Histopathologic and ultrastructural observations of acute and convalescent myocarditis: a serial endomyocardial biopsy study.

Serial endomyocardial biopsy findings were analyzed in ten cases with acute myocarditis of possible viral origin. The histopathologic findings were analyzed at the acute (0-10 days after the onset), subacute (11-21 days), and convalescent stages (22-167 days). The incidence and severity of various cardiac myocyte and interstitial changes were compared at each stage of the disease. The time-course changes of the histopathologic findings in acute myocarditis were as follows. At the acute stage, we observed: interstitial cell infiltration composed of fibroblasts, macrophages, and lymphocytes, fragmentation of the muscle bundles, myocytolytic changes, swelling and scarcity of the cytoplasm and swelling of nuclei, variation in size of the myocytes, disarrangement of the muscle bundles, interstitial edema, increased glycogen deposition in the myocytes, abnormal branching of the myocytes, and interstitial fibrosis. At the convalescent stage, most of the above findings could still be seen except for myocytolytic changes, swelling of myocytes, and interstitial edema. At the subacute and convalescent stages, an increase in abnormal branching and in double nuclei in the myocytes and nuclear degeneration became observable. A further control study comparing the changes at the convalescent stage of myocarditis and the myocardial changes in cases with chronic right ventricular overload in 58 cases revealed that in the former fragmentation of the muscle bundles, abnormal branching, size variation, glycogen deposition, and large mononuclear cell infiltrations were significantly more frequent. Ultrastructural observation of the myocytes revealed that they have a great potentiality for regeneration; myocytes showing severe myocytolytic changes may recover to regain an architecture of almost normal appearance.