Multisystem Inflammatory Syndrome in Neonates Born to Mothers with SARS-CoV-2 Infection (MIS-N) and in Neonates and Infants Younger Than 6 Months with Acquired COVID-19 (MIS-C): A Systematic Review

(1) Introduction: There is an increasing literature describing neonates born to mothers with SARS-CoV-2 infection (MIS-N) and infants infected with SARS-CoV-2 who presented with a severe disease (MIS-C). (2) Methods: To investigate clinical features of multisystem inflammatory syndrome in neonates and infants under six months of age, we used a systematic search to retrieve all relevant publications in the field. We screened in PubMed, EMBASE and Scopus for data published until 10 October 2021. (3) Results: Forty-eight articles were considered, including 29 case reports, six case series and 13 cohort studies. Regarding clinical features, only 18.2% of MIS-N neonates presented with fever; differently from older children with MIS-C, in which gastrointestinal symptoms were the most common manifestation, we displayed that cardiovascular dysfunction and respiratory distress are the prevalent findings both in neonates with MIS-N and in neonates/infants with MIS-C. (4) Conclusions: We suggest that all infants with suspected inflammatory disease should undergo echocardiography, due to the possibility of myocardial dysfunction and damage to the coronary arteries observed both in neonates with MIS-N and in neonates/infants with MIS-C. Moreover, we also summarize how they were treated and provide a therapeutic algorithm to suggest best management of these fragile infants.


Introduction
The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) can infect all age groups. To date, children seem to have a more favourable clinical course of the related disease COVID-19, as observed during the last two years of pandemic [1]. Neonatal SARS-CoV-2 infections seem less frequent even now [2], probably due to the lower expression of SARS-CoV-2 entry receptors in nasal epithelium in both term and preterm neonates, compared with adults [3]. Neonatal cases are mostly linked to horizontal transmission due to familial clusters [4], although the rare possibility of a maternal fetal transmission has been demonstrated by Vivanti et al. [5].
However, there is an increasing literature describing SARS-CoV-2 infected children who become critically sick [6] because of the onset of a multisystem inflammatory syndrome named MIS-C. MIS-C has emerged as a significant COVID-19 related consequence, In this review we conducted a thorough analysis and synthesis of previously described cases of MIS-N and MIS-C in neonates and infants, aiming to describe clinical features of multisystem inflammatory syndrome in these delicate patients. Moreover, we also summarize how these infants were treated, in order to provide preliminary suggestions for management in this new clinical scenario.

Materials and Methods
We performed this systematic review following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines throughout the whole project. Prior to commencing the search, a detailed protocol was agreed to determine search modalities, eligibility criteria, and all methodological details. We searched for cohort, cross-sectional and case-control studies, as well as case series or case reports published as articles or letters to the editors describing neonates with multisystem inflammatory syndrome following maternal SARS-CoV-2 infection (MIS-N) or neonates In this review we conducted a thorough analysis and synthesis of previously described cases of MIS-N and MIS-C in neonates and infants, aiming to describe clinical features of multisystem inflammatory syndrome in these delicate patients. Moreover, we also summarize how these infants were treated, in order to provide preliminary suggestions for management in this new clinical scenario.

Materials and Methods
We performed this systematic review following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines throughout the whole project. Prior to commencing the search, a detailed protocol was agreed to determine search modalities, eligibility criteria, and all methodological details. We searched for cohort, cross-sectional and case-control studies, as well as case series or case reports published as articles or letters to the editors describing neonates with multisystem inflammatory syndrome following maternal SARS-CoV-2 infection (MIS-N) or neonates and infants within first six months of life infected with SARS-CoV-2 and with MIS-C features.
