Determining the etiology of cardiac dysfunction in pa-tients with heart failure influences management and prognosis. 1 Myocarditis, diagnosed by the current histopathological Dallas criteria, accounts for 10% of patients with new-onset cardiac dysfunction submitted to endomyocardial biopsy. 1, 2 Despite complete evaluation including history, physical examination, blood work, echocardiography, coronary angiography, and endomyocardial biopsy, 50% of patients with dilated cardiomyopathy have no etiology identified. 1 Recent data suggest that patients in the “idiopathic” category may be suffering from myocardial inflammation due to persistent viral replication or autoimmune activation after a viral infection. These studies raise the question of whether the current histopathological criteria for myocardial inflammation (the Dallas criteria) are sensitive enough to identify the population with viral or autoimmune-related heart compromise.

The Dallas criteria were proposed in 1986 and provided a histopathological categorization by which the diagnosis of myocarditis could be established. Dallas criteria myocarditis requires an inflammatory infiltrate and associated myocyte necrosis or damage not characteristic of an ischemic event. Borderline myocarditis requires a less intense inflammatory infiltrate and no light microscopic evidence of myocyte destruction. 3 These criteria have been used exclusively by American investigators over the last 2 decades. Sampling error, variation in expert interpretation, variance with other markers of viral infection and immune activation in the heart, and variance with treatment outcomes all suggest that the Dallas criteria are no longer adequate. Chow et al and Hauck et al4, 5 demonstrated by biopsying postmortem hearts of patients who had died with myocarditis that, from a single endomyocardial biopsy, histological myocarditis could be demonstrated in only 25% of samples. With 5 biopsies, Dallas criteria myocarditis could be diagnosed in approximately two thirds of subjects. A recent MRI study used focal imaging abnormalities to guide heart biopsy investigation of possible myocarditis. The authors showed that the earliest myocardial inflammatory abnormalities were evident in the lateral wall of the left ventricle, and only these sites revealed myocarditis by histological examination. 6 Therefore, there is considerable sampling error associated with establishing the diagnosis of myocarditis.