Urea/Creatinine Ratio’s Correlation with Creatine Kinase Normalization in Pediatric COVID-19 Patients with Myositis: Evaluating Prognostic and Predictive Value

Round 1

Reviewer 1 Report

Methods

Please add in the methods section the criteria to do the diagnosis of myositis.

The methods section is confusing. The paragraph provides a good amount of detail about the study's methodology, but it could indeed benefit from some reorganization for clarity and flow. Here are some suggestions:

Structure: Break the paragraph into several smaller ones, each focusing on a specific aspect of the study. For instance, one paragraph could describe the patient selection criteria, another could detail the diagnostic parameters, and a third could discuss data collection and analysis. In addition, I suggest relocating the sentence pertaining to the local Ethics Committee approval and informed consent to the end of the methods section.

Consistency: Maintain consistent tense and voice throughout. It seems to switch from past to present, which can confuse readers.

Specificity: Be specific where necessary. For example, when you mention "recurrence, complications, treatments, and outcomes," it might help to briefly specify what kinds of each were recorded.

Redundancy: Eliminate any redundant information to make the paragraph more concise.

Statistical analysis

“Categorical variables, such as gender and clinical presentations, were represented as percentages”. Please modify in absolute number (percentages).

Add how do you define statistical significance.

Result

 Line 133. Remove either.

Line 136, please be more clear than “episodes”.

Line 138 you wrote “All patients […] had an uneventful medical history prior to admission”. For this reason, I believe that the sentence “None had previous muscular diseases or episodes of myositis or myolysis.” Could be avoided.

You wrote “There was no elevation in CK-MB or troponin-I (Table 1)” However these data are not present in Table 1. Please check.

The sentence “Patients received intravenous normal saline hydration (1.2–1.5 times daily maintenance fluid)” is uclear.

You wrote “Using cut-offs described by Bohn et al.” Please specify here or in the methods section what are these cut-offs.

Conclusion

I suggest you to reduce a little bit the length of the conclusion.

Table 1.

White Blood Cells (…/uL) should be modified in White Blood Cells (cells/uL).

Please removes the dots for hemoglobin.

Please ad the explanation of UCR

Some sentences are wordy. I suggest revising the manuscript and reformulating some difficult-to-read sentences. One example is in the line 137 you wrote “All patients studied were not vaccinated for SARS-CoV-2”. It would be better write “No patient was vaccinated for SARS-CoV-2”:

Author Response

REVIEWER 1

Methods

Please add in the methods section the criteria to do the diagnosis of myositis.

Reply: Dear Reviewer, thank you for your work. As you suggested, we added a specific section and a flow chart figure about patients’ recruitment.

The methods section is confusing. The paragraph provides a good amount of detail about the study's methodology, but it could indeed benefit from some reorganization for clarity and flow. Here are some suggestions:

Structure: Break the paragraph into several smaller ones, each focusing on a specific aspect of the study. For instance, one paragraph could describe the patient selection criteria, another could detail the diagnostic parameters, and a third could discuss data collection and analysis. In addition, I suggest relocating the sentence pertaining to the local Ethics Committee approval and informed consent to the end of the methods section.

Reply: We divided and shortened the chapter as you kindly suggested, hoping this makes it clearer.

Consistency: Maintain consistent tense and voice throughout. It seems to switch from past to present, which can confuse readers.

Reply: We made an extensive revision of the manuscript fixing what you pointed out.

Specificity: Be specific where necessary. For example, when you mention "recurrence, complications, treatments, and outcomes," it might help to briefly specify what kinds of each were recorded.

Reply: We created a specific paragraph on the primary outcome and the complications that we investigated.

Redundancy: Eliminate any redundant information to make the paragraph more concise.

Reply: We amended it.

Statistical analysis

“Categorical variables, such as gender and clinical presentations, were represented as percentages”. Please modify in absolute number (percentages).

Reply: We amended it.

Add how do you define statistical significance.

Reply: We explained the linear correlation value, and the statistical value of the R square.

Result

Line 133. Remove either.

Reply: We amended.

Line 136, please be more clear than “episodes”.

Reply: We amended.

Line 138 you wrote “All patients […] had an uneventful medical history prior to admission”. For this reason, I believe that the sentence “None had previous muscular diseases or episodes of myositis or myolysis.” Could be avoided.

Reply: We fixed it

You wrote “There was no elevation in CK-MB or troponin-I (Table 1)” However these data are not present in Table 1. Please check.