We conducted an extensive search of the following databases (accessed on 10 October 2021): PubMed (https://pubmed.ncbi.nlm.nih.gov/), Scopus (https://www.scopus.com/ search/form.uri?display=basic#basic) and Embase (https://www.embase.com/?phase= continueToApp#search). We used the following terms: (("pediatric" AND "multisystem" AND "inflammatory" AND "syndrome") OR ("multisystem" AND "inflammatory" AND "syndrome" AND "in" AND "children")) AND "SARS-CoV-2". Additional studies were identified by authors based on their knowledge in the field, if not already included by literature search. We excluded (1) all retrieved articles written in non-English language; (2) articles which did not clearly report the number of infants under six months of age.
Articles were assessed by independent researchers (DUDR, FP, MC, SR, SC, CM, LM and AS). Investigators evaluated abstracts and (where necessary) the full text of each article, excluding those not meeting the eligibility criteria, and removing duplicates. The CARE (Consensus-based Clinical Case Reporting Guideline Development) recommendations, specifically dedicated to case reports and series, were followed during the evaluation process. If an article was eligible but reported data on neonates and infants mixed with those on older children, eligible data were directly extracted. If there were uncertainties, they were resolved by discussion between the independent researchers and, if no agreement was reached, with the senior researcher (CA). We developed a dedicated online data extraction sheet (Excel 16: Microsoft Corporation, Redmond, WA, USA). Data from included records were independently extracted by each investigator using this data extraction sheet and then cross-verified.
Since we expected the majority of analyzed articles to be case reports or case series, we used the Mayo Evidence-Based Practice Centre tool that is specifically dedicated to the evaluation of case report/series quality [12]. Two investigators (DUDR and CA) independently summarized the results of this evaluation by aggregating the eight binary responses into a 0-8 score. Evaluation results were also qualitatively summarized (Low-Intermediate-Good), as recommended by the tool creators. If discrepancies or uncertainties persisted, they were resolved by discussion between the two researchers (DUDR and CA). We performed calculations and statistics with Excel 16 (Microsoft Corporation, Redmond, WA, USA), reporting continuous data as median (interquartile range, IQR), and categorical data as numbers and percentages. Figure 2 shows the study flow chart with included and excluded items (and the reasons for their exclusions). Finally, 48 articles were considered, consisting of 29 case reports [5,, six case series [11,[41][42][43][44][45] and 13 cohort studies [34,[46][47][48][49][50][51][52][53][54][55][56][57].

Workflow of Review and Synthesis
We describe in Table 1 the characteristics of the included papers: we assessed as intermediate (median score 5 (5,6)) the methodological quality of case reports, case series and cohort studies which provided supplemental data with case descriptions. Ninety cases of neonates and infants under 6 months of age with multisystem inflammatory syndrome were described in the literature at the time of the search.
Chest X-ray or chest CT discovered a pulmonary involvement in 12 neonates (36.4%), whereas echocardiography showed depressed ventricular functions and coronary anomalies in 17 (51.5%).
Twenty-seven neonates (81.8%) received intravenous immunoglobulins (IVIG: a total of 2 g/kg splitted in two infusions of 1 g/kg/day, without a further dose) and intravenous steroids (mostly methylprednisolone). Eighteen neonates (54.5%) needed inotropic support. None was treated with biologic medications or antivirals. The use of large-spectrum antibiotics was described in 10/13 cases (76.9%). Acetylsalicylic acid was given to 6 neonates (18.2%) while thromboprophylaxis was used in 13 cases (39.4%).        The median length of stay was 16 [13][14][15][16][17][18][19][20][21][22] days. The outcome was favourable in 30 neonates (90.9%); of these, a full-term male without comorbidities survived but required amputation of the right leg (because of an acute thrombosis of abdominal aorta) and a late preterm survived after peritoneal dialysis. Only two neonates died (the first on day 8 due to a multi-organ dysfunction and the second on day 11 because of necrotizing enterocolitis).
Eight infants (25%) presented pulmonary involvement by chest X-ray and/or CT scan, whereas depressed ventricular function and coronaries anomalies were detected by echocardiography in 21 (65.6%).
The use of large-spectrum antibiotics was described in 17 cases (53.1%). Acetylsalicylic acid was given to 14 infants (43.8%); thromboprophylaxis was used in 14 cases (43.8%) and warfarin was given to one infant (3.1%).
