The diagnostic criteria of myocarditis by endomyocardial biopsy.

The difficulties in diagnosing acute idiopathic myocarditis have been highlighted. Only about 30% of clinically suspected cases show morphologic evidence of an inflammatory infiltrate. Difficulties experienced in obtaining positive results include timing of the biopsy in relation to the acute symptoms of the patient, sampling error, and quantitative criteria. In addition, pressure from the referring physician may influence the pathologic interpretation, i.e., in making a morphologic diagnosis on slender evidence. Caution is also necessary in the interpretation of end-stage disease of dilated cardiomyopathy and "chronic myocarditis." This is important as it influences therapy with immunosuppressive agents. The importance of obtaining a detailed history of drugs to which the patient might have been exposed and can result in myocarditis is also stressed. Only if an accurate and unbiased pathologic evaluation can be made will a prospective, randomized multicenter trial yield useful information. The Dallas Myocarditis Panel has set forth useful criteria and guidelines in an attempt to classify the morphologic diagnosis of myocarditis. Semantic and diagnostic criteria for myocarditis can still be challenged, but the Dallas criteria for evaluation does allow an accurate assessment by all pathologists, in spite of individual variation.