Distinct characteristics of multisystem inflammatory syndrome in children in Poland

During the winter months of 2020/2021 a wave of multisystem inflammatory syndrome in children (MIS-C) emerged in Poland. We present the results of a nationwide register aiming to capture and characterise MIS-C with a focus on severity determinants. The first MIS-C wave in Poland was notably high, hence our analysis involved 274 children. The group was 62.8% boys, with a median age of 8.8 years. Besides one Asian, all were White. Overall, the disease course was not as severe as in previous reports, however. Pediatric intensive care treatment was required for merely 23 (8.4%) of children, who were older and exhibited a distinguished clinical picture at hospital admission. We have also identified sex-dependent differences; teenage boys more often had cardiac involvement (decreased ejection fraction in 25.9% vs. 14.7%) and fulfilled macrophage activation syndrome definition (31.0% vs. 15.2%). Among all boys, those hospitalized in pediatric intensive care unit were significantly older (median 11.2 vs. 9.1 years). Henceforth, while ethnicity and sex may affect MIS-C phenotype, management protocols might be not universally applicable, and should rather be adjusted to the specific population.


SARS-CoV-2 tests and anamnesis
Contact with a confirmed COVID- 19  www.nature.com/scientificreports/ Clinical presentation. The median time between first symptoms and hospital admission was 5 days, and the median fever length was 7 days. The vital signs at admission and at their respective peaks are presented in Table 2 and Supplementary Table 5. Complete clinical characteristics of children with MIS-C are presented in Table 3 and Supplemental Table 4. Mucocutaneous and lymph node involvement was observed in 95.6% of children, and 66.7% fulfilled American Heart Association (AHA) KD or atypical KD (aKD) diagnostic criteria, irrespective of age. Rash was less common in children >12 years of age (71.2% vs. 86.4%, p < 0.01), whereas conjunctival injection, distal extremity changes and cervical lymphadenopathy were equally prevalent across age groups. The second most common group of associating symptoms was gastrointestinal (92.6%), with no significant differences across age groups, except for nausea and vomiting, which were more prevalent in children 5-12 years of age (74.6% vs. 48.8%, p < 0.01). Ten children underwent abdominal surgery due to acute abdominal symptoms.
Hypotension was present in 30/213 (14.1%) of patients at admission, while 99/243 (40.7%) developed it at some point during hospitalization. At least one echocardiogram was reported for 255 children, of whom 85 (33.3%) had any of the following abnormalities: decreased ejection fraction (EF), coronary artery abnormality (CAA) or pericardial effusion. EF < 55% was reported in 58/255 (22.7%) of children (four had EF < 35%), and the age of children with heart dysfunction overall was significantly higher (median [IQR] from 7.9 [4.9-11.3] to 11.0 years [8.8-13.6], p < 0.01). CAA developed in 21/255 (8.2%) of patients irrespective of age or KD or aKD phenotype. In particular, eight children developed coronary artery aneurysms; three of them resolved in the follow up before discharge. Pericardial effusion was present in 24/255 (9.4%) of reports.
Neurological Laboratory results. Laboratory parameters at admission and at respective peaks are summarized in Sex-dependent clinical and laboratory characteristics. Male patients were diagnozed with MIS-C more often than expected from demographic structure, but only in the older age bracket (Fig. 2). We have identified some characteristics which corresponded with this discrepancy (Fig. 3, Table 3).
On the other hand, teenage girls more frequently presented osteoarticular and muscular symptoms (12.6% vs. 2.9%, p < 0.01), but less frequently rash (5.0% vs. 15.2%, p=0.01). KD/aKD phenotype prevalence did not differ between reported girls and boys of any age.
Pediatric intensive care treatment. Statistics regarding PICU treatment are presented in Tables 4 and 5. PICU treatment was required in 23/274 (8.4%) of children, ten of whom were mechanically ventilated. There were no children treated with extracorporeal membrane oxygenation (ECMO), neither with renal replacement therapy. Two deaths were reported: one in a severely immunocompromised child, and one in previously healthy teenager with fulminant multiorgan dysfunction, both with positive real-time polymerase chain reaction (RT-PCR) test result for SARS-CoV-2. In either case, it was impossible to determine whether the cause of death was a cytokine storm due to COVID-19 or MIS-C; they fulfilled the MIS-C criteria though, and hence were included in this analysis.
Children who were hospitalised in PICU were significantly older (median [ www.nature.com/scientificreports/ Table 2. Vital signs and laboratory results of MIS-C cohort at admission and at respective peaks. Binary data given as count (per-cent), and numerical data as median (interquartile range). Values at admission are marked with adm, lowest obtained with min, while highest with max. Troponin is considered elevated at > 50 ng/l, while BNP/NT-proBNP at > 150 ng/ml. AlAT, alanine transaminase, AST, aspartate transaminase, AVPU AVPU scale, BNP/NT-proBNP brain natriuretic peptide or N-terminal-pro-BNP, CK creatinine kinase, CRT capillary refill time, CRP, C-reactive protein, eGFR estimated glomerular filtration rate, ESR, erythrocyte sedimentation rate, MIS-C multisystem inflammatory syndrome in children, NT-proBNP N-terminal prohormone of brain natriuretic peptide, PICU, pediatric intensive care unit, SBP systolic blood pressure, WBC white blood cell count, P|Age age-adjusted p-value, P|Sex sex-adjusted p-value.        www.nature.com/scientificreports/ Moreover, Payne estimated the incidence of MIS-C as approximately 9-fold higher among Black and Hispanic or Latino Americans than among White Americans in general population and this trend sustained (with slightly lower rates) among SARS-CoV-2 infected children 14 . In this context, the high number of cases captured in Poland is even more surprising, as all but one child were White.

