Author's response to reviews Title:Determinants of high sensitivity cardiac troponin T elevation in acute ischemic stroke Authors: Kashif W Faiz (kashif.faiz@medisin.uio.no) Bente Thommessen (bente.thommessen@ahus.no) Gunnar Einvik (gunnar.einvik@medisin.uio.no) Pål H Brekke (paul.brekke@gmail.com) Torbjørn Omland (torbjorn.omland@medisin.uio.no) Ole M Rønning (o.m.ronning@medisin.uio.no) Version:2Date:20 December 2013 Author's response to reviews: see over
Lørenskog, December 20 th 2013 BMC Neurology Dr. Fabienne Perren Cover letter regarding the revised version of the manuscript no. MS: 1733418827105584 Determinants of high sensitivity cardiac troponin T elevation in acute ischemic stroke Dear Dr Perren, We are grateful for the thorough and insightful reviews of our paper, and appreciate the opportunity for resubmission. Please find below our point-to-point response to the reviewers comments (in italics). Our reply is given in bold, and the revised sections of our manuscript are given below in red colour. In our opinion the suggested changes from the reviewers have improved the manuscript and we hope you will find it acceptable for publication in BMC Neurology. On behalf of the authors, Yours sincerely Kashif Waqar Faiz, MD
Reviewer 1 1. In the introduction of the abstract I would not focus on the mortality as a major aim of the study, as for this purpose higher number of patients is needed. The main focus is: a) Frequency of stroke patients with increase in hs-ctnt b) How many are depicted with a real cardiac ischemia with an therapeutic relevance c) Which factors are associated with an increase in hs-ctnt In this context the conclusion of the abstract does not mention the mortality issue. The mortality should be mentioned, but with referral to methodological limitations. Reply: Thank you for these comments regarding the abstract. We have revised the Background section of the Abstract: The aims of this study were to assess the frequency of high sensitivity cardiac troponin T (hs-ctnt) elevation, to identify determinants and ECG changes associated with hsctnt elevation, to identify patients with myocardial ischemia and to assess the impact of hs-ctnt elevation on in-hospital mortality. We agree that the uncertainty regarding the mortality because of limited number of events. We have rephrased the Conclusion section in the Abstract. Established CHD and cardiovascular risk factors are associated with hs-ctnt elevation. Acute MI is likely underdiagnosed in acute ischemic stroke patients. Dynamic changes in troponin levels seem to be related to poor short-term prognosis. 2. I would avoid the term cohort as it is suggestive to a cohort study. Reply: We have removed the term cohort from the Methods section. 3. I would keep the presentation of the results simple. I think the multiple testing is problematic. In a stepwise logistical regression analysis the factors considered for the analysis should be selected restrictively (not all!!). Interpreting only the relative frequencies, the message is very clear: with increasing vascular burden the likelier is an elevated hs-ctnt to detect. In this direction the presentation and the discussion should be guided. Reply: Thank you for this question. Table 3. We have used both backward stepwise regression and enter method, with the same results regarding significant and nonsignificant factors. Different factors emerged as significant for i) all patients, ii) females and iii) males. If we had only used the data from the final step of the backward analysis in the table, some categories would be left blank (i.e. diabetes significant for all patients and males, but not for females). Therefore we opted to include both significant and nonsignificant values in the table. If we had omitted the male and female categories in the table, a simpler table with the final factors in the backward stepwise model would have been included, meaning fewer factors. 4. Regarding the multiple testing for example the congestive heart failure could be associated with a coronary heart disease; so it should be decided which factor is worth for consideration for the logistical analysis. Therefore the stepwise manner is here important. An advice from a professional statistician would be of benefit. Reply: There are several factors which may be associated with each other (i.e. congestive heart failure, coronary heart disease, atrial fibrillation, egfr).
