Background: Although antimyosin scintigraphy detects myocyte necrosis associated with myocarditis, it has also been reported to yield positive results in a large number of patients with clinical dilated cardiomyopathy without histologic evidence of myocarditis. The question to be resolved is whether this discordance represents false-positive results of antimyosin scans or whether antimyosin scintigraphy more accurately identifies the presence of myocyte necrosis than does endomyocardial biopsy testing.
Methods and results: Forty patients with the acute onset of dilated cardiomyopathy (left ventricular ejection fraction < 45%; mean 27% +/- 11%) but no endomyocardial biopsy evidence of myocarditis, were identified from a consecutive series of 50 patients who had undergone indium 111 antimyosin antibody scintigraphy and endomyocardial biopsy for suspected myocarditis. The endomyocardial biopsy specimens were analyzed to identify features correlating with antimyosin uptake or improvement in left ventricular ejection fraction (LVEF) over time. Twenty-five patients showed left ventricular myocardial uptake of radiolabeled antimyosin antibody by both planar and tomographic imaging. The remaining 15 patients had no antimyosin uptake. Of the 25, 22 (88%) patients with positive findings on antimyosin scans had degenerated, myofibrillarlytic myocytes in their biopsy specimens. Of the 15 patients with negative findings on antimyosin scans, only 6 (40%) had similar myofibrillarlytic myocytes (chi 2 = 8.13; p < 0.0047). No other histological feature correlated with the antimyosin positivity. Stepwise multiple regression analysis was performed for identification of predictors of short-term improvement in LVEF. Patients with positive findings on antimyosin scans showed a trend toward improvement with time (F = 3.97; p > 0.05). None of the histologic features predicted improvement in the LVEF. However, the combination of positive findings on an antimyosin scan and myofibrillarlysis did correlate significantly with spontaneous improvement in ejection fraction (F = 4.53; 0.01; < p < 0.05).
Conclusions: This study identifies myofibrillarlysis as a common pathologic alteration in patients with recent onset of dilated cardiomyopathy and positive findings on antimyosin scan, who lack right ventricular biopsy evidence of myocarditis. Because myofibrillarlytic cell population may represent a histologic spectrum of viable to necrotic myocytes, it appears that antimyosin uptake detects necrotic myofibrillarlytic myocytes that are not identified by light microscopy.