Cardiomyopathy

Cardiomyopathy is a type of heart disease in which the heart becomes abnormally enlarged (enlarged heart), thickened and/or stiffened. As a result, the heart muscle’s ability to pump and/or receive blood is impaired. This condition is generally progressive and may lead to heart failure.

Cardiomyopathies may be caused by a wide range of conditions, including chronic diseases, alcoholism, viral diseases, heart attacks and many others. An affected heart may grow larger either by dilatation, thickening (hypertrophy) or both. Additionally, the heart may suffer from a reduced ability to relax. Abnormalities found in cardiomyopathy include:

• Thickened and/or dilated ventricles, especially the left ventricle. The upper chamber (atria) may also be involved and enlarged.
• Scar tissue, after a heart attack or inflammation of the heart muscle.
• Overall enlargement of the heart.
• Tendency to form blood clots within the heart due to stagnation of blood in the heart chambers and/or atrial fibrillation. This may lead to embolism with a reduced blood supply to the affected regions, such as the brain, extremities and gut.

Cardiomyopathies cause symptoms including shortness of breath, chest pain, fainting, dizziness and a reduced ability to exercise. In addition, people with cardiomyopathy are at increased risk of abnormal heart rhythms, or arrhythmias. Particularly severe arrhythmias may lead to fainting (syncope) or even sudden cardiac death. According to the American Heart Association's 2006 Heart and Stroke Statistical Update, more than 27,000 deaths each year are caused by cardiomyopathy.

Physicians classify cardiomyopathies in a number of ways, and over the years, the definition of cardiomyopathy has evolved. In 2006, the American Heart Association offered a new definition for cardiomyopathy that generally grouped cardiomyopathies into two categories: primary, which are essentially limited to the heart, and secondary, which are caused by disease in other organ systems. For example, a cardiomyopathy that is caused by coronary artery disease is a primary cardiomyopathy (as well as an ischemic cardiomyopathy), while a cardiomyopathy caused by an autoimmune disorder is a secondary cardiomyopathy.

These definitions are not meant to help physicians diagnose cardiomyopathy, but to increase understanding of exactly what the condition is. Typically, physicians will approach the diagnosis and treatment of cardiomyopathy depending on exactly how it affects the anatomy of the heart and what underlying causes are present. Anatomical descriptions of cardiomyopathy include:

• Dilated cardiomyopathy (including peripartum cardiomyopathy and alcoholic cardiomyopathy), which involves dilation or enlargement of the heart’s ventricles and an increase in the size of the heart overall. This condition may be caused by heart disease, viruses, abuse of illicit drugs and alcohol, and genetic disorders. Additionally, idiopathic dilated cardiomyopathy (also called dilated cardiomyopathy of unknown cause) is not uncommon. Dilated cardiomyopathy causes about 10,000 deaths annually in the United States and is a major cause for heart transplantation.
• Hypertrophic cardiomyopathy, which involves an abnormal growth of muscle fibers in the heart muscle, usually in the left ventricle. In this case, the volume of the left ventricle is normal or reduced, but the additional muscle fibers prevent the chamber from relaxing completely after contraction (diastole), making it a diastolic dysfunction. This is usually considered a genetic disorder, but can also be caused by high blood pressure and disease of the aortic valve. Severe thickening of the heart muscle may cause obstruction of blood flow from the left ventricle to the aorta, resulting in low heart output, fatigue, fainting and arrhythmias.
• Restrictive cardiomyopathy, which means the heart muscle cannot adequately fill with blood after contraction, making this a diastolic disorder. This disease is more common in the tropics, where it may be caused by a condition known as endomyocardial fibrosis.
• Arrhythmogenic right ventricular dysplasia (ARVD). Less common types of cardiomyopathy include arrhythmogenic right ventricular dysplasia, a condition in which muscle in the right ventricle is replaced by fat and fibrous tissue.

Cardiomyopathy may also be classified by its underlying cause. These may include:

• Ischemic cardiomyopathy. This condition is caused by a lack of blood flow to the heart, usually as a result of coronary artery disease or heart attacks.
• Valvular cardiomyopathy. This may be caused by heart valve disease, which restricts the natural flow of blood through the heart.
• Hypertensive cardiomyopathy. This condition is caused by high blood pressure and is characterized by heart failure and hypertrophy of the left ventricle.
• Inflammatory cardiomyopathy. This is caused by an inflammatory condition that affects the heart, such as an autoimmune disease or infection. An autoimmune response occurs when the body's immune system mistakes body cells for foreign objects and attacks them. If this occurs in the heart, it may result in dilated cardiomyopathy.

Researchers are continually searching for more information on the potential causes, treatments and methods of preventing cardiomyopathy. Recent research includes:

• Researchers believe that many cases of idiopathic dilated cardiomyopathy (DCM), which account for about 50 percent of all DCM cases, may be genetically related. In recent years, a number of genes have been uncovered that are linked to DCM, including genes that code for heart and skeletal muscle. Scientists have also found that a significant number of people with dilated cardiomyopathy have autoantibodies in their bloodstream that target heart cells, and their family members also have these autoantibodies. This research may increase our ability to recognize people at greater risk for DCM and may one day even provide new treatment options.
• Researchers discovered the presence of a type of virus called Coxsackie B in heart tissue samples taken from patients with dilated cardiomyopathy who had previously been classified as having idiopathic DCM. In a significant number of patients, the virus was self-replicating. Further research may result in the use of antiviral agents to combat the Coxsackie B virus as part of the treatment of DCM.
• Studies have linked hypertrophic cardiomyopathy with irregularities in the connective tissue of the heart. Some autopsies have shown that patients with hypertrophic cardiomyopathy have increased amounts of connective tissue in their hearts. The connective tissue may be arranged in a disorganized and abnormal pattern, leading researchers to speculate that this pattern itself may be a cause of hypertrophic cardiomyopathy.

Clinical studies have also focused on a number of factors that may affect the development of cardiomyopathy in both children and adults. Other clinical studies have examined prevention and treatment of the disease.

Scientists have discovered more than 100 gene mutations that may play a role in the development of cardiomyopathy. The following genes have been linked to cardiomyopathy:

• Actin. A type of sarcomere protein gene, a group of genes that form the basic structure of muscle functions. Actin is believed to affect the contractions of the heart, and defects in this gene have been found to lead to hypertrophic cardiomyopathy.
• Desmin. An intermediate filament between cardiac and skeletal muscle. A mutation of this gene has been linked to cases of inherited cardiomyopathy.
• Dystrophin. A gene that plays a role in muscle function. Defects in this gene can lead to an inability for muscle to regenerate. Specific dystrophin gene mutations have been linked to cardiomyopathy, muscular dystrophy and other disorders.
• Tafazzin. A gene involved in musculoskeletal function.
• ß-myosin. One of a family of genes responsible for creating myosin, a protein involved in the contraction and relaxation of the heart muscle.
• Troponin T. A gene responsible for creating troponin T, a protein involved in the contraction and relaxation of the heart muscle.

Additional genetic research may lead to new tests, treatments and prevention methods for cardiomyopathy.