Correlation matrix showed r = -0.114 between coronary heart disease and congestive heart failure, which is acceptable. All Hosmer-Lemeshow goodness-of-fit tests had p > 0.05. 5. The analysis regarding the mortality should be kept simple (only univariate analysis) with 12 patients a logistical regression analysis do not make sense. Reply: We agree that the low number of in-hospital mortality events is a weakness regarding the conclusion of short term prognosis. We have chosen to keep the multivariate results in the table (Table 4), but have addressed the issue as a limitation in the Discussion section: ( ) because of the relatively low number in-hospital mortality events, the results should be considered preliminary. 6. In the manuscript the authors report on stroke in general. In my Opinion it is important to stratify according to stroke subtypes (TOAST). It is of relevance to distinguish, cardioembolic, large artery occlusion, small artery occlusion, undetermined stroke and other determined stroke. In this context a comparison of cardioembolic strokes versus non-cardioembolic stroke would be of interest. Reply: Thank you for this important remark. We have available data on stroke etiology (TOAST), and have included the results in Table 1: All patients (n=287) hs-ctnt 14 ng/l (n=131) hs-ctnt > 14 ng/l (n=156) p value TOAST Small vessel 65 (22.6) 37 (28.2) 28 (17.9) 0.038 Large artery 51 (17.8) 23 (17.6) 28 (17.9) 0.931 Cardioembolic 84 (29.3) 20 (15.3) 64 (41.0) <0.001 Other/ undetermined 87 (30.3) 51 (38.9) 36 (23.1) 0.004 We have briefly commented this in the Results section: Regarding stroke subtypes, patients with elevated hs-ctnt had significantly higher frequency of cardioembolic strokes (p<0.001). 7. Seizures following stroke could also lead to an increase in troponin. Are data available on this issue? I think this could be derived from care files. Since seizures lead to an increase in troponin, this possibility needs to be considered for the analysis [1]. Reply: We do not have data on seizures before or after the ischemic stroke, but in our experience seizures rarely coincide with ischemic stroke, more often with hemorrhagic stroke or cerebral venous thrombosis. In the revised manuscript, we have included the reference as one of the several causes of troponin elevation. Some colleagues at our department are planning a prospective study on patients with first-ever epileptic seizures, including repeated troponin measurements. 8. As you report on stroke patients, brain imaging (MRI, CT-scans) must be available. This issue is important (infarct localisation, size, pattern, cortex-involvement) [2,3]. Since some
authors describe specific regions for generating a troponin release, this aspect need to be considered in paper like yours. Reply: This is a very important comment. Unfortunately we do not have CT/ MRI information on the site of stroke, and it would have been interesting to assess whether troponin elevation is related to i.e. insular strokes. All patients had a CT scan on admission, but not all patients were referred to MRI or a new CT, therefore data are insufficient in this retrospective analysis. We are planning a prospective study on troponin elevation in acute ischemic stroke, where this element will be included. Reviewer 2 1.The universal definition of myocardial infarction requires a clinical setting consistent with myocardial ischemia in combination with a rise/fall of ctn. The #> 20% criteria or absolute change >=9.2ng/l relate to early ctn measurements within 3-6 hours after the first sample. The authors should discuss this. Reply: Thank you for this important comment. Because of the retrospective method, this is a major limitation of the study. We have stated this briefly in the Discussion section. It is recommended that blood samples for the measurement of ctn should be drawn on first assessment and repeated 3 6 hours later [1], while the median time interval was 16 hours in our study. 2. The authors excluded patients with MI during the hospital stay. How was acute MI defined in these patients? Reply: Patients diagnosed with acute MI within the last 7 days before admission or during hospitalization were excluded, as it is expected that they will have elevated troponin levels. In all, 16 patients were excluded because of this criterion, of which 1 patient was diagnosed with AMI 3 days before admission, and 15 during the hospital stay. The 15 patients diagnosed with a cute MI during hospital stay were all referred to and evaluated by a cardiologist and underwent a echocardiography, and had higher levels of troponin than the non-referred patients. 3. In order to analyze the association of ctn with in-hospital mortality Cox proportional hazards should be calculated, since observation period ranges between 3 and 8 days. Reply: We have used in-hospital mortality as dichotomous variable ( not dead or dead ) to assess whether hs-ctnt was related to in-hospital mortality, regardless of hospital stay duration and when the death occurred during hospitalization. 4. A certain limitation is the small sample size and low event rate (12 deaths). This should be discussed. Hence, the regression model includes too many factors. Was the model calculated in a backward stepwise fashion?