Reply: We amended.

The sentence “Patients received intravenous normal saline hydration (1.2–1.5 times daily maintenance fluid)” is unclear.

Reply: We reformulated the line

You wrote “Using cut-offs described by Bohn et al.” Please specify here or in the methods section what are these cut-offs.

Reply: We inserted the table along with reference values

Conclusion

I suggest you to reduce a little bit the length of the conclusion.

Reply: We amended.

Table 1.

White Blood Cells (…/uL) should be modified in White Blood Cells (cells/uL).

Please removes the dots for hemoglobin. Please ad the explanation of UCR

Reply: We fixed what you suggested

Comments on the Quality of English Language:

Some sentences are wordy. I suggest revising the manuscript and reformulating some difficult-to-read sentences. One example is in the line 137 you wrote “All patients studied were not vaccinated for SARS-CoV-2”. It would be better write “No patient was vaccinated for SARS-CoV-2”

Reply: We revised the entire manuscript focusing on language with the help of a native english speaker. Thank you for your suggestions.

Reviewer 2 Report

1.       Introduction: It is not very clear if you plead for a covid-19 related myositis or myolisis. Please reconsider to be more specific. Please highlight the particularithis of covid-19 related myopathis with regard to other viral infections

2.       Introduction: please specify other clincial markers, scores etc which could be used to differentiate myositis from myolisis

3.       Results: Please consider to add some MRI findings that soustained your diagnosis of myositis

4.       Discussion: since all, CK, BUN, urea/crea ratio, are easy to be assessed, please justify the importance to know the correlation of CK to urea/crea? How could this finding change the clinical practice

5.       Disscusion: you mentioned that none of your patient developed myolisis, which is not supported by your results. Please adda dditional data to support this information.

6.       Disscusion: please consider to move the raws 197-210 in the introduction section

Author Response

REVIEWER 2

Introduction: It is not very clear if you plead for a covid-19 related myositis or myolisis. Please reconsider to be more specific. Please highlight the particularithis of covid-19 related myopathis with regard to other viral infections

Reply: Thank you for your valuable point of view. We changed the text trying to make it more understandable and clearer, thank you for the precious suggestion.

Introduction: please specify other clincial markers, scores etc which could be used to differentiate myositis from myolisis

Reply: We added what you suggested.

Results: Please consider to add some MRI findings that soustained your diagnosis of myositis

Reply: Only one patient underwent MRI and we added the examination findings

Discussion: since all, CK, BUN, urea/crea ratio, are easy to be assessed, please justify the importance to know the correlation of CK to urea/crea? How could this finding change the clinical practice

Reply: We stated that the urea/cr ratio is useful in distinguishing an increase in the catabolism of nitrogen products from a reduced excretion of nitrogen.

Discussion: you mentioned that none of your patient developed myolisis, which is not supported by your results. Please adda additional data to support this information.

Reply: Thank you for your evaluation. We stated within the text how then diagnosis of myolysis was excluded

Disscusion: please consider to move the rows 197-210 in the introduction section

Reply: We fixed it.

Reviewer 3 Report

In this study it was seen that in children with myositis and who were SARS-CoV-2 positive there was a notable correlation between CK levels, blood urea nitrogen and the urea/creatinine ratio. It was suggested that it is crucial that specific markers are monitored to inform early intervention and management.

The main issue I have with the paper is that the authors suggest that the myositis was COVID-19 associated, as the patients in the paper were also tested COVID-19 positive. But this in itself does not mean association, as one could as well select children with myositis who also happen to have glasses; we then don’t speak about ‘pediatric glasses-associated myositis’. It should be noted that there have always been children with myositis. Too much a suggestion is given throughout the paper that the myositis was precipitated by COVID-19, which I find highly questionable.

The study really addresses markers of myositis that are equally applicable to COVID-negative and COVID-positive children. If COVID-19 was really important for your story, you at least would have needed to compare children with myositis that were COVID- positive to those that were COVID- negative. Since this comparison is missing, there is no possibility to ascribe any observation to COVID, but rather the disease itself, myositis. You can say they are COVID-positive, but don’t mention it in the title, or make a big point of it. Once again, the design of your study precludes you from ascribing anything to a positive COVID test.

