Giant cell myocarditis.

Giant cell myocarditis (GCM) as a distinct disease entity has been questioned. The superficial morphologic resemblance to cardiac sarcoidosis and incomplete histopathologic assessment of extracardiac organ systems in reported cases has suggested that GCM represents a predominant cardiac manifestation of generalized sarcoidosis. The morphologic and immunocytochemical features at autopsy of this rare disorder were seen in a 15-year-old boy. All other organs were free of granulomatous inflammation. Transition from normal myocardial fibers to giant cells was observed, without a limiting plasma membrane between the two different aspects of the fiber. Immunocytochemistry for cytoplasmic muramidase (CM) showed CM in neutrophils and tissue macrophages. Cytoplasmic muramidase was distinctly absent from the characteristic giant cells. The classification of GCM as a disease separate from generalized sarcoidosis may be justified.