Reply: We agree that the low number of in-hospital mortality events is a weakness regarding the conclusion of short term prognosis. We have chosen to keep the multivariate results in the table (Table 4), but have addressed the issue as a limitation in the Discussion section: ( ) because of the relatively low number in-hospital mortality events, the results should be considered preliminary. We used backward stepwise logistic regression model. 5. Based on the data provided by the authors the conclusion that MI is underdiagnosed may probably not be made. Imagine an elderly female patient with history of congestive heart failure and chronic kidney disease. There is no information about chest pain and ctn is 15 ng/l on admission and 18ng/l 16h later. ECG on admission shows T inversion in two contiguous leads. Is this MI or chronic elevation with small individual variation? Reply: This is a very interesting remark. In this example, the patient has two values above the upper reference limit (>14), rise of troponin (20%, from 15 to 18), and T inversion in two contiguous leads (assuming T inversion 0.1 mv in absence of LVH and LBBB). The patient thus meets the criteria for acute MI (Thygesen et al, 2012). Symptoms of ischemia are no longer compulsory. A limitation of the study is the long duration (median 16 hours) between the two blood samples, as stated in remark #1, and stated in the revised manuscript. Because of the definition, acute MI may be overdiagnosed in patients, particularly borderline cases, as this example. On the other hand, we excluded patients with LVH and LBBB, of which some patients may have had MI but not detected in our study. Minor: Is there imaging information on the site of stroke (e.g. the insular cortex)? Reply: Unfortunately we do not have CT/ MRI information on the site of stroke, and it would have been interesting to assess whether troponin elevation is related to insular strokes. We are planning a prospective study on troponin elevation in acute ischemic stroke, where this element will be included. Reviewer 3 1.) Why were patients that have been diagnosed during hospitalization with acute myocardial infarction excluded. How many of those 16 patients were excluded for AIM that was diagnosed after admission to the hospital? Reply: Patients diagnosed with acute MI within the last 7 days before admission or during hospitalization were excluded, as it is expected that they will have elevated troponin levels. Inclusion of these patients may influence the results, and therefore they were excluded. In all, 16 patients were excluded because of this criterion, of which 1 patient was diagnosed with AMI 3 days before admission, and 15 during the hospital stay.
2.) What were the criteria for the diagnosis of AMI that results in exclusion of the patient from the study? According to the 3rd universal definition of the IM, each IM has to meet criterion as follows: rise and / or fall of cardiac biomarker values (preferable cardiac troponin) with at least one value above the 99th percentile of the upper reference limit. In the present study, besides 16 excluded patients, there were also additional 156 with elevated ctnt and 81 of them had a decrease or an increase of more than 20%. All of these patients (if confirmed by other method as angiography, echocardiography etc.) possibly could meet the criteria for AIM. Reply: This is a very interesting remark and one of the key messages of the manuscript, that acute MI is underdiagnosed in stroke patients. Patients with moderate elevation (e.g. hs-ctnt 20-50) are often not referred to a cardiologist, even in the presence of rise or fall. The patients diagnosed with acute MI during hospitalization (and thus excluded from the present study) were all referred to and evaluated by a cardiologist. 3.) Why authors diagnosed AMI only in cases when the combination of ctnt decrease or increase (by more than 20%) and a positive ECG findings occurs in enrolled patients? Reply: This is another interesting remark. We chose to use a strict definition of acute MI, where we excluded patients with left ventricular hypertrophy (LVH) and left bundle branch block (LBBB). Indeed, some of the patients with LVH and LBBB could also have had an acute MI with troponin elevation and a rise or fall pattern, and the actual number of patients with undiagnosed acute MI could be higher than the 6.6% we report. We did unfortunately not have ECG s before admission of all patients, and could thus not conclude if the LVH or LBBB changes were new or old. 4.)Overall NIHSS score is very low median was 4, respectively 3. How could authors explain this fact? It seems that patients with MCA or BA occlusions are missing in the cohort. That suggests question, if there was some kind of pre-selection of patients (potential important limitation of this study). Does the Akershus University Hospital department of Neurology admit truly all ischemic stroke patients? Or patients with severe deficit are admitted to different hospital, e.g. to comprehensive stroke center (accessibility of mechanical thrombectomy or i.a. trombolysis)? Reply: All patients in the catchment area hospitalized for stroke are admitted to our hospital regardless of severity. We have a liberal approach to admission of patients with suspected stroke, and the NIHSS score indicates that a significant proportion of patients have minor strokes. Only patients requiring surgical intervention or intra-arterial thrombolysis/ thrombectomy are transferred to another hospital (but first admitted at our hospital). Minor Essential Revisions: 1.) I would prefer to extend the patient s characteristics with data of past myocardial infarction, not only of coronary heart disease, and correlate this data too (Table1. and 3). This could be another important fact regarding possibility of severe coronary artery atherosclerosis. Reply: The definition of previous coronary heart disease in this study was: previous acute MI and/ or coronary intervention. Unfortunately, we do not have data on just MI.
