Infantile histiocytoid cardiomyopathy--myocardial or conduction system hamartoma: what is the cell type involved?

Primary myocardial diseases in the pediatric age group encompass a variety of metabolic, infectious, congenital, and acquired disorders, one of which is "histiocytoid cardiomyopathy." We describe clinical and pathologic features in two infants. Autopsy findings in the first case were consistent with sudden cardiac death. The second infant has survived for 2 years on antiarrhythmic therapy with amiodarone. Microscopically, cells with vacuolated to granular cytoplasm were grouped in fascicles, imparting a pseudonodular appearance, but following a distribution reminiscent of conduction fibers. Ultrastructurally, the cells lack a T-tubule system, possess scattered lipid droplets and desmosomes rather than side-to-side junctions, and contain leptomeric fibrils that predominantly marginate to the cell periphery without sarcomeres. Immunostaining of paraffin-embedded tissue reveals perimembranous immunoreactivity for muscle-specific actin, but not for the histiocytic markers CD68 (KP1) and lysozyme. Immunohistochemistry may be a practical alternative when tissue is not saved for electron microscopy. The clinical and pathologic features of this disorder in light of the current literature suggest that it may be hamartoma, possibly of conduction system origin.