Necropsy diagnosis of myocarditis: a retrospective study using CD45RO immunohistochemistry

AIM: To use CD45RO immunohistochemistry to investigate the numbers of T lymphocytes found in sections of myocardium from a routine necropsy series, and to determine the incidence of myocarditis in this series. METHODS: Myocardial sections from 163 routine hospital necropsies were stained with CD45RO and the numbers of positive lymphocytes/mm2 were counted. The results were correlated with the H/E opinion and the clinical context of the necropsy. RESULTS: Most (143) cases showed low numbers (0-3) of CD45RO positive lymphocytes/mm2. Fifteen cases showed 7-13 positive lymphocytes/mm2, comprising a wide variety of clinical conditions, generally with no specific cardiac pathology. Five cases showed 14 or more positive lymphocytes/mm2, comprising one case of active myocarditis, three cases of cardiac transplant rejection, and one post-transplant lymphoproliferative disorder, all conditions in which large numbers of lymphocytes would be expected. CONCLUSIONS: The incidence of myocarditis in our series was 0.6%. In most cases the normal myocardium has a low T lymphocyte count (0-3/mm2). In some cases immunohistochemistry shows more positive cells than would have been expected on light microscopy. Immunohistochemistry is a useful and reliable means of confirming a diagnosis of myocarditis. The results support the conclusion of the 1997 ISFC task force that 14 or more lymphocytes or macrophages/mm2 of myocardium in the appropriate clinical context is a reliable threshold for the diagnosis of chronic myocarditis.