Abstract: Given the above comments you can’t state in the abstract line 29-30 ‘…aimed to analyze clinical and laboratory factors associated with COVID-19-induced myositis in pediatric patients’. How, except for being COVID+ can you be so sure that the myositis was COVID-19 induced? The only reason is that you selected patients who were COVID+ and subsequently hospitalised (page 2 line 96), but how long was the period between being positive and being hospitalised? Was it for all the same duration? It is good you tested for other viruses, as long before COVID, viral infections and other agents have been shown to increase pediatric myositis (Buss BF, Shinde VM, Safranek TJ, et al. Pediatric influenza-associated myositis—Nebraska, 2001-2007. Influenza Other Respir Viruses 2009; 3(6): 277–285; Crum-Cianflone NF. Bacterial, fungal, parasitic, and viral myositis. Clin Microbiol Rev 2008; 21(3): 473–494). In other words, there may be other causes. However, you tested for the other viruses while in the hospital, and it could be that the children were influenza etc -negative, while earlier, before hospitalisation, they might have been positive for those viruses that then could have induced the myositis. In other words, measuring that other viruses were absent at the moment of hospitalisation is not proof they did not cause the myositis as they may have been infected with these before. Page 5 line 158-159 makes me wonder whether the COVID test was done on nasal swaps before (page 2 line 97 suggests so ‘… positive for SARS-CoV-2 using a nasal swab and were subsequently hospitalized’) or during hospitalization. Or was it in both instances. You need to show this, or if it was done twice, these COVID test outcomes.

While your methods describe that (page 3 line 109-113) all patients were tested for other viruses, this does not match with what you write on page 8 line 220 ‘In our cohort, we tested only one patient for additional pathogens’.

Further on it says ‘Neverthless, the probability (so, not measured) of concurrent influenza was deemed low (but was it?) due to minimal influenza cases in the community, which was a result of stringent infection control measures during the pandemic’. This of course begs the question why these measures were then so effective for influenza, but not for the target of all those measures, COVID? I suspect cases were low as people tested primarily for COVID (hence high recorded numbers) and not for influenza (hence the apparent, but not real, almost complete disappearance of influenza).

Introduction

Most of the studies mentioned in line 58 suffer from the same issue as in this paper: their patients suffered from a muscle problem and were COVID+, but this does not mean a causal relationship. In many cases they did not compare with non-COVID people who had the same issues, or were even case studies! Is the incidence elevated in COVID+ children? And then the issue arises, ‘who has not been COVID+ in these years, begging the question that therefore everything can be ascribed to COVID?’ In other words, these studies are not convincing that COVID caused all those issues, particularly when one considers that they also occurred before COVID. Same applies to references in line 77: all case studies.

Line 60-61 shows many more causes of myositis, also in children.

Line 68-71: myolysis not commonly associated with viral infections. So, what does make you believe then that it is associated with a viral COVID-19 infection?

Line 72-74: Is there a higher prevalence/incidence of myolysis in COVID+ people? You have provided no reference to backup this statement. Also, this is not ‘evidenced by the observed heightened mortality….’ The way it is written suggests that the myolysis is caused by COVID that then in turn causes kidney problems and then death, where really most deaths had other causes!

Line 87 applies, irrespective of COVID.

Line 92-93: It is dubious at best that the condition you investigated was COVID-induced. It is interesting that 70% of your patients acquired myositis in winter (page 3 line 136), typically a season with a higher incidence of flu (your reference 21 and typically accounting for 80% of cases (page 7 line 215)! I highly recommend to just remove this concept of COVID-induced myositis as the study you did is relevant for any, no matter the cause, child with myositis. You don’t need COVID to be able to ‘sell’ your paper. You can mention in the introduction that it may play a role, but I want to see much, much more reservation about the claim of ‘complication of COVID infection…’ Your study is simply not designed to assess a causal relationship.

Methods

Page 2 Line 97: what was done with the nasal swap? A PCR test or a lateral flow test? If with PCR, how many cycles was the PCR run? What was the period between being tested positive and hospitalisation?

Page 3 line 118: you were concerned here with ‘sex’ not ‘gender’. Gender is a social construct, and here the interest was in the biology.

Table 1 patient 6. I think you miss a ‘.’ In the value. Throughout the table, in some cases decimals are indicated with a ‘,’ and others with a ‘.’. Make it consistent. For CRP more decimals are required. I suggest to add the leucocyte count to the table, if it is the same as the white blood cell count, then the statement ion page 5 line 154 is incorrect as none of the counts was <1500/µL. It is said on page 5 that also troponin is presented in Table 1, but I did not see it. Line 157: what was the negative in the urine test?

