In one form of heart muscle disease, the muscle of the ventricle becomes considerably thickened to the point that the cavity of the left ventricle becomes nearly filled with muscle mass; thus less blood enters the chamber and less blood is expelled into the circulation. Because the muscle is enlarged, or hypertrophied, the disease is called hyper-trophic cardiomyopathy. This is a disease of young adults. See chapter entitled ‘‘Athletes and Sudden Cardiac Death.’’
The muscle enlargement may be so severe that it obstructs the flow of blood into the aorta, and death may occur suddenly, particularly in individuals from age 12 to 36 years. Some athletes who have died suddenly have had this disease. In some families, hypertrophic cardiomyo-pathy is caused by mutation in the cardiac myosin gene. Approximately 60% of cases occur in families with an autosomal dominant pattern and 40% of cases are sporadic.
In another form of heart muscle disease, the heart dilates without increasing the size of the muscle. The chambers are swollen, and the muscle becomes weak. This condition often results in failure of the heart to pump blood, which results in heart failure. Heart transplants are required in some of these patients (see the chapter Heart Failure).

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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.