ANTIMYOSIN UPTAKE AND MYOFIBRILLARLYSIS IN DILATED CARDIOMYOPATHY

Background. Although antimyosin scintigraphy detects myocyte necrosis associated with myocarditis, it has also been reported to yield positi ve results in a large number of patients with clinical dilated cardiom yopathy without histologic evidence of myocarditis. The question to be resolved is whether this discordance represents false-positive result s of antimyosin scans or whether antimyosin scintigraphy more accurate ly identifies the presence of myocyte necrosis than does endomyocardia l biopsy testing. Methods and Results. Forty patients with the acute o nset of dilated cardiomyopathy (left ventricular ejection fraction < 4 5%; mean 27% +/- 11%) but no endomyocardial biopsy evidence of myocard itis, were identified from a consecutive series of 50 patients who had undergone indium 111 antimyosin antibody scintigraphy and endomyocard ial biopsy for suspected myocarditis. The endomyocardial biopsy specim ens were analyzed to identify features correlating with antimyosin upt ake or improvement in left ventricular ejection fraction (LVEF) over t ime. Twenty-five patients showed left ventricular myocardial uptake of radiolabeled antimyosin antibody by both planar and tomographic imagi ng, The remaining 15 patients had no antimyosin uptake. Of the 25, 22 (88%) patients with positive findings on antimyosin scans had degenera ted, myofibrillarlytic myocytes in their biopsy specimens. Of the 15 p atients with negative findings on antimyosin scans, only 6 (40%) had s imilar myofibrillarlytic myocytes (X(2) = 8.13; p < 0.0047). No other histological feature correlated with the antimyosin positivity, Stepwi se multiple regression analysis was performed for identification of pr edictors of short-term improvement in LVEF. Patients with positive fin dings on antimyosin scans showed a trend toward improvement with time (F = 3.97; p > 0.05). None of the histologic features predicted improv ement in the LVEF. However, the combination of positive findings on an antimyosin scan and myofibrillarlysis did correlate significantly wit h spontaneous improvement in ejection fraction (F = 4.53; 0.01; < p < 0.05). Conclusions. This study identifies myofibrillarlysis as a commo n pathologic alteration in patients with recent onset of dilated cardi omyopathy and positive findings on antimyosin scan, who lack right ven tricular biopsy evidence of myocarditis. Because myofibrillarlytic cel l population may represent a histologic spectrum of viable to necrotic myocytes, it appears that antimyosin uptake detects necrotic myofibri llarlytic myocytes that are not identified by light microscopy.