Page 5 line 162-164: The CK levels did not peak day 5 and 7 after onset of COVID symptoms, but rather at day 5 and 7 after the onset of myositis symptoms!

Page 5 description of figure 1 and figure 1: I think a linear relationship is not the most appropriate one. What the graph seems to show to me is that there is a cut-off value. If UCR ranges between 0-8, it will take 5-10 days for normalising CK. It is only when UCR exceeds that level that the time needed to normalise CK takes longer. Similar thing applies to Figure 2 and description that only above a BUN of 5 the time needed to normalise CK increases with increasing BUN. The same for Figure 3. If you want to use it as markers you can then thus say that if the UCR, BUN or UCR value is above a certain cut-off the recovery will take longer.

Discussion

The opening paragraph is a good reflection of your data and I agree with that interpretation, bar that I would talk about cut-offs rather than linear relationships! In fact, on page 5 line 254 you seem also to suggest cut-off values.

The discussion relates a whole host of mechanisms that may play a role, but they have not been assessed in the study, as no comparison is being made between COVID-negative and COVID positive patients.

Line 208-209: The muscle damage is a consequence of inflammation. Yes, that is clear as that is the definition of myositis and applies to any myositis and is not specific to SARS-CoV-2.

Line 224-225 in fact says that it is to all intents and purposes just the same as myositis seen in influenza.

Line 261 yopur approach helped to understand more about pediatric myositis, and not so much about SARS-CoV-2 with other health parameters.

Page 9 line 292: It not just bears resemblance, they are just the same presentation. The study does not reveal something specific to SARS-CoV-2 in the pediatric myositis patients, but just reveals (important enough!) how the progression and recovery from pediatric myositis can be predicted.

Page 9 line 293: You did not show any ‘distinguishing characteristics of SARS-CoV-2 associated myositis’. Line 301-303 exactly describes what should have been done, and my prediction/hypothesis is that there is no difference. If my hypothesis is true then it also solves the issue mentioned in 303-306, where there will be no difference in the presentation of myositis depending on the variant of the SARS-CoV-2 virus, or any other virus; it will all just be the same.

Page 9 line 297-298 and 308-310 Your study did not provide a ’clearer picture of the interplay between SARS-CoV-2 and myolysis’ or myositis as first, you had no comparison with COVID negative patients and secondly you had in none of your patients myolysis (see page 7 line 193).

Author Response

REVIEWER 3

In this study it was seen that in children with myositis and who were SARS-CoV-2 positive there was a notable correlation between CK levels, blood urea nitrogen and the urea/creatinine ratio. It was suggested that it is crucial that specific markers are monitored to inform early intervention and management.

The main issue I have with the paper is that the authors suggest that the myositis was COVID-19 associated, as the patients in the paper were also tested COVID-19 positive. But this in itself does not mean association, as one could as well select children with myositis who also happen to have glasses; we then don’t speak about ‘pediatric glasses-associated myositis’. It should be noted that there have always been children with myositis. Too much a suggestion is given throughout the paper that the myositis was precipitated by COVID-19, which I find highly questionable.

The study really addresses markers of myositis that are equally applicable to COVID-negative and COVID-positive children. If COVID-19 was really important for your story, you at least would have needed to compare children with myositis that were COVID- positive to those that were COVID- negative. Since this comparison is missing, there is no possibility to ascribe any observation to COVID, but rather the disease itself, myositis. You can say they are COVID-positive, but don’t mention it in the title, or make a big point of it. Once again, the design of your study precludes you from ascribing anything to a positive COVID test.

Reply: Thank you for your critical assessment of our manuscript and for your valuable scientific point of view. We understand your concerns about establishing a direct causal link between COVID-19 and myositis in children. However, we firmly believe that our study adds valuable insight into this emerging field, despite the acknowledged limitations.

Firstly, the association between viral infections and myositis is well-documented in medical literature. Viruses such as influenza, enterovirus, and adenovirus have been associated with myositis in pediatric populations (Attaianese, F.; Costantino, A.; Benucci, C.; Lasagni, D.; Trapani, S. Benign Acute Children Myositis: 5 Years Experience in a Tertiary Care Pediatric Hospital. Eur J Pediatr 2023; Szenborn, L.; Toczek-Kubicka, K.; Zaryczański, J.; Marchewka-Kowalik, M.; Miśkiewicz, K.; Kuchar, E. Benign Acute Childhood Myositis during Influenza B Outbreak. Adv Exp Med Biol 2018). Given this established precedent, it is scientifically plausible to hypothesize a similar association with SARS-CoV-2, especially considering its widespread impact and diverse clinical manifestations.

