patients. In Korea, the KDCA constructed a surveillance system for MIS-C through a consultative process with academic societies and clinicians, and we need to be continuously vigilant about ensuring the early diagnosis and treatment of patients with MIS-C.