The median length of stay was 15 (10-24) days. The outcome was favourable in 29 infants (90.6%), whereas four infants (12.5%) died: two neonate females within one month of life without comorbidities, a 2-months-old male with a familial hemophagocytic lymphohistiocytosis secondary to Griscelli syndrome type 2, and a 3-months-old baby with isolated coronary artery disease.

Differences between Neonates with MIS-N and Neonates with MIS-C
When SARS-CoV-2 infection was acquired postnatally, fever was a frequent feature (60%). rather than in neonates with MIS-N (18.2%). All neonates had a severe course, requiring NICU admission. Organ system involvement was similar for both neonate groups, with prevalent cardiovascular and respiratory signs and symptoms. We found no significant differences between the two groups for laboratory tests and treatments.
Two neonates died in both groups, with a slightly higher incidence in the MIS-C group although not significantly (2/33 MIS-N neonates versus 2/5 MIS-C neonates, p = 0.07).
Dufort [51] 191 1 A neonate with MIS-N, born to a positive mother (asymptomatic at delivery) who presented with fever and left breast cellulitis between 14 and 28 days of age. Laboratory work-up showed increasing troponin levels; echocardiogram showed good ventricular function and unremarkable coronary arteries. Two molecular tests for SARS-CoV-2 were negative. The discharge diagnoses were cellulitis, myocarditis, and shock.
The specific clinical characteristics of infants < 6 months of age are not described.
Grewal [55] 92 9 Nine infants under the age of 6 months, none with acute kidney injury. A 1-month-old girl, followed up with aortic coarctation, resulted to be positive for SARS-CoV-2 and had fever and poor feeding. A 3-months-old boy, followed up with ventricular septal defect and pulmonary hypertension, had fever, diarrhoea (leading to a severe dehydration and lack of urine). A 6-days-old boy, who applied with fever, vomiting, decreased feeding and respiratory distress for 2 days. It was learned that his aunt and grandfather had been diagnosed with COVID-19 and they had loved and cared for the baby.

Discussion
This systematic review is the first to synthesize the current data regarding MIS-N in neonates and MIS-C in neonates and infants younger than six months of age. The literature relating to multisystem inflammatory syndrome in these patients is mainly centred on case reports or small case series: we summarized the most frequent clinical features to describe this syndrome in neonates and provide some therapeutic suggestions.
Regarding clinical features, fever is a milestone in older children with MIS-C (99.3%) [59]: we confirmed this trend in young infants with MIS-C (84.4%). Conversely, only the 18.2% of neonates with MIS-N presented with fever, but temperature changes are not a constant finding in the onset of infectious and even in the inflammatory febrile pathologies in preterm and term neonates [60,61]. Therefore, differences in age of MIS-N and MIS-C infants could explain this disparity in fever incidence. Differently from older children with MIS-C, in which gastrointestinal symptoms were the most common manifestation (87.3%) [62], we demonstrated that cardiovascular dysfunction and respiratory distress are the prevalent findings both in neonates with MIS-N and in neonates/infants with MIS-C (Figure 3).