Our study's findings, showing elevated markers of myositis in COVID-19 positive children, align with emerging literature indicating a potential link. For instance, Saud et al. (2021) in the Journal of Rheumatology and Giraudo et al (20203) in Frontiers in pediatric journal both report observations supporting this hypothesis. These studies, coupled with our findings, suggest a pattern that warrants further investigation, rather than dismissal.

While we acknowledge the absence of a control group of COVID-negative children with myositis in our study, it is important to recognize the challenges and constraints faced in conducting research during a global pandemic. The logistics of enrolling and comparing COVID-positive and COVID-negative children with myositis would have significantly delayed crucial insights at a time when the medical community is urgently seeking information about the impact of COVID-19 on pediatric populations.

Therefore, while we agree with your suggestion to mention the limitations of our study more prominently, we respectfully assert that our findings contribute meaningful preliminary data to the ongoing discourse about the potential implications of COVID-19 in pediatric myositis. We propose to revise our manuscript to include a more detailed discussion on the broader context of viral myositis and the emerging evidence of a potential link with SARS-CoV-2, while clearly stating the limitations and the need for further research. In addition, we changed the title, as you suggested, removing the term “associated”.

In conclusion, our study, though preliminary, provides a crucial starting point for future research that can more definitively explore the relationship between COVID-19 and myositis in children. We believe that omitting or diminishing the mention of COVID-19 in our study would undermine the potential significance of these findings in the current global health context.

Stat rosa pristina nomine, nomina nuda tenemus (De contemptu mundi by Bernardo form Cluny).

Abstract: Given the above comments you can’t state in the abstract line 29-30 ‘…aimed to analyze clinical and laboratory factors associated with COVID-19-induced myositis in pediatric patients’. How, except for being COVID+ can you be so sure that the myositis was COVID-19 induced? The only reason is that you selected patients who were COVID+ and subsequently hospitalised (page 2 line 96), but how long was the period between being positive and being hospitalised? Was it for all the same duration? It is good you tested for other viruses, as long before COVID, viral infections and other agents have been shown to increase pediatric myositis (Buss BF, Shinde VM, Safranek TJ, et al. Pediatric influenza-associated myositis—Nebraska, 2001-2007. Influenza Other Respir Viruses 2009; 3(6): 277–285; Crum-Cianflone NF. Bacterial, fungal, parasitic, and viral myositis. Clin Microbiol Rev 2008; 21(3): 473–494). In other words, there may be other causes. However, you tested for the other viruses while in the hospital, and it could be that the children were influenza etc -negative, while earlier, before hospitalisation, they might have been positive for those viruses that then could have induced the myositis. In other words, measuring that other viruses were absent at the moment of hospitalisation is not proof they did not cause the myositis as they may have been infected with these before.

Reply: Thank you for what you pointed out. We took a careful medical history and none of the patients enrolled had had fever, malaise, asthenia or other signs and symptoms of infection in the previous month. If we had suspicion or evidence of previous infection, we would not have enrolled the patients.

Page 5 line 158-159 makes me wonder whether the COVID test was done on nasal swaps before (page 2 line 97 suggests so ‘… positive for SARS-CoV-2 using a nasal swab and were subsequently hospitalized’) or during hospitalization. Or was it in both instances. You need to show this, or if it was done twice, these COVID test outcomes.

Reply: Nasal swab for COVID 19 RNA molecular research was performed at the entrance of our pediatric emergency department. The patients were subsequently hospitalized in isolation. Furthermore, a film-array swab was performed to search for Covid and respiratory viruses every 48 hours until the results were negative.

While your methods describe that (page 3 line 109-113) all patients were tested for other viruses, this does not match with what you write on page 8 line 220 ‘In our cohort, we tested only one patient for additional pathogens’.

Reply: We fixed what you kindly suggested, all enrolled patients were tested for respiratory viruses. Sorry for the mistake.

Further on it says ‘Neverthless, the probability (so, not measured) of concurrent influenza was deemed low (but was it?) due to minimal influenza cases in the community, which was a result of stringent infection control measures during the pandemic’. This of course begs the question why these measures were then so effective for influenza, but not for the target of all those measures, COVID? I suspect cases were low as people tested primarily for COVID (hence high recorded numbers) and not for influenza (hence the apparent, but not real, almost complete disappearance of influenza).

