Demographic features and prevalence of idiopathic myocarditis in patients undergoing endomyocardial biopsy

From January 1985 through December 1990, 534 patients underwent endomyocardial biopsy at Johns Hopkins Hospital for suspected myocarditis. One hundred thirty-eight (26%) biopsy specimens were diagnosed histologically by 2 cardiac pathologists as either active (n = 85, 16%) or borderline (n = 53, 10%) myocarditis. Of the 138 patients, 60 were excluded based on either specific concurrent clinical conditions or noncongestive heart failure presentations. Immunohistochemical staining for common leukocyte antigen infiltrating cells performed on the remaining 78 specimens confirmed the presence of focal or multifocal inflammatory infiltrates in 58, of which 49 had histologic evidence of active myocarditis. All 49 patients presented with congestive heart failure and left ventricular ejection fractions of <40%. Compared with patients with either idiopathic dilated cardiomyopathy (n = 207) or ischemic cardiomyopathy (n = 44), these patients with myocarditis had a less striking male predominance (58 vs 69 and 83%, respectively) (p = 0.02) and were younger (43 ± 16 vs 50 ± 17 and 55 ± 13 years, respectively, p = 0.005). Racial distributions were similar. A recent history of a discrete flu-like illness was obtained in 52%, two-thirds of which were clustered between the months of December and March. Onset of heart failure peaked between December and April (63% and was low between May through September (22%). A peak in the proportion of patients found to have active myocarditis on biopsy occurred in 1986 (17 vs 7 to 10% in other years). In conclusion, patients presenting clinically with idiopathic dilated cardiomyopathy with histologic evidence of active myocarditis confirmed by immunohistochemical staining accounted for 9% of all patients undergoing endomyocardial biopsy. Both seasonal and yearly variations in the proportion of such patients with biopsy-proven myocarditis appear significant.