RCM is a rare type of cardiomyopathy that is characterized by stiffening of the heart muscle, which impairs its ability to fill with blood. In restrictive cardiomyopathy (RCM), the muscle cells in the heart become replaced with abnormal tissue (such as scar tissue). This causes the muscle walls of the ventricles (the lower chambers of the heart) to become stiff, which restricts the movement of the heart. The pumping action of the heart is not usually affected, and the heart can contract to pump blood around the body. However, it is the filling function of the heart, where the heart muscle relaxes and the ventricles fill with blood, that is affected.
This poor filling function means that the ventricles receive less blood than normal, and so blood flow around the body and the heart is reduced. Also, blood gets 'backed up' as it cannot enter the heart as normal, which causes a build-up of pressure. This can cause the atria (top chambers of the heart) to enlarge.