Leukocytic infiltrates in idiopathic dilated cardiomyopathy. A source of confusion with active myocarditis.

The histologic criteria for the endomyocardial biopsy diagnosis of idiopathic dilated cardiomyopathy (IDCM) and active idiopathic/viral myocarditis are unclear. The present study was undertaken to characterize the nature of the inflammatory cell infiltrates in IDCM and thereby refine the differential diagnostic criteria for distinguishing IDCM from myocarditis using endomyocardial biopsy. We examined a mean of 6.2 large random sections from excised hearts of all cardiac transplant recipients at Stanford University with a diagnosis of IDCM, from June 1968 through June 1984. The 108 cases were evaluated for inflammatory cell type, extent, and location. Thirteen percent had no infiltrate, 32.5% had 1-5 foci of at least five inflammatory cells, 47% had 6-30 foci, and 7.5% had 30 or more foci. The infiltrates were primarily lymphocytic; while they were usually in the myocardial parenchyma, infiltrates were also located in zones of fibrosis, the endocardium, the epicardium, and surrounding vessels. Pretransplant biopsies in 56 of the 108 cases were available for review, and 55% of these contained inflammatory cell infiltrates. Agreement between the presence of infiltrates in the biopsy and the resected heart was obtained in 64%. This study highlights the high incidence of inflammatory cell infiltrates in the hearts of patients with IDCM. It reinforces the need for interpreting lymphocytic infiltrates in an endomyocardial biopsy with caution, as their mere presence does not necessarily imply a diagnosis of active myocarditis.