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.