From the diagnostic point of view (Figure 4), although the current diagnostic reference standard for neonatal respiratory distress includes chest X-ray [63], only a third of patients with MIS-N or MIS-C showed pathologic chest X-ray. Recently, Musolino et al. suggested performing lung ultrasound (LUS) in patients with clinical suspicion of MIS-C, or without a certain diagnosis: the finding of many B-lines and pleural effusion would support the diagnosis of a systemic inflammatory disease [64]. Moreover, LUS can also reduce X-ray exposure, but its efficacy as diagnostic tool has been described only in a 6-months-old infant with MIS-C: the authors found an irregular pleural line, B-lines, with bilateral patchy distribution, and small peripheral consolidations [38]. pathologies in preterm and term neonates [60,61]. Therefore, differences in age of MIS-N and MIS-C infants could explain this disparity in fever incidence. Differently from older children with MIS-C, in which gastrointestinal symptoms were the most common manifestation (87.3%) [62], we demonstrated that cardiovascular dysfunction and respiratory distress are the prevalent findings both in neonates with MIS-N and in neonates/infants with MIS-C ( Figure 3). From the diagnostic point of view (Figure 4), although the current diagnostic reference standard for neonatal respiratory distress includes chest X-ray [63], only a third of patients with MIS-N or MIS-C showed pathologic chest X-Ray. Recently, Musolino et al. suggested performing lung ultrasound (LUS) in patients with clinical suspicion of MIS-C, or without a certain diagnosis: the finding of many B-lines and pleural effusion would support the diagnosis of a systemic inflammatory disease [64]. Moreover, LUS can also reduce X-ray exposure, but its efficacy as diagnostic tool has been described only in a 6months-old infant with MIS-C: the authors found an irregular pleural line, B-lines, with bilateral patchy distribution, and small peripheral consolidations [38].  All infants with suspected inflammatory disease should undergo echocardiography, due to the possibility of myocardial dysfunction and damage to the coronary arteries observed both in neonates with MIS-N [11,19,25,26,29,33,38,39] and in neonates/infants with MIS-C [13,18,22,24,28,32,[35][36][37][40][41][42][43]45,47,52,53]. Indeed, functional echocardiography can provide a direct bed-side assessment of cardiovascular anomalies and hemodynamics [65]. We suggest performing echocardiography in neonates with MIS-N after 24-48 h of life (or previously in case of symptoms), considering hemodynamic changes that physiologically occur during transition from fetal to neonatal life, while in MIS-C infants echocardiography should be carried on admission. Furthermore, neonates with MIS-N had a higher need of inotropic support (54.5%) than infants with MIS-C (15.6%); targeted echocardiography also offers the advantage of longitudinally assessing infants and their re- All infants with suspected inflammatory disease should undergo echocardiography, due to the possibility of myocardial dysfunction and damage to the coronary arteries observed both in neonates with MIS-N [11,19,25,26,29,33,38,39] and in neonates/infants with MIS-C [13,18,22,24,28,32,[35][36][37][40][41][42][43]45,47,52,53]. Indeed, functional echocardiography can provide a direct bed-side assessment of cardiovascular anomalies and hemodynamics [65]. We suggest performing echocardiography in neonates with MIS-N after 24-48 h of life (or previously in case of symptoms), considering hemodynamic changes that physiologically occur during transition from fetal to neonatal life, while in MIS-C infants echocardiography should be carried on admission. Furthermore, neonates with MIS-N had a higher need of inotropic support (54.5%) than infants with MIS-C (15.6%); targeted echocardiography also offers the advantage of longitudinally assessing infants and their response to therapeutic intervention [66]. The increase in cardiac enzymes (troponin) and cardiac function-related proteins (NT-proBNP) must induce a strong suspicion of myocardial involvement and requires careful monitoring [67]. Additionally, the presence of microvascular dysfunction has recently been characterized in pediatric patients with SARS-CoV-2 pneumonia [68].
Inflammatory markers could be also raised, but we identified that neonates with MIS-N had lower levels than neonates/infants with MIS-C. Similarly, Zhao et al. assessed that younger children with MIS-C had lower levels of inflammatory markers when compared to middle-age children and adolescents with MIS-C [69].
