Myocarditis is defined as myocardial inflammation presenting with edema, cellular infiltration, apoptosis and necrosis of cardiomyocytes[
17]. In up to 20% it accounts for postinflammatory dilated cardiomyopathy in children[
18,
19]. It is mostly diagnosed clinically, however in some cases its diagnosis has to be verified by Endomyocardial Biopsy (gold standard). Due to a diffuse to focal inflammatory pattern the sampling error results in low sensitivity of EMB[
2]. Usually five biopsies are taken from myocardium; increasing the number of biopsies to 17 per patient could improve sensitivity up to 80%[
20] however this is associated with increased invasiveness. Instead, in some centers biopsies are taken from myocardium that presented as LGE to improve sensitivity[
3,
4].
Pathophysiology of LGE
LGE has been established in diagnosing myocarditis in the past[
4,
21‐
23]. Hypotheses concerning pathophysiology of LGE in myocarditis have been made for a long time. Necrotic cardiomyocytes, as found in animals of experimental group, with ruptured membranes could be able to take up contrast media such as Gadolinium-DTPA, whereas Gadolinium as an extracellular molecule cannot penetrate into healthy cardiomyocytes, resulting in contrast media enhancement of necrotic foci. Abdel-Aty et al.[
24] explained this process during their study of differentiating acute and chronic myocardial infarction that as being related to the increased volume of distribution of gadolinium chelates secondary to extracellular space expansion in myocardial scars. The underlying mechanism of this extracellular space enlargement, however, differs in relation to infarct age. In acute myocardial infarction, there is loss of membrane integrity of the already edematous cardiomyocytes, allowing communication between the extracellular and intracellular spaces. Moreover, the induction of reperfusion marks the rapid evolution of an inflammatory-like response of which interstitial edema is a substantial feature, even though edema does not seem case LGE. In chronic myocardial infarction, on the other hand, enlargement of the extracellular space is mostly the result of the relatively large collagen matrix in the absence of myocardial edema. These considerations explain that LGE directly correlated to the extent of myocardial cell necrosis and this is a feature of myocardial necrosis regardless of its acuity or chronicity. This conclusion is also agreed by Friedrich et al. [
25].
Correlations between CMR and serology
TroponinT seems to have a correlation with the size of LGE. In all measured MR sequences we found statistically significant positive correlations (Turbo-FLASH without fat saturation: r = 0.64, p < 0.05; Turbo-FLASH with fat saturation r = 0.75, p < 0.05; TSE without fat saturation r = 0.95, p < 0.05; TSE with fat saturation: r = 0.75, p < 0.05; see table
3). Being dependent on the degree of cellular necrosis, troponin level is directly correlated to the extent of LGE.
Even though animals of the experimental group had significantly higher haptoglobin levels on days 7 and 21, haptoglobin had no correlation to the extent of LGE. Pathologcially raised levels of haptoglobin in control group can be explained by the unspecific inflammation caused by Complete Freud's Adjuvance injected in control animals.
Comparing topographic distributions of LGE and inflammation
LGE was found exclusively inside the left ventricular wall, despite cardiac lesions of the right ventricle found in histology. Probably, because the rat's heart is of small size, the right ventricular wall with its fewer muscular mass compared to the left ventricle could not be displayed properly by CMR-examination. Thus identifying possible LGE inside the right ventricle failed.
