The heart is almost always affected in Duchenne and Becker muscular dystrophies, or DMD and BMD respectively, although the degree to which it is affected and the age at which it becomes a problem vary.

The main problem in both conditions is degeneration of the heart, which is, of course, a muscle. The medical term for this condition is cardiomyopathy. The problem caused by cardiomyopathy is heart failure, which means the heart isn't effective enough at pumping blood through the body. (In BMD, boys with mild skeletal muscle involvement can have severe cardiac involvement. The mildness of the skeletal muscle effects may give parents and doctors a false sense of security regarding the health of the heart.)