Reply: Thank you for your help. Evaluating the epidemiology of influenza is not the objective of our study, however there is numerous evidence in the literature that a reduction in influenza and respiratory viruses during the pandemic. Furthermore, Galli et al. in the work 'On the lookout for influenza viruses in Italy during the 2021-2022 season: Along came A (H3N2) viruses with a new phylogenetic makeup of their hemagglutinin' they state a delayed peak of influenza and lower than pre- pandemics.

Introduction

Most of the studies mentioned in line 58 suffer from the same issue as in this paper: their patients suffered from a muscle problem and were COVID+, but this does not mean a causal relationship. In many cases they did not compare with non-COVID people who had the same issues, or were even case studies! Is the incidence elevated in COVID+ children? And then the issue arises, ‘who has not been COVID+ in these years, begging the question that therefore everything can be ascribed to COVID?’ In other words, these studies are not convincing that COVID caused all those issues, particularly when one considers that they also occurred before COVID. Same applies to references in line 77: all case studies.

Reply: We answer to this issue in the first response. We changed this paragraph by introducing the more information about pathophysiological pathways leading to myositis in COVID-19 patients. We hope this way make the paper clearer.

Line 60-61 shows many more causes of myositis, also in children.

Reply: We also introduced childhood-onset or juvenile idiopathic inflammatory myopathies (JIIMs), metabolic myopathies, Neurogenic myositis ossificans.

Line 68-71: myolysis not commonly associated with viral infections. So, what does make you believe then that it is associated with a viral COVID-19 infection?

Reply: We fixed this paragraph and meant to say myositis and not myolysis. Furthermore, we also specified in the discussion chapter that none of the patients enrolled was diagnosed with myolysis.

Line 72-74: Is there a higher prevalence/incidence of myolysis in COVID+ people? You have provided no reference to backup this statement. Also, this is not ‘evidenced by the observed heightened mortality….’ The way it is written suggests that the myolysis is caused by COVID that then in turn causes kidney problems and then death, where really most deaths had other causes!

Reply: We have removed this unclear line. Thank you for highlighting that.

Line 87 applies, irrespective of COVID.

Reply: There are several literature evidence about SARS-CoV-2 associated myositis. dos Santos et al. show specifically how inflammation triggered by covid promotes muscle damage.

Line 92-93: It is dubious at best that the condition you investigated was COVID-induced. It is interesting that 70% of your patients acquired myositis in winter (page 3 line 136), typically a season with a higher incidence of flu (your reference 21 and typically accounting for 80% of cases (page 7 line 215)! I highly recommend to just remove this concept of COVID-induced myositis as the study you did is relevant for any, no matter the cause, child with myositis. You don’t need COVID to be able to ‘sell’ your paper. You can mention in the introduction that it may play a role, but I want to see much, much more reservation about the claim of ‘complication of COVID infection…’ Your study is simply not designed to assess a causal relationship.

Reply: Thank you again for your point of view. We reported other studies which highlight the concept of myositis associated with SARS-CoV-2 infection. We agree with you, it is difficult and challenging to correlate the two entities. We changed the title as you suggested and modified the discussion section to reduce the strength of this association. We hope the manuscript is clearer without misunderstandings.

Methods

Page 2 Line 97: what was done with the nasal swap? A PCR test or a lateral flow test? If with PCR, how many cycles was the PCR run? What was the period between being tested positive and hospitalisation?

Reply: The cycle threshold number (CT) in SARS-CoV-2 RT-PCR ranged from a minimum of 25 to a maximum of 30 cycles.

Patients were swabbed upon entering the Emergency department. Subsequently, the text was repeated in the ward under isolation regimen and then every 48 hours.

Page 3 line 118: you were concerned here with ‘sex’ not ‘gender’. Gender is a social construct, and here the interest was in the biology.

Reply: We amended.

Table 1 patient 6. I think you miss a ‘.’ In the value. Throughout the table, in some cases decimals are indicated with a ‘,’ and others with a ‘.’. Make it consistent. For CRP more decimals are required. I suggest to add the leucocyte count to the table, if it is the same as the white blood cell count, then the statement ion page 5 line 154 is incorrect as none of the counts was <1500/µL. It is said on page 5 that also troponin is presented in Table 1, but I did not see it. Line 157: what was the negative in the urine test?

