The diagnosis and classification of myocarditis are challenging. Use of the endomyocardial biopsy for the evaluation of patients with new-onset congestive heart failure or arrhythmias places the surgical pathologist in a critical role in the diagnosis and management of myocarditis. Direct and open communication with clinicians is essential for accurate clinical-pathologic assessment. Recognition of the architectural alterations in the myocardium and the predominant inflammatory cell type narrow the diagnostic possibilities. Lymphocytic infiltrates are found in lymphocytic myocarditis, some viral types, toxic myocarditis, sarcoidosis, hematopoietic malignancies, myocarditis associated with collagen vascular diseases, and postpartum myocarditis. Infiltrates composed predominantly of neutrophils suggest suppurative myocarditis, pressor effect, ischemic necrosis, and early viral and idiopathic myocarditis (particularly in children). Eosinophils may be a minor component of idiopathic and giant cell myocarditis but are predominant in hypersensitivity and parasitic myocarditis and in hypereosinophilic syndrome. Giant cells are seen in IGCM, sarcoidosis, rheumatic fever, and granulomatous infections and occasionally in idiopathic myocarditis (often of myogenic origin).12 The treatment and prognosis for many of these types of myocarditis differ significantly, and therefore accurate classification is important.
The relationship of idiopathic myocarditis to the subsequent development of idiopathic dilated cardiomyopathy remains controversial. The application of molecular techniques has broadened our concepts about the pathogenesis of both disease processes. These clues should lead to therapeutic strategies for their treatment and prevention. The surgical pathologist will continue to play a central role in these efforts.
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