Immunohistochemical diagnosis of myocarditis on (infantile) autopsy material: Does it improve the diagnosis?

The standard for the histopathologic diagnosis of myocarditis has been the Dallas criteria. Recently, immunohistochemical studies that include the specification and quantification of interstitial inflammatory cells have been proposed as the diagnostic approach for myocarditis. Cut-off limits regarding inflammatory cell numbers for the positive diagnosis of myocarditis have been recommended. However, it is unclear whether these can be applied to postmortem tissues or to infants, as they were established from endomyocardial biopsies and for adults. Nevertheless, cut-off limits for the postmortem diagnosis of myocarditis in the first year of life have been proposed. Studies using these cut-off limits identified myocarditis in a high percentage of presumed sudden infant death syndrome (SIDS) cases. These results were re-evaluated by the present study, which examined heart specimens from infants less than 1 year of age. The study had a test group of 92 SIDS cases and a control group of 15. Myocardial tissue was examined from eight standardized locations, stained with hematoxylin-eosin and for three different immunohistochemical reagents (LCA for leukocytes, CD68 for macrophages, CD45-RO for T-lymphocytes). Histopathological assessment of the number of inflammatory cells was carried out on an aggregate of 80 mm² of myocardial tissue per case. Myocarditis, based on the Dallas criteria, was histologically diagnosed in only two cases. Immunohistochemical quantification revealed elevated cell counts in the SIDS group for LCA and CD45-RO. However, those differences were neither statistically significant nor clinically relevant as the mean cell counts per mm² were low. The density of inflammatory cells differed considerably from section to section and even within single sections. Therefore the commonly used arithmetic mean value was not diagnostically relevant, suggesting cut-off values based on the arithmetic mean value as recommended in the literature, cannot be regarded as valid. At least in infants, the diagnosis of myocarditis from autopsy tissues still requires application of the Dallas criteria. Immunohistochemical methods cannot replace the conventional diagnosis of myocarditis.