Discussion
Age and sex-related differences in MIS-C presentation. The mucocutaneous, gastrointestinal or respiratory manifestations and laboratory picture in our cohort were more uniform across age groups than in others [4][5][6][8][9][10]15,16 . Cardiovascular involvement significantly increased with age however, which is in line with Dufort and Belay findings 10,17 . In opposition to Payne 14 we found sex-related differences in MIS-C prevalencehigher for boys, but only from pubertal age. Teenage boys also more commonly had cardiac involvement, fulfilled MAS definition and required PICU hospitalisation. More severe course of COVID-19 in adult males is well established 12 . While some authors postulate that it is linked to genetic and immunological background 18 , others suggest that sex hormones play a role 19,20 . COVID-19 and MIS-C are separate entities, but share some similarities being hyperinflammatory conditions. In our cohort, the sex-related differences appeared from pubertal age, which might support the hormonal theory. www.nature.com/scientificreports/ Kawasaki-like disease as the most common presentation of MIS-C. The exact pre-pandemic incidence of KD in Poland was not known (there had been no national reporting effort before the pandemic), though it was not likely to substantially differ from other European countries 21 . Similarly as in other countries 7,22,23 , we have observed a sharp increase in KD cases following COVID-19 wave. The data limited to two reporting sites supporting these observations are presented in Supplementary Table 6. Despite being initially reported as Kawasaki-like disease, MIS-C appeared to be a distinct entity soon after 8,9,[23][24][25] . Children in our cohort, more frequently than in others, fulfilled KD/aKD diagnostic criteria but concomitantly presented unique features typical for MIS-C 7,15-17,26,27 . More prevalent KD/aKD phenotype in our cohort could be explained in several ways. Firstly, we have included both: typical and atypical KD presentations. In previous studies which included both clinical variants, KD prevalence was two times more common than in those in which only typical KD definition was used [15][16][17]27 . Moreover, the comparison of patients with KD from pre-pandemic and pandemic periods in Spain revealed more www.nature.com/scientificreports/ frequent atypical presentation (71%) among patients with documented SARS-CoV-2 history 23 . Secondly, KD/ aKD phenotype could have been over-represented due to our inclusion criteria and the fact, that the cases were identified and reported by pediatricians, who were more familiar with recognising KD than either TSS, MAS or new inflammatory syndrome. Thirdly, once in the database, the fulfillment of the KD/aKD criteria was verified by the dedicated software, not by clinicians. Thus, we captured cases of atypical presentation that might had been overlooked if identified by clinician's diagnosis only (as in other reports 10,11,[15][16][17]. Fourthly, some distinct clinical features of our cohort e.g., more prevalent KD phenotype, could be due to specific, homogeneous ethnic background, differing from all other cohorts 10,11,16,26,27 . Clinical phenotype of MIS-C has not been analyzed in relation to ethnicity/race thus far and needs to be further studied. www.nature.com/scientificreports/

Milder course of MIS-C in Poland.
In Western Europe and the USA more than half of children with MIS-C required intensive care 10,11,[15][16][17]27 , contributing to a multifold increase in the PICU admissions number 7 . Approximately 1.4-3% of children with MIS-C died 7,10,16,28 . In our study, the course of the disease appeared substantially milder-only 8.4% of patients were hospitalised in PICU and two deaths were reported. Treatment used in our cohort did not differ substantially from other reports-most children received IVIG and a large proportion also got steroids. Fewer children required more than two different immunomodulatory agents [4][5][6][7][8][9][15][16][17]27,29 . Moreover, the median day of hospital admission since the first symptoms was similar to other reports 6,17 . Hence, the therapeutic approach is an unlikely factor responsible for more favourable outcome among Polish children with MIS-C. The data about cardiovascular complications from previous reports are inconsistent. This is partially due to different (sometimes unspecified) definitions used by authors 4,6,17,30 , and various inclusion criteria-either broader than World Health Organization (WHO) MIS-C case definition 30 , or narrowed only to the most severe cases 7 . Decreased SBP, occasionally defined as a shock, was reported in 36-86% of patients with MIS-C, whereas heart failure-in 20-45% 4-8,10,30 . Our findings place Polish children with MIS-C within the "milder end" of the acute cardiovascular complications spectrum described above. Similarly, the prevalence of coronary artery involvement in MIS-C is debatable. Undoubtedly aneurysms may appear, cases of giant aneurysms have been described 30 . The true prevalence of CAA is unknown though and may be over-estimated, as febrile condition or myocarditis can cause transient coronary dilation too 31 . Both coronary artery aneurysms and dilations were less prevalent in our group than in other reports [4][5][6][8][9][10]30 .
It is not established whether race/ethnicity is associated with the severity of the disease 4,6,32 . Non-Hispanic White children comprised only 13-30% of cases in the most numerous MIS-C cohorts and systematic reviews [4][5][6][8][9][10]14 . Predominance of White children in our cohort could be considered as a possible explanation of milder clinical presentation with more favourable outcome. However, due to the lack of a control group of other ethnicities in our study, this conclusion should be treated with caution and requires further analysis.
Another distinguishing feature of our cohort was the small proportion of obese children as compared to reports from other countries (6.7% vs. 18-26%) 4,6,9 . This could have possibly resulted from lower obesity prevalence in Polish children (up to 13%) 33 . It is unknown whether obesity is a risk factor for developing MIS-C nor if it is connected to its severity. In our study, we found no association of body mass index (BMI) Z-score or obesity with severity of the disease. Severity predictors. We aimed to identify clinical and laboratory features specific to patients who required intensive care. Older age (in line with other reports 4,6 ) and male sex were the only demographic characteristics associated with PICU admission. The median time from the first symptoms to hospital admission did not differ significantly for PICU patients. They could be distinguished already at admission by their vital signs: decreased level of consciousness, longer CRT, higher respiratory rate and lower SBP and by the laboratory results. Apart from previously established high inflammatory markers 6,8 , high D-dimers, low eGFR, and presence of the heart injury markers turned out to be severity predictors.