2.) The ECG findings are described in detail, however the overall impression is little bit confusing. Nevertheless at the end of ECG paragraph, authors summarize ECG changes suspected of acute myocardial ischemia. It would be nice, if they add summary for ECG changes suspected of old myocardial infarction too. Reply: We have added a sentence in the ECG paragraph in the Results section: In all, 34 of 276 (12.3%) patients had pathological Q waves, suggestive of prior MI, but there were no significant differences between patients with hs-ctnt within the reference limit and elevation (p=0.096). 3.) More published articles studied correlation between ctnt and risk factors or ECG findings (e.g. Kral M. Troponin T in acute ischemic stroke. Am J Cardiol. 2013 Jul 1;112(1):117-21. doi: 10.1016/j.amjcard.2013.02.067. Epub 2013 Apr 6). Results should be discussed and compared with results of the present study in the Discussion. Reply: The article by Kral et al. covering many of the same topics as our manuscript was published after the preparation of the present manuscript. We have included the reference in the Discussion section of the revised manuscript. In a recently published study by Kral et al [27], 36% of the patients had hs-ctnt higher than the URL, and 22% of the patients presented with elevated hs-ctnt with new or presumed new significant ST-T segment changes or new LBBB. Reviewer 4 Major comments Values below the lower limit of detection of an assay should not be reported as numbers. All values <5 ng/l should be reported as this and the dataset has to be reanalysed accordingly and the manuscript has to be adopted accordingly. Reply: Thank you for this comment. We have replaced the limit of blank (LoB=3 ng/l) with the limit of detection (LoD=5 ng/l). All values below the LoD have been classified as 4.99 ng/l. We have performed a new multivariate linear regression analysis with hsctnt as the dependent variable (logarithmically transformed continuous variable because of a non-normal distribution). Smoking no longer emerged as a significant factor (p=0.040 -> p=0.082), but the five other factors remained statistically significant. Accordingly, we have made changes in the manuscript. Methods section: The limit of detection (LoD) was 5 ng/l (pg/ml), and values under this limit were routinely classified as 4.99 ng/l Results section: In the multivariate linear regression (Supplementary Table 2), the same five factors were significantly associated with lower hs-ctnt as a continuous variable.
Supplementary Table 2. B Coefficient Standard error t statistic P value Age 76 (median) 0.330 0.091 3.634 <0.001 Coronary heart disease 0.237 0.100 2.361 0.019 Congestive heart failure 0.343 0.148 2.315 0.021 Diabetes mellitus 0.280 0.120 2.340 0.020 egfr -0.009 0.002-4.568 <0.001 Minor comments: Abstract and Discussion: The authors found a high incidence of acute myocardial injury whether this has been caused by ischemia is uncertain, and even more uncertain is whether it is caused by Type I or II myocardial infarction. The conclusions should be reworded. Reply: Thank you for this comment. In the Discussion, we have reworded the following sentence: We did not record patient symptoms such as chest pain or shortness of breath, but the presence of hs-ctnt elevation and ECG manifestations of myocardial ischemia indicate that acute MI is likely underdiagnosed in patients with acute ischemic stroke, although the findings are insufficient to conclude whether the ischemia has been caused by type 1 or type 2 MI. The conclusion has also been reworded: We have identified determinants and ECG changes associated with hs-ctnt elevation in patients with acute ischemic stroke, and this study demonstrates a possible role of dynamic changes in hs-ctnt for prediction of in-hospital mortality. Acute MI is likely underdiagnosed in patients with acute ischemic stroke, but it is uncertain whether myocardial ischemia is caused by type 1 or type 2 MI. These results need to be verified in prospective studies. The conclusion in the Abstract has also been reworded: Acute MI is likely underdiagnosed in acute ischemic stroke patients, but it is uncertain whether myocardial ischemia is caused by type 1 or type 2 MI. Results: Multivariate prediction of in-hospital mortality: a table showing the odds ratios and p values of all variables included into the model would be of interest. Reply: Table 4. Because of simplicity and focus of interest, we chose not to include the OR s and P values of all variables included in the model (14 variables x 6 different troponin variants x 2 [unadjusted and adjusted]), only the different troponin variants, but we can provide such a table if required by the Editor. Discussion: Previously published studies on hs-ctnt for outcome prediction have been incompletely cited and discussed. Reply: We have included a reference where hs-ctnt predicts 90-day clinical outcome (Furtner M et al. The high-sensitivity cardiac troponin T assay is superior to its previous assay generation for prediction of 90-day clinical outcome in ischemic stroke. Clin
Chem Lab Med 2012, 50:2027-2029), but have chosen not to include references using the previous troponin assay. Previously, it has been demonstrated that elevated hs-ctnt is a predictor of 90-day clinical outcome [24] and long-term mortality [13] in ischemic stroke, thus it is of clinical relevance to identify determinants of elevated hs-ctnt.