Giant cell myocarditis, a rare, fatal, and poorly understood cause of myocarditis, requires pathological examination for diagnosis. It is considered to be an autoimmune disease and is frequently associated with other conditions, in particular thymoma and myasthenia gravis. The typical patient with giant cell myocarditis is young and has severe, progressive congestive cardiac failure that is unresponsive to standard medical therapy and ultimately requires cardiac transplantation. Hence giant cell myocarditis is the most dangerous form of myocarditis. Here we report an unusual presentation of giant cell myocarditis, which mimicked acute myocardial infarction in an elderly woman with myasthenia gravis and a previous diagnosis of thymoma. This patient had evidence of anti-myocyte antibodies, consistent with an autoimmune mechanism.

A clinical and histopathologic comparison of cardiac sarcoidosis and idiopathic giant cell myocarditis. J Am Coll Cardiol. 2003 Jan 15;41(2):322-9.

OBJECTIVES: The goal of this study was to determine the prognostic value of clinical data available at presentation and histology in cardiac sarcoidosis (CS) and idiopathic giant cell myocarditis (IGCM). BACKGROUND: The prognosis of patients with nonischemic cardiomyopathy is partly dependent on the histologic diagnosis. Survival in IGCM is poor. The prognosis of a histologically related entity, cardiac sarcoidosis (CS), is less well established, and the prognostic value of the distinction between CS and IGCM on endomyocardial biopsy (EMB) is unknown. METHODS: We identified 115 patients from the Multicenter IGCM Registry with CS (n = 42) and IGCM (n = 73). We compared the clinical data for these two groups using Cox proportional-hazards models to assess the association between histologic diagnosis and survival. In order to determine whether histologic features could reliably differentiate these two entities, two cardiac pathologists semiquantitatively graded the inflammatory infiltrate components and compared the results between groups. RESULTS: Black race was more frequent in the CS group (31% vs. 4%, p < 0.0001). Syncope and atrioventricular block were also more frequently observed in CS than IGCM (31% vs. 5%, p = 0.0002 and 50% vs. 15%, p < 0.0001, respectively). Left-sided heart failure was more common in IGCM (40% vs. 64%, p = 0.013). In CS patients diagnosed by EMB, the five-year transplant-free survival after diagnosis was 69.8% versus 21.9% for IGCM (p < 0.0001, log-rank test). In multivariate models, presentation with heart failure predicted IGCM, and presentation with heart block or more than nine weeks of symptoms predicted CS. Eosinophils, myocyte damage, and foci of lymphocytic myocarditis were more frequent in IGCM, while granulomas and fibrosis were more frequent in CS. CONCLUSIONS: Transplant-free survival is better for patients with CS than for IGCM diagnosed by EMB. Presentation with heart failure predicted IGCM, and presentation with heart block or more than nine weeks of symptoms predicted CS.