Reply: We have arranged the table as indicated. The phrase <1500 refers to the number of neutrophils and not of leukocytes, as specified in parentheses. Also, we made the line clearer. The urine test was negative for bacteria, leukocytes, nitrates. We specified to prevent an infection from altering the urinary sediment and affecting proteinuria.

Page 5 line 162-164: The CK levels did not peak day 5 and 7 after onset of COVID symptoms, but rather at day 5 and 7 after the onset of myositis symptoms!

Reply: We amended.

Page 5 description of figure 1 and figure 1: I think a linear relationship is not the most appropriate one. What the graph seems to show to me is that there is a cut-off value. If UCR ranges between 0-8, it will take 5-10 days for normalising CK. It is only when UCR exceeds that level that the time needed to normalise CK takes longer. Similar thing applies to Figure 2 and description that only above a BUN of 5 the time needed to normalise CK increases with increasing BUN. The same for Figure 3. If you want to use it as markers you can then thus say that if the UCR, BUN or UCR value is above a certain cut-off the recovery will take longer.

Reply: The graphs are only the consequence of a linear correlation as can be seen from the function of the graph. We also obtained the d R2 values.

The R square in fact measures the strength of the linear relationship between the independent variables included in the regression model and the dependent variable. Stronger relationships, R∼1 indicate less dispersion of the data around the regression line.

Discussion

The opening paragraph is a good reflection of your data and I agree with that interpretation, bar that I would talk about cut-offs rather than linear relationships! In fact, on page 5 line 254 you seem also to suggest cut-off values.

Reply: We wanted to calculate cut-offs but to do so, we gave up because the sample of 10 patients could be small. In fact, to have cut-offs with good sensitivity, larger samples are needed. We tried to calculate cut offs with ROC (Receiver operating characteristic curve) curves but we were not satisfied with the sensitivity values obtained from these theoretical cut offs.

The discussion relates a whole host of mechanisms that may play a role, but they have not been assessed in the study, as no comparison is being made between COVID-negative and COVID positive patients.

Reply: Our interest was to investigate SARS-CoV-2 positive population, the literature offers numerous data regarding the non-covid population. Furthermore, it was not easy to find a matched cohort of covid-negative patients during a pandemic.

Line 208-209: The muscle damage is a consequence of inflammation. Yes, that is clear as that is the definition of myositis and applies to any myositis and is not specific to SARS-CoV-2.

Reply: We modified as much as possible the paper also with the help of the other reviewers to underline the concept you pointed out.

Line 224-225 in fact says that it is to all intents and purposes just the same as myositis seen in influenza.

Reply: In fact, I reiterate that the purpose of this manuscript is to differentiate between Covid and flu myositis; It is also significant to note that Covid myositis has a similar trend to other viral myositis.

Line 261 your approach helped to understand more about pediatric myositis, and not so much about SARS-CoV-2 with other health parameters.

Page 9 line 292: It not just bears resemblance, they are just the same presentation. The study does not reveal something specific to SARS-CoV-2 in the pediatric myositis patients, but just reveals (important enough!) how the progression and recovery from pediatric myositis can be predicted.

Reply: We believe this is also a significant fact, as you pointed out.

Page 9 line 293: You did not show any ‘distinguishing characteristics of SARS-CoV-2 associated myositis’. Line 301-303 exactly describes what should have been done, and my prediction/hypothesis is that there is no difference. If my hypothesis is true then it also solves the issue mentioned in 303-306, where there will be no difference in the presentation of myositis depending on the variant of the SARS-CoV-2 virus, or any other virus; it will all just be the same.

Reply: Thank you for your analyses. We think theta this fact could not be taken for granted a priori

Page 9 line 297-298 and 308-310 Your study did not provide a ’clearer picture of the interplay between SARS-CoV-2 and myolysis’ or myositis as first, you had no comparison with COVID negative patients and secondly you had in none of your patients myolysis (see page 7 line 193).

Reply: I changed the sentence using the term covid exposed children. I tried to use lighter terms, to better express this concept. Furthermore, I added in the conclusions the need to conduct double-blind studies to better understand the similarities and differences between the covid and non-covid population.

Round 2

Reviewer 2 Report

Many thanks for the revised form which is in accordance to the recommandation 

Author Response

Thank you for your time and precious suggestions.

Reviewer 3 Report

see attached file

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf