Multisystem inflammatory syndrome in children (MIS-C): a mini-review

Multisystem inflammatory syndrome in children (MIS-C) is a novel, life-threatening hyperinflammatory condition that develops in children a few weeks after infection with severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). This disease has created a diagnostic challenge due to overlap with Kawasaki disease (KD) and KD shock syndrome. The majority of patients with MIS-C present with the involvement of at least four organ systems, and all have evidence of a marked inflammatory state. Most patients show an increase in the level of at least four inflammatory markers (C-reactive protein, neutrophil count, ferritin, procalcitonin, fibrinogen, interleukin-6, and triglycerides). Therapy is primarily with immunomodulators, suggesting that the disease is driven by post-infectious immune dysregulation. Most patients, even those with severe cardiovascular involvement, recover without sequelae. Since coronary aneurysms have been reported, echocardiographic follow-up is needed. Further study is needed to create uniform diagnostic criteria, therapy, and follow-up protocols.


Background
A large percentage of children infected with the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) are asymptomatic or mildly symptomatic [1].
In April 2020, a novel life-threatening hyperinflammatory condition named multisystem inflammatory syndrome in children (MIS-C) as a complication of SARS-CoV-2 in children was first recognized [2][3][4].

Case definition (May 2020)
The European and US Centers for Disease Prevention and Control (CDC) criteria for the diagnosis of MIS-C are as follows: age < 21 years, fever ≥ 1 day, laboratory evidence of inflammation, and multisystem (≥ 2) organ involvement (cardiac, renal, respiratory, hematologic, gastrointestinal, dermatologic, or neurological) requiring hospitalization; no alternative plausible diagnoses; and positive for SARS-CoV-2 infection by RT-PCR, serology, or antigen test or exposure to suspected or confirmed SARS-CoV-2 infection within the 4 weeks prior to the onset of symptoms [4]. The World Health Organization (WHO) criteria are somehow different: 0-19 years of age with fever ≥ 3 days, multisystem (≥ 2) organ involvement (mucocutaneous, hypotension/shock, cardiac, gastrointestinal), and no microbial cause of inflammation, including bacterial sepsis, staphylococcal, or streptococcal shock syndromes. The SARS-CoV-2 epidemiologic link is based on RT-PCR, antigen test, or serology positive or likely contact with patients with SARS-CoV-2 [10].
Cardiac biomarkers (troponin, brain natriuretic peptide [BNP], or pro-brain BNP [proBNP]) are elevated in a large proportion of patients (64-95% and 73-95%) and indicate cardiac involvement [3,7,9,11]. Cardiac and inflammatory biomarkers not only suggest a diagnosis of MIS-C but may also reflect the severity of illness [9,11]. A recent study suggests that high ED levels of proBNP can serve as early warning indicators for the requirement of inotropic/vasoactive support in MIS-C [14]. Point-ofcare ultrasound or complete echocardiography may be useful to promptly recognize patients with cardiac dysfunction and to adjust therapy or to evaluate for alternate diagnoses [3,9]. Other imaging tests should be guided by clinical judgment, and repeating the laboratory evaluation during the course of illness may be useful to identify patients at risk for deterioration.

Comparison of MIS-C and similar syndromes
The clinical overlap of SARS-CoV-2, MIS-C, and Kawasaki disease (KD) creates a formidable diagnostic challenge.

MIS-C and SARS-CoV-2
Patients with MIS-C, compared to SARS-CoV-2 patients, are younger and less likely to have one or more underling medical conditions. Presenting symptoms and signs are similar among the two groups with the exception of mucocutaneous findings. Patients with MIS-C are more likely to have both cardiovascular and mucocutaneous involvement, and the majority test positive for SARS-CoV-2 antibody. MIS-C patients have a higher median neutrophil-to-lymphocyte ratio and higher CRP and more frequently have thrombocytopenia [5][6][7][8][9][11][12][13].

MIS-C, atypical KD, and KD shock syndrome (KDSS)
KD generally presents in children less than 8 years old and is more common in Asians [6]. MIS-C presents with a greater variety of signs and symptoms and is more often characterized by gastrointestinal and neurologic symptoms. Children with MIS-C have lower absolute lymphocyte and platelet counts, higher ferritin and Ddimer levels, and a higher likelihood of having elevated troponin or ProBNP levels [7].
The typical cardiac features in the two syndromes differ: MIS-C more typically presents with ventricular dysfunction and shock (more than 50% in MIS-C versus 5-10% in KD) [5]. The development of coronary artery aneurysms in both disorders may increase diagnostic uncertainty [7].

Treatment
Due to lack of evidence, published guidelines are based on expert consensus.
Therapy for MIS-C is primarily with immunomodulators, suggesting that the disease is driven by postinfectious immune dysregulation [2]. Many children were successfully treated with high dose intravenous gamma globulins (IVIG) alone or combined with corticosteroid. These regimens were adopted from KD guidelines, and combining IVIG with corticosteroid led to a lower rate of treatment failure [5,7]. Treatment with IVIG and methylprednisolone vs IVIG alone was associated with a more favorable fever course [15]. Additional first-line therapies included antiplatelet medications (58-74%) and anticoagulants (37-44%) [5,[7][8][9].

Prognosis and follow-up
The majority of MIS-C patients, even after critical care and with severe cardiovascular involvement, recovered without sequelae (70-97%) [5,9,11,15]. Since coronary aneurysms have been reported to develop in the convalescent phase of illness, echocardiographic follow-up is needed, even in patients with no cardiac involvement in the acute phase of illness [7]. Until more is known about long-term morbidity, it is reasonable to use KD guidelines to guide outpatient follow-up [5,7].

Conclusion
MIS-C, a SARS-CoV-2-related condition, is a novel pediatric syndrome characterized by the presence of fever, elevated inflammatory markers, and multi-organ involvement. Clinical characteristics are shared to some extent with KD, and therapy is guided by KD guidelines. Despite a high level of morbidity, the majority of patients experience a good outcome. As this is a new pediatric syndrome, much is yet to be learned about long-term prognosis, and further research is needed to create uniform diagnostic criteria and to optimize therapy protocols.