Moreover, we also tried to summarize how these infants were treated, in order to provide preliminary suggestions in management of this new clinical scenario ( Figure 5), although data did not come from randomized studies [70]. The best treatment approach and the appropriate timing should be defined on an individual basis, as suggested by Cattalini et al. [71]. MIS-C and MIS-N seem to originate from immune-mediated mechanisms, in the setting of a suspected or confirmed SARS-CoV-2 infection: however, in MIS-N the primary source is the maternal infection in pregnancy, with transplacental passage of maternal antibodies [62], whereas in MIS-C the infant contracts a postnatal infection and mounts an antibody response with intact neutralization capability [72]. The efficacy of intravenous immunoglobulins (IVIG) in MIS-C, a common approach to activating inhibitory Fc-receptors and preventing membrane-attack complexes by complement factors, thereby mitigating autoantibody-mediated pathology [73], lends support to the hypothesis that autoantibodies contribute to MIS-C pathogenesis [74].  MIS-C and MIS-N seem to originate from immune-mediated mechanisms, in the setting of a suspected or confirmed SARS-CoV-2 infection: however, in MIS-N the primary source is the maternal infection in pregnancy, with transplacental passage of maternal antibodies [62], whereas in MIS-C the infant contracts a postnatal infection and mounts an antibody response with intact neutralization capability [72]. The efficacy of intravenous immunoglobulins (IVIG) in MIS-C, a common approach to activating inhibitory Fc-receptors and preventing membrane-attack complexes by complement factors, thereby mitigating autoantibody-mediated pathology [73], lends support to the hypothesis that autoantibodies contribute to MIS-C pathogenesis [74]. The majority of infants described in the literature received IVIG [11,13,14,19,20,24,25,27,28,34,35,[37][38][39][40][42][43][44][45]47,52,53] and steroids (mostly methylprednisolone) [11,14,15,18,22,25,26,28,31,32,34,[36][37][38][39][40][41][42]44,45,52]. According to results of a French study, the treatment with IVIG and methylprednisolone vs. IVIG alone was associated with a more favourable course among children with MIS-C [18,22,45]. However, data interpretation is limited by the observational design of the study [75].
Most neonates and infants were also treated with large-spectrum antibiotics, given the overlapping clinical signs and symptoms with those of sepsis. Recently, Yock-Corrales and colleagues set the alarm regarding the high rate of antibiotic prescriptions (24.5%) in children with COVID-19 (in particular in those with more severe forms) [76]. Considering the high need for ICU admission in neonates with MIS-N (100%) and infants with MIS-C (40.6%) described in the included studies of this review, and given that an antibiotic treatment is often guaranteed in younger infants, the length of antibiotic therapy should be carefully evaluated: reducing patient exposure to large-spectrum antibiotics can result in avoiding the spread of multi-drug resistant organisms [77]. The time to positivity of blood cultures performed on admission could guide decisions on antibiotics administration in neonates, because most bacterial pathogens grow within 48 h [78,79].
Although the outcome was favourable in the majority of cases, the observed mortality was 9.2% in neonates with MIS-N and infants younger than under six months with MIS-C. Conversely, the reported mortality rate was 1.9% when all pediatric MIS-C cases reported in the literature were considered [59]. This is an expected finding, considering the higher cardiovascular and respiratory involvement in younger infants.
This study had several limitations. First, the available studies were mostly case reports or case series, with possible selection biases. Second, data on some variables were not accessible in all papers or were not uniformly reported. Third, there is no consensus about diagnostic criteria in MIS-C, whereas recently Pawar et al. proposed modified criteria for MIS-N [11]. Fourth, most of these data have been obtained in the "pre-Omicron" period. Indeed, since late November 2021, with the emergence of the new SARS-CoV-2 variant B.1.1.529 (named Omicron), the number of COVID-19 cases increased substantially. Although the symptoms of the new cases are reported to be mild to moderate [80], we do not know what the actual impact will be in younger infants, for whom a vaccine is not yet available.

Conclusions
Multisystem inflammatory syndrome, named MIS-N or MIS-C, related to SARS-CoV-2 exposure can occur in a high percentage of neonates and infants. In affected neonates common findings are cardiac dysfunction and the coronary artery dilation or aneurysms; thus, a complete echocardiography is strongly recommended in the diagnostic approach.
The studies that we have consulted reported an overall good prognosis, despite the frequent need of NICU and ICU admissions.
Further epidemiological, clinical, immunological, and neurodevelopmental studies are needed to better clarify short-and long-term outcomes of neonates and infants with this inflammatory condition related to SARS-CoV-2 exposure.