Limitations.
We have noted the following limitations of the presented work. The study relied on voluntary participation, hence a number of MIS-C cases might have been missed or biased by non-random sampling. Some patients meeting the MIS-C criteria may have been misclassified, e.g. due to unequal access to SARS- Table 5. Therapy and outcome of MIS-C cohort respective of age. Binary data given as count (per-cent), and numerical data as median (interquartile range). ASA acetylsalicylic acid, GCS glucocorticoids, IVIG intravenous immunoglobulin, y.o. years old, P|Sex sex-adjusted p-value. *All signs and symptoms resolved at discharge.  www.nature.com/scientificreports/ CoV-2 testing or missing data. Outliers in our data were verified at source whenever possible. Because of the broad MIS-C case definition and highly prevalent COVID-19 in the society, children with alternative diagnoses and coincidental positive SARS-CoV-2 results could be included in our cohort. Precise epidemiological data on COVID-19 prevalence among age groups in Poland is lacking. We also assumed the risk of contracting the virus in the juvenile population to be homogeneous.

Conclusion
The severity of MIS-C is not as uniform as it seemed based on previous reports. In particular, race/ethnicity, age, and sex may affect MIS-C phenotype. Consequently, management protocols might not be universally applicable, and should rather be adjusted to the specific population. Patients with altered vital signs and higher inflammatory markers, lower eGFR and markers of heart injury at admission or lower lymphocyte count and albumin concentration during hospitalisation have greater risk of deterioration. Anonymised patients' data (demographic, clinical characteristics, laboratory parameters, cardiovascular evaluation results, treatment and outcome data) were extracted from health records and collected through an online questionnaire developed for that purpose. Vital signs and laboratory parameters were obtained at admission and at their respective peaks.

Methods
Here we retrospectively report and analyze data covering the period from 4th March 2020 (when the first COVID-19 case was confirmed in Poland) to 20th February 2021. Nine of the presented cases were included in our previous, cursory report 34 .
MIS-C case definition. We adopted WHO MIS-C case definition 3 , which requires all of the following: • children 0-18 years old with fever lasting at least 3 days; • at least two of the following: -rash or bilateral conjunctivitis, or mucocutaneous inflammation signs; -hypotension defined by a minimal SBP below 70 + 2× age (in years) mmHg or below 90 mmHg for children older than 10 years 35 ; -features of myocardial dysfunction, pericarditis, or CAA, based on echocardiographic findings or elevated B-type natriuretic peptide (BNP)/N-terminal-pro-BNP (NT-proBNP), or troponin; -evidence of coagulopathy (by international normalised ratio (INR) > 1.1, activated partial thromboplastin time > 40 s or D-dimer > 500 g/ml); -acute gastrointestinal problems.
Standardised study definitions. All standardised study definitions and measures, including laboratory and echocardiographic abnormalities, consciousness level and nutritional status are presented in Supplementary  Table 2. They were automatically evaluated by a dedicated software.
Echocardiographic abnormalities. Echocardiography results were categorised based on left ventricular EF and coronary artery measurements whenever available. The worst available EF and the largest coronary Z-scores were included. The echocardiography results were assessed by two independent cardiologists. Dilation was defined as a Z-score between 2 and less than 2.5, while aneurysm as Z-score ≥ 2.5 36 . Hypotension was defined by a minimal SBP below 70 + 2×age (in years) mmHg or below 90 mmHg for children over 10 years old 35 .
Clinical definitions. Diagnostic criteria of KD in its typical and atypical form were adapted from AHA guidelines 36  www.nature.com/scientificreports/ Statistical methods. We described variables in relation to the sum of cases for which the variable was