Other types of heart muscle diseases may be caused by viruses. Patients with AIDS have had HIV viral infection of the heart muscle. The heart muscle may also be damaged by cocaine, an overload of iron (hemochroma-tosis), and some inherited conditions.
With different varieties of involvement of the heart muscle, classification became necessary. In the 1970s and 1980s cardiomyopathy was defined as heart muscle disease of unknown cause. The current classifications are:
cardiomyopathy
1. Hypertrophic cardiomyopathy
2. Dilated cardiomyopathy
3. Restrictive cardiomyopathy
4. Arrhythmogenic right ventricular (right ventricular dysplasia)
5. Unclassified cardiomyopathy: diseases that do not have features of 1through 4 and include fibroelastosis and mitochondrial disease
6. Specific cardiomyopathies (specific heart muscle diseases formerly called secondary cardiomyopathy).
Each of these cardiomyopathies will be discussed. Most of these diseases are rare, but hypertrophic cardiomyopathy has become well-known because it is one of the causes of sudden death in young athletes and young individuals.

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• I. HYPERTROPHIC CARDIOMYOPATHY
  Hypertrophic cardiomyopathy (HCM) is found through¬out the world with a prevalence in North America of 0.2%. Before the diagnosis of HCM is considered hyper¬tensive heart disease, a major cause of the left ventricle hypertrophy, and other causes of hypertrophy must be excluded. In practice HCM is defined and diagnosed by the demonstration of unexplained left ventricular hypertrophy.
• II. SUDDEN DEATH
  Death is most often sudden in HCM and unfortunately this may occur in asymptomatic patients, in those who were unaware that they have the disease, or in individuals with an otherwise stable course.
• III. DILATED CARDIOMYOPATHY
  Heart failure is rare in individuals younger than age 20. If congenital heart disease is excluded, the most common cause of heart failure in the young is idiopathic dilated cardiomyopathy (DCM). More than 50 known specific diseases of heart muscle can produce the signs, symptoms, and manifestations of idiopathic DCM. Some of these diseases include the following:
• IV. RESTRICTIVE CARDIOMYOPATHY
  Restrictive cardiomyopathy is rare in the western world and in Europe. Diseases that cause damage to the muscle and restrict the flow of blood into the ventricle include amyloidosis, sarcoidosis, hemochromatosis, scleroderma, Adriamycin toxicity, and heart involvement by infectious agents. The most common cause of restrictive cardiomyo-pathy, especially in tropical regions, is endomyocardial fibrosis.
• V. SPECIFIC HEART MUSCLE DISEASE
  Specific heart muscle disease usually produces a dilated form of cardiomyopathy with impaired systolic function. Restrictive physiology is seen with amyloid, sarcoid, neoplasm, radiation, scleroderma, hemochromatosis, and eosinophilic endomyocardial disease, in which eosino-philia is usually present. Rarely, myocardial tuberculosis is present with restrictive features. Amyloid heart disease and EMF are usually considered examples of RCM, but when cardiac involvement is associated with multiple organ disease, they qualify as specific heart muscle disease (see Table 1).
• BIBLIOGRAPHY
  Ackerman, M. J., McKenna, W. J., Thierfelder, L. et al. Mutations in the genes for cardiac troponins T and alpha tropomyosin in hypertrophic cardiomyopathy. N. Engl. J. Med., 332:1058–64, 1995.