As described in results, sensitivity as well as positive predictive values varied widely dependending on the location of LGE and examined CMR-sequence. In general, inflammations located in the left anterior and lateral ventricular wall were identified by Turbo-FLASH-sequences in 61.54-76.92% whereas sensitivity dropped to 0-20% for inflammations located in the left inferior myocardium and in the septum; LGE located in the left anterior and lateral ventricular wall had high positive predictive values (75%-80%) in Turbo-FLASH-examination. Possible hypotheses for dependence on location of LGE (positive predictive value) and inflammation (sensitivity) could be: We believe that in Experimental Autoimmune myocarditis inflammation occurs primarily in the left anterior and lateral ventricular wall and delayed in the left inferior myocardium and septum and thus on day 21 inflammation could have been on different stages inside the myocardium as proved by histology. Cardiac lesions located "left inferior" and "septum" were of small size and only found in three animals. As it is challenging to compare histological segments with exactly corresponding MR-slices, we think that finding cardiac lesions with a small extent (e.g. located left inferior and septum) by MR examination is more difficult and resulted in low sensitivity. On the other hand it is possible that myocardial inflammation located in the left inferior ventricular wall and in the septum had caused mainly extracellular edema and hardly histologically verifiable myocardial damage as found in histology. In consequence raised interstitial space could lead to LGE without a histologically verifiable, corresponding myocardial inflammation, resulting in low positive predictive values. Furthermore, technical issues such as field inhomogenity of reception coil and motion artefacts could have an influence on our described results and might lead to differences in accuracy of CMR for different myocardial locations.
In their study Mahrholdt et al.[
23] could show a sensitivity for LGE in diagnosing acute viral myocarditis of 95%. In this study LGE was compared to histological findings of CMR-guided Endomyocardial Biopsy. In our study we compared LGE to histopathological analysis of the whole myocardium, which consequently has a higher sensitivity than EMB. This explains lower values for sensitivity in our study. Nevertheless we found higher sensitivity (up to 76%) for the locations "left anterior" and "left lateral" compared to EMB.
In another study Mahrholdt et al.[
4] were able to increase sensitivity of EMB by taking biopsy close to contrast enhanced myocardium. With our results for positive predictive value (up to 80%) we agree, that EMB should be performed "CMR-guided". Nevertheless, clinical studies should investigate, if sensitivity and positive predictive value vary depending on location of LGE and histological inflammation in humans, too.
Adequate histopathological criteria predicting progression into chronic disease or death are missing[
27] and further, using Dallas criteria often results in false-negative cases of myocarditis[
28]. Concluding, diagnostic information delivered by EMB seems to be reduced to displaying myocardial inflammation. Taking biopsies from myocardium that presents with contrast enhancement is described to be easier for the right ventricle but appears complex for the left ventricular wall. We were able to show that histological severity of myocarditis-induced injury correlates with the regional distribution and extent of LGE. Accordingly, CMR seems to provide similar diagnostic information (unmasking myocardial inflammation, its distribution and information about severity) as EMB. Kuhn et al.[
29] see no indication for EMB in routinely diagnosing myocarditis and at least we think that indication for EMB should be revised.
Munk et al.[
30] mentioned that troponin T levels below 0.1 μg/l predict absence of contrast enhancement in acute phase of myocarditis. In this aspect the current study results regarding the correlation between troponin T and size of LGE agree with Munk et al.
Study limitations
Differences in slice thickness of histopathological (2 μm) and MR examination (3 mm) has to be seen critically. As the histologically proven area of inflammation stretched across several slices, we do not think that differences in slice thicknesses had a strong influence on data analyses. Another limitation of the study is that criteria used for evaluation is center specific which may be difficult to put the results into perspective with other reports. We did not use Dallas criteria as their use often results in false-negative cases of myocarditis and they seem to be no predictor of outcome in myocarditis[
31]; instead we used above mentioned criteria which are standard in our department of pathology and were used in our previous research [
11].
The current study is based on an animal model for acute autoimmune myocarditis. In humans acute myocarditis is mostly caused by viral infection. However, in our animal model it is possible to perform CMR-examination and histolgical analysis of the whole myocardium on the same day. As we wanted to show the reliability of CMR in showing topography of myocardial inflammation, we do not think that different aetiologies (autoimmune in the current study vs. viral in humans) inhibits transfer of current study results to humans.
Using a 1.5T clinical scanner in combination with clinical MR-sequences has to be seen as a limitation regarding spatial resolution. However most centers use 1.5T scanners, thus we think, in this way our result can be compared to further studies more easily. Finally, we did not provide data about early enhancement, nor about T2-weighed images as proposed by Friedrich et al [
32].