The least common of the cardiomyopathies, restrictive cardiomyopathy is a condition in which the walls of the lower chambers (ventricles) are abnormally rigid, but not necessarily thickened or enlarged. These rigid walls lack the flexibility to expand as the ventricles fill with blood. Restrictive cardiomyopathies are characterized by normal ejection fractions (50 to 70 percent), but the pressure within the ventricle remains high through the relaxation phase of the heartbeat, due to increased stiffness of the ventricular walls. As a result, it is known as a diastolic cardiomyopathy, or one that affects the relaxation phase of the heart cycle.

Although the cause of restrictive cardiomyopathy is sometimes unknown (idiopathic), known causes include:

• Buildup of fat and proteins (amyloidosis) around the heart muscle, a condition that is frequently inherited, but may be associated with acquired diseases such as multiple myeloma and chronic infections.
  
  
• Iron overload (hemochromatosis) in the heart, a condition that may be inherited or may be the result of another condition that required multiple blood transfusions (e.g. Cooley’s anemia) and later resulted in iron overload and restrictive cardiomyopathy
  
  
• Scarring of the heart from illness or open-heart surgery.
  
  
• Chest exposure to radiation or chemotherapy.
  
  
• Inflammation of the inner lining of the heart due to abnormal activity of eosinophils (a type of white blood cell), a condition known as hypereosinophilic syndrome.
  
  
• Connective tissue diseases or various other illnesses.

Although restrictive cardiomyopathy is rare in the United States, it is a somewhat more common cause of death in the tropics, including Africa, India, South and Central America, because of the higher prevalence of endomyocardial fibrosis, a disease of the heart lining.