Endomyocardial biopsy findings in 50 patients with idiopathic atrioventricular block: presence of myocarditis.

In patients with primary atrioventricular (AV) conduction disturbances, lesions are commonly believed to be limited to the conduction system. In the present study, myocardial lesions were analyzed histologically in patients with AV conduction disturbances to determine the presence of myocarditis. We studied 50 patients with second or third degree AV block in whom the cause was not clear (AVB group). Endomyocardial biopsy was performed from the right ventricle and examined by light microscopy. Tissue specimens also were obtained from 12 normal hearts at autopsy as a control group (N group). The diagnosis of myocarditis was based on the Dallas criteria. The myocyte transverse diameter was 15.4+/-4.1 microm in the AVB group and 11.7+/-3.1 microm in the N group (P<0.01). The fibrosis area ratio also was significantly higher in the AVB group than in the N group (10.1+/-6.7% vs 5.1+/-2.0%). The mean number of lymphocytes per 400-fold magnified field was significantly greater in the AVB group than in the N group (1.9+/-1.6 vs 1.3+/-0.4). In addition, disorganization of the cardiac myocytes was noted in 8 patients in the AVB group (16%), myocyte disarrangement in 39 patients (78%), myocytolysis in 23 patients (46%), and nuclear deformity in 21 patients (42%). Myocarditis was diagnosed in 3 of the 50 patients (6%). The present study demonstrates that myocardial lesions can be detected in a large proportion of patients with AV conduction disturbances. Furthermore, myocarditis is present in 6% of the cases.